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physiology of digestive system.ppt

The document provides an overview of the physiology of the digestive system. It discusses the basic functions of the digestive system which include ingestion, digestion, absorption, and defecation. It describes the organs that make up the gastrointestinal tract (GIT) and their roles, including the mouth, esophagus, stomach, small intestine, large intestine, liver, gallbladder and pancreas. It also discusses the layers of the GIT wall, regulation of digestive functions by nerves and hormones, and the roles of saliva, stomach secretions, bile, and pancreatic juices in digestion.

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Physiology of the Digestive System By: Yerukneh Solomon (MSc.)
Outline: • Digestive System • Role of GI-system in Homeostasis • Basic functions of the digestive system • General characteristics of the GIT • Functional structure, layers of the GIT • Accessory organs and their functions • Regulation of GIT • Receptors of the GI Tract • Functional movement of the GIT • Salivary Glands and Saliva • Stomach • Secretary function of GIT • Small Intestine: • Large Intestine
Digestive System
Digestive System (GIT) What are the basic functions of the GIT (esophagus-to-Anus)?... It provide the body with a continuous supply of: • Nutrients • Water and • Electrolytes Digestive organs
Overview: Functions of the Gastrointestinal Organs • Four processes carried out by the gastrointestinal tract: digestion, secretion, absorption, and motility
Role of Digestive system in Homeostasis - Body cells require nutrients to obtain energy and function properly. - This supply of energy is accomplished by gastrointestinal system (GIS) that breaks food by: a. Mechanical and b. Chemical processes until the food is changed into its usable form - Therefore, the GI-system is one of the important homeostatic organs involved in maintaining the day-to-day balance of food intake and excretion, thereby sustaining life.
Basic functions of the digestive system The GI system carries out the following basic activities: 1. Ingestion: food intake, which is controlled by the feeding and satiety center in the HT. 2. Mastication or chewing: mechanical grinding of food with the aid of the teeth. 3. Swallowing or deglutition: propulsion of food from the mouth to the stomach. 4. Chemical digestion of food 5. Secretion of enzymes, electrolytes (HCl, NaHCO3), mucous, and hormones 6. Absorption of nutrients, water & electrolytes into the blood vessels 7. Defecation: excretion of fecal matter
Digestive Process Figure 23.2 ~ Mouth Food ingestion controlled by Hypothalamus ~ Esophagus Simple passage of food ~ Stomach Temporary storage, Mixing, digestion, Secretion and emptying ~ Small intestine Digestion, secretion Absorption ~ Large Intestine Storage, absorption, Passage of faecal matter
GIT and its natural defense • GIT is hollow at both ends (mouth- to - Anus). – Therefore, can possibly harbor microorganisms in its luminal surfaces. • However, the GI-system can protect itself from such hazards by some defense mechanisms found in: – a. Mouth: Saliva contains lysozymes, IgA etc – b. Stomach: HCl, Pepsin etc. have bactericidal effect – c. SI (e.g., Payer's patches): Immuno-competent lymph tissues – d. Macrophages: located in intestinal walls act to defend from bacterial invasion etc
Fig.1: GIT-organs and its accessories 1. Mouth 2. Pharynx 3. Esophagus 4. Stomach 5. Small intestine 6. Large intestine 7. Rectum (Anus) Main GI-Organs Accessory Organs 1. Salivary glands 2. Pancreas 3. Liver & 4. Gallbladder I. Functional structure of the gut
Digestive System… Organs involved in the process of digestion include: 1. The alimentary canal or gastrointestinal tract (GIT) that used to digests and absorbs food – mouth, pharynx, esophagus, stomach, small intestine, and large intestine – teeth, tongue, gallbladder, salivary glands, liver, and pancreas • Teeth aids mechanical breakdown of food • Tongue assists chewing and swallowing and speech • Other accessory organs produce and send secretions that facilitate chemical breakdown of food 2. Accessory organs
Accessory organs • Lies deep to the greater curvature of the stomach • The head is encircled by the duodenum and the tail adjacent to the spleen • Divided into: Head, body and tail • Connected to the duodenum via the main pancreatic duct and accessory duct Anatomy of the Pancreas
THE PANCREAS… • Pancreas contains two types of secretary glands: 1. Endocrine cells (islets of Langerhans) * secrete hormones 2. Exocrine cells (acinar cells) * secrete a mixture of fluid rich in NaHCO3 and digestive enzymes called pancreatic juice. Involved in digestion processes by producing: a. Digestive enzymes: necessary to digest CHO, fat, and protein b. Bicarbonates : to neutralize the gastric juice c. Water and electrolytes (Na+, K+ etc): Fig. Acinus of the Pancreas
Digestive enzymes 1. Proteolytic enzymes: enzyme that helps to digest proteins. * Trypsin: activated by enterokinase, a duodenal enzyme that converts trypsinogen to trypsin * Chymotrypsin: protein-digesting enzyme in pancreatic juice, activated by trypsin * Carboxypolypeptidase: activated by trypsin * Ribonuclease: to digest nucleic acids * Deoxyribonuclease, Elastase, collagenase 2. Pancreatic enzymes involved in CHO digestion: * Pancreatic amylase 3. Pancreatic enzymes involved in fat digestion: * Pancreatic lipase, Cholesterol esterase, Phospholipase
Hormonal regulation of pancreatic secretion • Fats and amino acids stimulate the duodenum to releases CCK and Secretin. • These hormones flow through the blood and reach the pancreas. • In the Pancreas two processes take place a . CCK stimulates acinar cells to release digestive enzymes b. Secretin stimulates duct epithelium to release H2O & HCO3- • Both flow through common bile duct to duodenum to do their specific job.
LIVER AND GALLBLADDER Liver • The liver is the heaviest gland in the body and the second largest organ in the body after the skin. • Weighs 1.36kg. • Located below diaphragm in the abdomen • Right lobe larger • Gallbladder on right lobe • Size causes right kidney to be lower than left
Liver and Gallbladder… • The liver is divisible into left and right lobes, separated by the falciform ligament. • The gallbladder is a sac located in a depression on the posterior surface of the liver.
Liver: Associated Structures… Bile leaves the liver via: • Bile ducts, which fuse into the common hepatic duct • The common hepatic duct, which fuses with the cystic duct • These two ducts form the common bile duct
Liver: Microscopic Anatomy • Hexagonal-shaped liver lobules are the structural and functional units of the liver. – Composed of hepatocyte (liver cell) – Portal triads are found at each of the six corners of each liver lobule Portal triads consist of: 1. Bile duct 2. Hepatic artery – supplies O2 rich blood to the liver 3. Hepatic portal vein – carries venous blood with nutrients from digestive viscera
Liver: microscopic structures… • Liver sinusoids – enlarged, leaky capillaries located between hepatic plates Cells in the liver • Kupffer cells – hepatic macrophages found in liver sinusoids. • Hepatocytes’ functions include: – Production of bile – Processing blood-borne nutrients – Storage of fat-soluble vitamins – Detoxification
Functions of the liver 1. On Carbohydrate Metabolism:  It is the site of glycogenesis, gluconeogenesis and glycogenolysis • Turn proteins into glucose • Turn triglycerides into glucose • Turn excess glucose into glycogen & store in the liver • Turn glycogen back into glucose as needed 2. On Lipid Metabolism  It is the site of β-oxidation, formation of phospholipids, lipoproteins, synthesis of cholesterol, and conversion of CHO into fat
Functions of Liver… 3. Protein Metabolism – Deamination of amino acids = removes amine group (NH2) from amino acids so can use what is left as energy source. – Converts resulting toxic ammonia (NH3) into urea for excretion by the kidney. – Synthesizes plasma proteins utilized in the clotting mechanism (fibrinogen) and immune system (gamma globulin for antibody production) – Convert one amino acid into another
Other Functions of the liver 4. Inactivation of drugs & hormones (Sulfonamide, penicillin, thyroid .H, steroids) 5. Removes the waste product; bilirubin 6. Releases bile salts that help digestion of fat by emulsification. 7. Stores: fat soluble vitamins (A, B12, D, E, K), iron, copper and blood (a major blood reservoir) 8. Filtration of blood: phagocytizes worn out blood cells & bacteria. Removes blood clots and toxins from portal circulation 9. Activates vitamin D 10. Synthesis of blood clotting factors (F-I, II, VII, IX, X)
Bile Production • Bile is an alkaline fluid (pH 8) • secreted in the liver and functions in emulsification of fat in the duodenum. • Emulsification means changing greater fat globules into smaller fat-droplets called micelles
Secretion of Bile Bile is secreted by hepatocytes in the liver for two purposes 1. It facilitates fat digestion and absorption of in the SI 2. Serves as a means of excretion of waste products (bilirubin & cholesterol) Components of Bile • Bile contains the following constituents: 1. Bile salts (bile acids), ~11% 2. Bile pigment (bilirubin), ~1% 3. Others organic constituents like: Cholesterol, protein etc ( ~3%) 4. Electrolytes (Na+, K+, Ca2+, Cl-, and greater HC03- than plasma) ~1%. 5. H2O (~ 84%)
Regulation of Bile Release • Acidic, fatty chyme causes the duodenum to release: – Cholecystokinin (CCK) and – secretin into the bloodstream • CCK and secretin transported in blood stimulate the liver to produce bile • Vagal stimulation causes weak contractions of the gall bladder
The Gallbladder • Thin-walled, green muscular sac on the ventral surface of the liver • Stores and concentrates bile by absorbing its water and ions • Releases bile via the cystic duct, which flows into the bile duct
Facts… • The average adult consumes about 800 g of food and 1200 ml of water per day. • An additional 7000 ml of fluid from salivary glands, gastric glands, pancreas, liver, and intestinal glands is secreted into the tract each day. • 99 % is absorbed; only about 100 ml is normally lost in the feces. Fig: Average amounts of solids and fluid ingested, secreted, absorbed, and excreted from the gastrointestinal tract daily
Characteristics, Length of the GIT - ~ 9 meters long. - Is open at both ends and highly susceptible in harboring microbial agents. - Voluntary segments of the GIT are mouth, tongue, upper 1/3 of the esophagus and the anal regions. - The GI-wall possesses several glands that secrete digestive juices and enzymes into the lumen.
Functional structure, layers of the GIT Functions 1. Mucosa: protection, secretion & absorption of the luminal surfaces 2. Submucosa: blood vessels, glands, lymph, nerve plexuses are found.  Submucosa necessary for motility + secretion 3. Muscularis externa : Circular & Longitudinal smooth mus., mix and propel the chyme. 4. Serosa: outer most protective layer, consists of connective tissues.
1. Mucosa: Functions of the Mucosa The mucosa is divided into 3-sub- layers a. Epithelium:covers the mucosa and is columnar or stratified squamous cells. b. Lamina propria: is under the epithelium and contains capillaries for absorption & some lymph nodules for immunological defenses. C. Muscularies mucosa: Located underneath the lamina. • It is a thin layer of smooth muscle and its contraction and relaxation changes the degree or folding of the luminal diameter.
(Cont… GIT) Function of Submucosa 2. Submucosa - is located beneath the mucosa - Contains rich supply of  blood vessels,  nerves,  lymph nodes,  glands, and  nerve plexuses of the enteric nervous system called Submucosa (Meissiner’s) plexus.
(Cont…GIT) Function of Muscularis externa 3. Muscularies Externa (has 2- sublayers) and also contains myenteric nerve plexus. a. The inner circular layer their contractions decrease the diameter of the lumen. – At some regions, the circular SM thicken and forms a sphincter that prevents backflow of food contents. b. The outer longitudinal layer when it contracts, it decreases the length of the tract and cause shortening and lengthening of the tract. – The effect of both contractions helps to mix and propel the chyme (i.e., towards the anus).
(Cont… GIT) Function of the Serosa 4. Serosa – Is an outermost layer consisting of connective tissues. – It protects the underlying tissues and supplies blood vessels, lymph and nerves to the gut wall. – Its squamous epithelial cells secrete serous fluid that helps in moistening & lubricating the tubes outer surface. – This helps the abdominal cavity to slide freely against one another, there by decreasing friction.
Regulation of GIT Neural & Hormonal Regulation of the GIT • Regulation of GI-activities is the role of Neural & Hormonal mechanisms: A. Neural regulation of GI-activities: includes that of a. Extrinsic (Autonomic N fibers) b. Intrinsic (Enteric fibers) B. Hormonal regulation: Includes secretion of different hormones like Gastrin, CCK, Secretin, GIP etc. C. Paracrine Regulation: • Paracrine secretions flow into the interstitial fluid and diffuse to neighboring cells where they exert their effects. E.g.: - Histamine (stimulates parietal cells to secrete HCl) - Somatostatin (inhibit secretion of gastrin by G-cells)
Neural Innervations of GIT A. Somatic innervations: The voluntary muscle fibers at the upper esophagus and lower end of the GIT are controlled by somatic nerves that are voluntarily in action. B. Autonomic innervations (extrinsic regulation): - have generally a modulatory role (cause strong smooth muscle contractions by peristalsis) C. Enteric NS innervations (intrinsic regulation)
A. Autonomic Innervations (extrinsic regulation): • Functions of the parasympathetics & sympathetics nerve fibers. a. Parasympathetic fibers (cholinergic, Ach): - is excitatory and causes strong contractions on longitudinal muscles. - At sphincters (at circular SM): PS-fibers cause dilation. b. Sympathetic fibers (adrenergic, NA & adrenaline): - is generally inhibitory in action and causes dilatation of the alimentary tract. - At sphincters, it causes constriction of the circular SM
Neural control of the GIT, cont,d B. Enteric NS: shows local, intrinsic regulation: - Can act independently to cause motility & secretion of the gut - Are very numerous in No (approx. > 100 mill. neurons) and are sometimes called little brain. Types of NTs secreted from these neurons: Several e.g., Ach, NE, Dopamine, CCK, Substance P, Vasoactive intestinal polypeptide (VIP), Somatostatin, etc. Components of ENS includes: 1. Myenteric (Auerbach’s) plexus: - mainly control motility of the GIT 2. Submucosa (Meissner’s) plexus: - mainly control glandular secretions
Receptors of the GI Tract • Receptors initiate GI-reflexes that: – Activate or inhibit digestive glands – Mix lumen contents and move them along • Mechanoreceptors: ~ respond to distension, spastic contraction • Chemoreceptors: ~ respond to osmolarity, irritation, pH, presence of fat and protein food and end products of digested food • Thermoreceptors: respond to warm food/drinks Diffusion of gasses and other food contents are affected by temperature. When T- Increases rate of diffusion also increases • Pain receptors: respond to tissue injury in the GIT.
Nervous Control of the GI Tract: Intrinsic and extrinsic control I. Intrinsic control - Involves the enteric nerve plexuses present with in the GIT. Nerve plexuses near the GI tract initiate short reflexes: • Submucosal plexus (plexus of Meissner) – controls GI secretary activities • Myenteric plexus (plexus of auerbach) – controls motility of the gut
cont,d II. Extrinsic control Involve CNS centers and extrinsic autonomic nerves • They control GI motility and secretion • They also modify the action of enteric nerves ~ Sympathetic NS ↓GI function, ↓Motility ↓Secretions ~ Parasympathetic NS ↑GI function, ↑Motility,↑Secretions
GIT Reflexes: What do u understand from this Fig.? Figure 23.4
GIT reflexes 1. Reflexes that occur entirely within the enteric nervous system. These include reflexes that control  GI-secretion, peristalsis and mixing-contractions, local inhibitory effects. (Short reflexes)
Reflexes: cont,d 2. Reflexes that arise from the gut go to the sympathetic ganglia and then back to the GI-tract. (intermediate reflexes) Examples: a) The gastro-colic reflex: signals send from the stomach to cause evacuation of the colon (LI) b) The entero-gastric reflexes: signals from the colon & small intestine to inhibit stomach motility and secretion. c) The colono-ileal reflex: reflexes from the colon to inhibit emptying of ileal contents into the colon.
Reflexes: cont,d 3. Reflexes from the gut to the spinal card or brain stem and then back to GIT: (long reflexes) • Example: defecation reflex that travel to – the spinal cord and back again to produce the powerful colonic, rectal, abdominal contraction required for defecation (the defecation reflexes).
Hormonal control of GI function • Entero-endocrine cells produce several GIT- hormones, capable of regulation motility and secretary activities. 1. Cholecytokinin (CCK, 33aas) • secreted by the mucosa of the jejunum in response to the presence of fatty food in the intestine. • has a very potent effect on gallbladder contractility for expelling bile into the intestine in order to facilitate fat digestion and absorption. • Increase the release of pancreatic lipase from acinar cells. • CCK inhibits stomach motility in order to give adequate time for fat digestion. 2. Secretin (27aas) • Secreted by the mucosa of the duodenum in response to acidic gastric juice pumped from the stomach. • It increases NaHCO3 secretion by the pancreas, a buffer that used to neutralize the acid..
Functional movement of the GIT • Two basic types of movements occur in the GIT: 1. Propulsive movements: • which cause food to move forward along the tract at an appropriate rate for digestion and absorption. * Peristalsis: is the basic propulsive movement of the GIT that appears in the form of contractile rings around the gut and propels to the anal ward direction. • Peristalsis is an intrinsic property of the smooth muscles in the GIT that generate action potential rhythmically (basic electrical rhythm, BER). Peristalsis and Segmentation
Peristalsis Factors that initiate peristalsis • Distension (overstretching) • Irritation of epithelial lining • Nervous mechanisms: ANS:- Parasympathetic nerve stimulation ENS:- myenteric plexus • Hormonal mechanisms: Gastrin, CCK, Insulin, Motilin • Drugs: Ach-like drugs: carbacoline, pilocarpin Factors that inhibit peristalsis • Congenital absence of myenteric plexus Drugs: cholinergic antagonists • Nervous mechanism: stimulation of SNS • Hormonal mechanisms: secretin, GIP, somatostatin, VIP, sub-P
2. Mixing movements : • which keep the intestinal contents thoroughly mixed at all times. • Mixing contractions are beneficial to mix the food contents with gastric juice (chyme). - Pacemaker cells are normally located at Fundus region and fire spontaneously causing the so called basic electrical rhythm (BER). - BER are slow waves that result in tonic/energized and weak contractions of the stomach walls. • When slow waves progress to the antrum, the peristalitic contractions becomes strong and cause mixing of the stomach contents. • The contractions also close the pyloric sphincter, which aids in the repulsion of the chyme back to the stomach. • This process helps in mixing the chyme.
Clinical Paralytic ileus: • Paralysis of intestinal motility that is caused by increased in sympathetic stimulation and damage to the smooth muscles Hunger pangs (no food in stomach) can sometimes produce strong peristalitic contractions. • Such contractions fuse together and cause pain sensations.
Mastication (Chewing) • Chewing is a process of mechanical breakdown of food. • Salivary secretion containing amylase involves chemical digestion and lubrication of chewing. • Teeth, tongue, jaws and lips are involved in chewing. • The movt of these organs are controlled by such centers located in the brainstem, HT, amygdala and cerebral cortex. • Mastication muscle are supplied mainly by the motor branch of the trigeminal nerve. • Chewing center is located in the pons
Deglutition (Swallowing) • It is the propulsion of food from mouth to the esophagus that is controlled by the swallowing center in the medulla. • Involves the coordinated activity of the tongue, soft palate, pharynx, esophagus and many separate muscle groups. Has 3 stages: 1. Voluntary stage of swallowing: Buccal/oral phase – bolus is forced into the oropharynx 2. Involuntary stage A. Pharyngeal stage of swallowing: – contributes the passage of food through the pharynx to the esophagus. – Controlled by the medulla and lower pons. B. The esophageal stage of swallowing: – promote the passage of food to the stomach.
Secretion of the esophagus • Mucous glands that surround the esophagus secrete mucus that provide lubrication and protection from gastric digestion (HCl) that may arise from reflux of stomach contents.
Lower esophageal (Gastroesophageal) sphincter • It is a thickened circular smooth muscle at the junction b/n the esophagus and the stomach. • Function: prevents the reflux of gastric contents into the esophagus.
Common Disorders of the Esophagus 1. Gastro-esophageal reflux disease (heart burn). • It occurs when the cardiac sphincter remains open (incompetence of the lower esophageal sphincter) . 2. Achalasia: • The lower esophageal sphincter remains contracted (closed). This effect results in accumulation of food in the esophagus causing several disturbances (bad smell) and dilation of esophageal walls. (lower esophagus)
Digestive Processes Mechanical Digestion: ~ The physical breakdown of food. Ex: Grinding of the teeth or churning of the stomach. Chemical Digestion: ~ The catabolic breakdown of food using enzymes & acids. Absorption: ~ The passage of digested food from the lumen of the GI Tract into the blood or lymph.
The Mouth • The mouth (oral cavity) is responsible for mechanical digestion of solid food by mastication. • Mastication helps mix food with saliva. Parts of the oral cavity: - The cheeks, - The lips - The tongue - The hard and soft palate - The teeth - Saliva is released into the oral cavity
Salivary Glands and Saliva • Saliva is produced in and secreted from salivary glands. • The basic secretory units of salivary glands are clusters of cells called an acini. • These cells secrete a fluid that contains; – water, – electrolytes, – mucus and – enzymes • All of which flow out of the acinus into collecting ducts.
Types of Salivary Gland cells • The secretary unit in salivary glands is the acini. There are two types • Serous acini: secret watery secretion rich in the enzyme pthyalin. • The gland is called serous salivary gland • Mucous acini: secret thick and sticky secretion rich in mucin, a glycoprotein, which disolves in water gives mucus. • The gland is known as mucus salivary gland.
Functions of Saliva a. Digestion: CHO-digestion begins in saliva . • The enzyme ptyline breaks starch- to-maltose. • Lingual lipase begins fat digestion in the mouth. b. Protection: has anti-microbial actions (contains Lysozyme & thiocyanate that kills microbes). c. Involved in speech: Clear & fluent articulation is possible in the presence of saliva. d. Secretes HCO3 - : good to maintains the pH to neutral range (6-to-7), the neutral pH is good for ptyline action. e. Lubrication: Mucin found in saliva facilitates moistening and swallowing of food. f. Endocrine function: Sex steroids and some peptides like VIP are found in saliva to plasma levels. – So, sex steroids in saliva help to diagnose hypogonadism/ sex gland secret little or no hormone.
Types of Salivary glands and their secretions a. Parotid 25%: Secrete mainly serous watery fluid rich in ptyline. b. Submandibular 70%: Produce both serous and mucous fluid. c. Sublingual ~5%: Secrete mainly thick mucous with little serous fluid
Constituents of saliva A. H2O (99.5%): B. Solid ( 0.5%) – Inorganic (0.2%) Electrolytes : Na+, Cl-, K+, HCO3- Mg, Iodine, etc. – organic substances (0.3%) include: Enzymes: amylase, lingual lipases, lysozymes, thiocyanate, Glycoproteins, (albumin, globulin), IgA, mucus, etc) (Total secretion = about 1-1.5 L/day)
Reflex control of salivation: (Nervous control) • Sight, smell, and test or thinking of food  Receptors in oral cavity or smell  Sensory fibers from the tongue to the nuclei in brain stem (MO), so called Salivatory nuclei  parasympathetic fibers act on salivary glands to increase copious salivary secretion and sympathetic fibers to increase thick and sticky secretion. • Salivation can also be controlled by higher centers like hypothalamus which has nerve connections with salivatory nuclei in the Medulla oblongata (MO). • Higher centers like appetite area in the hypothalamus are also involved in reflex control.
Phases of Salivary secretions Three phases of salivary secretions include 1. Cephalic (brain) phase: triggered by thought, smell, or sight of food (conditioned reflexes) 2. Oral phase: triggered by food that stimulate touch & test receptors in the mouth (unconditioned reflex) 3. Gastric phase: triggered by substances which stimulate the gastric mucosa (acids or sour tastes) in the stomach (unconditioned reflex) .
Stomach • Chemical breakdown of proteins begins and food is converted to chyme • Cardiac region: surrounds the cardiac orifice • Fundus: dome-shaped region beneath the diaphragm • Corpus (Body): midportion of the stomach • Antrum (Pyloric antrum): the dilated part of the stomach before the pyloric region. • Pyloric region: made up of the antrum and canal which terminates at the pylorus • The pylorus is continuous with the duodenum through the pyloric sphincter
Functions of the stomach  Storage of large quantities of food until it can be pumped into the duodenum.  Can accommodate large amount of food up to 1.5 liters .  Mixing of food with gastric secretion to form a semi-fluid chyme.  Slow emptying the food from the stomach into the small intestine at a rate suitable for proper digestion and absorption by the small intestine.  Secretary function: HCl, mucous, pepsin, gastrin, IF  Sterilization  Digestion  Absorption  Facilitates defecation
Functional layers of the stomach Mucosa: protection, secretion & absorption from the luminal surface. Submucosa: contains blood vessels, lymph nodes and some nerve fibers. Between Submucosa and circular muscles is found a nerve plexus called meissners plexus which is involved in GIT- secretions. Muscularis externa: contains circular & Longitudinal smooth muscle layers. Between them is found Myeinteric plexus involved in motility. Contraction of the circular muscles decreases the diameter of the lumen and contraction of the longitudinal, decreases the length of the GIT tract. The Serosa: outer most protective layer, consists of connective tissues
Microscopic Anatomy of the Stomach • Muscularis: Allows the stomach to churn, mix, and pump food physically. • Epithelial lining: contains Goblet cells that produce a coat of alkaline mucus • The mucous surface layer traps a bicarbonate- rich fluid beneath it • Gastric pits – contain gastric glands that secrete gastric juice, mucus, and gastrin.
Glands of the Stomach Fundus and Body • Gastric glands of the fundus and body have a variety of secretory cells – Mucous neck cells – secrete acid mucus – Parietal cells – secrete HCl and intrinsic factor – Chief cells – produce pepsinogen • Pepsinogen is activated to pepsin by: – HCl in the stomach – Pepsin itself via a positive feedback mechanism – Enteroendocrine cells – secrete gastrin, histamine, endorphins, serotonin, cholecystokinin (CCK), and somatostatin
Stomach walls, cont,d - Rugae, increases surface area as the stomach fills with chyme. - Folds have tube like depressions called gastric pits that contain different gastric glands. - The secretions are released into the base of the gastric pit.
Production of gastric juices in stomach cells Cells Secretions Location 1. Parietal cells (oxyntic cells) HCl + IF the body and fundus 2. Chief cells Pepsinogen the body and fundus 3. Mucous cells: Mucous the body and fundus 4. G-cells: Gastrin at pyloric antrum 5. D-cells Somatostatin 6. Enterochromaffin-like cells - Histamine throughout the tubular glands *Rennin in children (curdles milk)
HCl secretion by Parietal cells 1. H+ ions that result from the dissociation of H2O in the cytoplasm of the parietal cells are continuously pumped actively through the membrane of the gland into the gland lumen (pit). 2. intracellular) CO2 and OH- combine to produce HCO3-. 3. Cl- ions are transported from the blood into the parietal cell and finally into the lumen (pit) of the gland by facilitated diffusion. 4. HCO3 - in exchange to Cl- is transported in reverse direction (from the cytoplasm into the blood, charge balance). 5. Finally, H+ and Cl- ions combine in the lumen of the gland (pit) and produce HCl • HCl is collected and stored in the pit until used for different physiological functions. Secretion of HCl by parietal cells
Regulation and Mechanism of HCl Secretion • The four inputs to parietal cells that regulate acid secretion by controlling the transfer of the H-K ATPase pumps in cytoplasmic vesicle membranes to the plasma membrane.
Pepsin Secretion • Several protein-digesting enzymes collectively known as pepsin • Pepsin is secreted by chief cells in the form of an inactive precursor called pepsinogen. • The acidity in the stomach’s lumen converts pepsinogen to pepsin, the fully active form • Thus the activation of pepsin is positive-feedback process. Fig: Conversion of pepsinogen to pepsin in the lumen of the stomach.
Emptying of the stomach • The rate at which the stomach empties depends on the fluidity of the chyme and its contents. • Thus, liquids empty faster than solids (fast CHO> protein> fat). Factors that affect emptying include neural and hormonal 1. Neural factors that affect emptying: a. Parasympathetic: Causes increase rate of emptying by opening the pyloric sphincter. b. Sympathetics: Increases constriction of the pyloric sphincter through its adrenergic receptors, thus it has the effect of delaying emptying. c. Enteric nerves (Enterogastric reflex): When fat or protein chyme reaches the duodenumreceptors detect and send impulses to enteric nerves of the stomach cause the inhibition of stomach motility and secretion.
(Cont…) Stomach emptying 2. Hormonal factors that slow emptying: - CCK (stimulated by fat and peptides) - GIP (gastric inhibitory polypeptide), stimulated by CHO and fat - Secretin : stimulated by high acid (HCl) • The hormones after being secreted pass through the blood and reach the stomach, where they inhibit stomach motility and secretion. • They also cause constriction of the pyloric sphincters thereby delaying emptying.
Physiological advantage of delaying stomach contents 1. It gives ample time for nutrients (e.g., like fat) to remain longer in the stomach and be digested by gastric juices. 2. The delay prevents acids (HCl) not to be damped into the duodenum at higher rates to cause duodenal ulcers. 3. The delay also gives time for pancreatic secretions to reach duodenum and neutralize the acid.
Stomach Lining • The stomach is exposed to the harshest conditions in the digestive tract To keep stomach from digesting itself, the stomach has ; • A mucosal barrier with: – A thick coat of bicarbonate-rich mucus on the stomach wall – Epithelial cells that are joined by tight junctions – Gastric glands that have cells impermeable to HCl • Damaged epithelial cells are quickly replaced
Absorption from the stomach An insignificant absorptive functions takes place in the stomach, these are: a. Alcohol b. Certain drugs (aspirin, morphine etc.) c. Small quantities of H2O. NB: Organic nutrients (glucose, amino acids, and FFA) etc are not usually absorbed from the stomach.
Phases of gastric juice secretion The cephalic (~15%) (neural): • Afferent impulses from taste, smell, sight, or thought of food are carried to MO that causes reflex stimulation of the stomach to secrete gastric juices. The gastric phase (~70%) • When food distends the stomach, mechanoreceptors and chemoreceptor are activated by products of food. • Sensory information from the receptors pass to the brain stem (MO) • Reflex Vagal stimulation on the stomach causes increased gastric secretions through Ach . 3. The intestinal phase (~5%) • This stage begins when chyme reaches the duodenum. • This phase is mostly inhibitory to gastric juice secretion. • Secretion of hormones like Secretin, CCK, GIP that reflexly inhibit gastric secretion in the stomach.
Small Intestine: Gross Anatomy • Runs from pyloric sphincter to the ileocecal valve • Has three subdivisions: – duodenum, – jejunum, and – ileum • The bile duct and main pancreatic duct join the duodenum • The jejunum extends from the duodenum to the ileum • The ileum joins the large intestine at the ileocecal valve
Small Intestine: gross anatomy • SI is specialized for completion of digestion and absorption of nutrients. a. Duodenum: ~25 cm, mainly secretary, mucose, hormones, enzymes) b. Jejunum : ~2.5m mainly absorptive c. Ileum : ~3.7m mainly absorptive
Small intestine, structure • Structure of the wall is similar to others and consists of 4-layers (mucosa, Submucosa, Muscularis externa and Serosa). • Enteric and Vagal nerves abundantly innervate the Small intestine. • The inner lining is greatly folded to increase the surface area for absorption, thus – The microvillus + Villi + greater foldings (plicea circulares) increase the absorptive surface of Small intestine by ~ 600-fold).
Small Intestine: microscopic anatomy • Structural modifications of the small intestine wall increase surface area • Plicae circulares: ~ deep circular folds of the mucosa and sub mucosa • Villi ~ finger like extensions of the mucosa • Microvilli ~ tiny projections of absorptive mucosal cells’ plasma membranes
The Villi • Villi: Are numerous finger like projections that emerge from the mucous membrane. • Each Villi consist of a layer of absorptive epithelial cells that contain minute microvilli (brush border) at their surfaces. • The epithelial cells contain digestive enzymes like sucrase, maltase, lactase, and peptidases in the membranes of the microvilli.
Movement in the small intestine ~ Two types of movements occur in the SI: 1. Mixing movement 2. Propulsive movement Mixing movements (segmentation contractions) ~ Ring like contractions appear at regular intervals ~ Segmentation contractions exert chopping action on intestinal chyme and mix it with digestive juice
Propulsive movements • Are peristaltic waves that propel chyme anal wards • Initiated by intestinal distension. • Chyme is propelled in the SI until it reaches the terminal ileocecal sphincter
Ileocecal Sphincter • Function: prevents back flow of fecal matter from the cecum to the ileum Factors regulating the sphincter • Pressure and chemical irritation of ileum – relax it and initiates peristalsis. • Pressure and chemical irritation of cecum – inhibit peristalsis of ileum and closes the sphincter.
Digestive enzymes secreted in the SI 1. Peptidase: splits peptides into amino acids 2. Three enzymes hydrolyzing diasaccharides into monosaccharides: sucrase, maltase and lactase 3. Intestinal lipase: splits neutral fats into glycerol and fatty acids. Regulation of SI secretion • Local factors: tactile, distension, irritation, pH. • Hormonal: Secretin, CCK, VIP, Glucagon, GIP • Nervous: vagal stimulation increases intestinal secretion Sympathetic stimulation decreases intestinal secretion • Enteric reflexes: stimulation of submucosal plexus
Digestion in the Small Intestine • As chyme enters the duodenum: – Carbohydrates, proteins and fat digestion have taken place – Digestion continues in the small intestine – Chyme is released slowly into the duodenum – Because it is hypertonic and has low pH, mixing is required for proper digestion • Virtually all nutrient absorption takes place in the small intestine
Digestion in the Small Intestine 1. Digestion of Carbohydrates Involved Enzymes and their sight of Secretion • Mouth: salivary amylase • Esophagus & stomach: nothing happens • Duodenum: pancreatic amylase • Brush border enzymes (maltase, sucrase & lactase) act on disaccharides and produce monosaccharides: fructose, glucose & galactose
Starch - Salivary amylase (20-40%) - Pancreatic amylase (50-80%) Maltose and 3-9 glucose polymers Lactose Sucrose -Maltase -Lactase -Sucrase -Dextrase Glucose + Glucose Glucose and Glucose and Galactose Fructose Digestion of Carbohydrates…
2. Proteins Digestion: Involved Enzymes and their Sight of Secretion Enzymes used: pepsin in the stomach • Enzymes acting in the small intestine * Pancreatic enzymes – trypsin, chymotrypsin, and carboxypeptidase * Brush border enzymes – aminopeptidases, carboxypeptidases, and dipeptidases
Digestion of Proteins, cont,d Stomach • HCl denatures or unfolds proteins • HCl activates pepsinogen into pepsin • Pepsin turns proteins into peptides Pancreas • Digestive enzymes: split peptide bonds between different amino acids. Small Intestine • Brush border enzymes * enzymes break peptide bonds that attach terminal amino acids to carboxyl ends of peptides (carboxypeptidases) * enzymes break peptide bonds that attach terminal amino acids to amino ends of peptides (aminopeptidases) * enzymes split dipeptides to amino acids (dipeptidase)
3. Chemical Digestion of Fats • Happens mainly in the small intestine Two phases: 1. Emulsification – By bile salts – Emulsion droplets 2. Chemical digestion • Pancreatic lipase –Free fatty acids –Monoglycerides
Dietary source of fat • Neutral fats (triglycerides), Cholesterol and cholesterol esters Phospholipids Fat Emulsified fat FFA + Glycerides -Lingual lipase -Gastric lipase -Pancreas lipase Cholesterol Bile salt FFA + Glycerides Cholesterol esters Cholesterol Esterase Phospholipids-A2 Phospholipase FFA + Phopholipids Digestion of Lipids Bile salt
Absorption in the Small Intestine • Absorption ~ is the passage of the end products of digestion from the GI tract into blood or lymph Occurs by • diffusion, • facilitated diffusion, • osmosis, and • active transport.
Small intestine, Absorption • Absorption & transport – Carbohydrates are degraded into their simpler forms; glucose, galactose, and fructose are absorbed by secondary active transport (diffusion together with Na+ ion). – Fats are degraded into FFA + glycerol and taken by lacteals that enter the thoracic duct and finally join the circulation. – Protein are absorbed as amino acids by active transport. – Electrolytes Na+, Ca2+, iron etc are absorbed by active transport.
Where will the absorbed nutrients go?
Water Absorption • 95% of water is absorbed in the small intestines by osmosis • Water moves in both directions across intestinal mucosa • Net osmosis occurs whenever a concentration gradient is established by active transport of solutes into the mucosal cells • Water uptake is coupled with solute uptake, and as water moves into mucosal cells, substances follow along their concentration gradients.
Large Intestine
Large Intestine • Is subdivided into – the cecum, • appendix, – colon, – rectum, and • anal canal • The cecum: – Contains appendix
Colon • Has distinct regions: – ascending colon, – transverse colon, – descending colon, and – sigmoid colon • The sigmoid colon joins the rectum • The anal canal, the last segment of the large intestine, opens to the exterior at the anus
Valves and Sphincters of the Rectum and Anus • Valves of the rectum stop feces from being passed with gas • The anus has two sphincters: – Internal anal sphincter composed of smooth muscle – External anal sphincter composed of skeletal muscle • These sphincters are closed except during defecation
Large intestine (colon) Function of the large intestine Other than digestion of bacteria, no further digestion takes place 1. Water absorption 2. Electrolyte (like NaCl) absorption 3. Mucous & HCO3 - secretion 4. Storage, transport, and evacuation of feces 5. Absorption of some drugs 6. Bacterial fermentation in the colon 7. Stimulates synthesis of some vitamins
• The large intestine takes about 16 hours to finish up the remaining processes of the digestive system. • Food is no longer broken down at this stage of digestion. • The colon absorbs vitamins which are created by the colonic bacteria • It also compacts feces, and stores fecal matter in the rectum until it can be discharged via the anus in defecation.
Bacteria • The bacteria of the large intestine consist of: – Bacteria surviving the small intestine that enter the cecum – Those entering via the anus • The large intestine houses over 700 species of bacteria that perform a variety of functions. • The large intestine absorbs some of the products formed by the bacteria inhabiting this region.
Bacteria in the large intestine: – Ferment indigestible carbohydrates, undigested polysaccharides (fiber) are metabolized to short- chain fatty acids by bacteria in the large intestine and absorbed by passive diffusion. – Release irritating acids and gases (flatus). The bicarbonate that the large intestine secretes helps to neutralize the increased acidity – They produce large amounts of vitamins, especially vitamin K and biotin (a vitamin B), for absorption into the blood.
Large intestine, structure • The difference of large intestine from small intestine is that the large intestine does not have villi, and thus, has lower surface area • The large intestine mucosa contains deep crypts and the epithelium linings contain abundant goblet cells. • Goblet cells secrete mucus in response to tactile stimuli and parasympathetic stimulation.
Large intestine, Functional structures • The colon is about 1.3 m long and has the following parts: • Cecum,--> Ascending colon --> Transverse colon --> Descending colon --> Sigmoid colon Rectum --> and Anus • Ileocecal valve (sphincter) : connects ileum -to- cecum. It is mostly in a contracted state b/s: a. By remaining contracted, it prevents bacterial penetration back to the SI. b. It permits slow flow of chyme to the LI to increase transit time.
Movement in the large intestine * Two types of movements 1. Mixing movements (Haustration) 2. Propulsive movements (mass movements) * Mass movement :is initiated by local distension, gastro-colic reflex * Poor motility of the transverse colon causes greater absorption and constipation * Excess motility of the sigmoid colon causes less absorption and diarrhoea or loose stool
Defecation reflex • Rectal Stimulation distension of myenteric plexus Initiates peristalsis in descending, sigmoid colon and rectum Inhibition of internal anal sphincter (IAS) by • Parasympathetic stimulation • myenteric plexus  Peristaltic wave forces feces to the anus  Voluntary Relaxation of external anal sphincter ►Defecation
Secretion of the large intestine Glands • Cryps of Lieberkuhn Secret H2O, electrolytes • Goblet cells and mucous  Regulated by local (tactile) factors
physiology of digestive system.ppt

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physiology of digestive system.ppt

  • 1.
    Physiology of theDigestive System By: Yerukneh Solomon (MSc.)
  • 2.
    Outline: • Digestive System •Role of GI-system in Homeostasis • Basic functions of the digestive system • General characteristics of the GIT • Functional structure, layers of the GIT • Accessory organs and their functions • Regulation of GIT • Receptors of the GI Tract • Functional movement of the GIT • Salivary Glands and Saliva • Stomach • Secretary function of GIT • Small Intestine: • Large Intestine
  • 3.
  • 4.
    Digestive System (GIT) Whatare the basic functions of the GIT (esophagus-to-Anus)?... It provide the body with a continuous supply of: • Nutrients • Water and • Electrolytes Digestive organs
  • 5.
    Overview: Functions ofthe Gastrointestinal Organs • Four processes carried out by the gastrointestinal tract: digestion, secretion, absorption, and motility
  • 6.
    Role of Digestivesystem in Homeostasis - Body cells require nutrients to obtain energy and function properly. - This supply of energy is accomplished by gastrointestinal system (GIS) that breaks food by: a. Mechanical and b. Chemical processes until the food is changed into its usable form - Therefore, the GI-system is one of the important homeostatic organs involved in maintaining the day-to-day balance of food intake and excretion, thereby sustaining life.
  • 7.
    Basic functions ofthe digestive system The GI system carries out the following basic activities: 1. Ingestion: food intake, which is controlled by the feeding and satiety center in the HT. 2. Mastication or chewing: mechanical grinding of food with the aid of the teeth. 3. Swallowing or deglutition: propulsion of food from the mouth to the stomach. 4. Chemical digestion of food 5. Secretion of enzymes, electrolytes (HCl, NaHCO3), mucous, and hormones 6. Absorption of nutrients, water & electrolytes into the blood vessels 7. Defecation: excretion of fecal matter
  • 8.
    Digestive Process Figure 23.2 ~Mouth Food ingestion controlled by Hypothalamus ~ Esophagus Simple passage of food ~ Stomach Temporary storage, Mixing, digestion, Secretion and emptying ~ Small intestine Digestion, secretion Absorption ~ Large Intestine Storage, absorption, Passage of faecal matter
  • 9.
    GIT and itsnatural defense • GIT is hollow at both ends (mouth- to - Anus). – Therefore, can possibly harbor microorganisms in its luminal surfaces. • However, the GI-system can protect itself from such hazards by some defense mechanisms found in: – a. Mouth: Saliva contains lysozymes, IgA etc – b. Stomach: HCl, Pepsin etc. have bactericidal effect – c. SI (e.g., Payer's patches): Immuno-competent lymph tissues – d. Macrophages: located in intestinal walls act to defend from bacterial invasion etc
  • 11.
    Fig.1: GIT-organs andits accessories 1. Mouth 2. Pharynx 3. Esophagus 4. Stomach 5. Small intestine 6. Large intestine 7. Rectum (Anus) Main GI-Organs Accessory Organs 1. Salivary glands 2. Pancreas 3. Liver & 4. Gallbladder I. Functional structure of the gut
  • 12.
    Digestive System… Organs involvedin the process of digestion include: 1. The alimentary canal or gastrointestinal tract (GIT) that used to digests and absorbs food – mouth, pharynx, esophagus, stomach, small intestine, and large intestine – teeth, tongue, gallbladder, salivary glands, liver, and pancreas • Teeth aids mechanical breakdown of food • Tongue assists chewing and swallowing and speech • Other accessory organs produce and send secretions that facilitate chemical breakdown of food 2. Accessory organs
  • 13.
    Accessory organs • Liesdeep to the greater curvature of the stomach • The head is encircled by the duodenum and the tail adjacent to the spleen • Divided into: Head, body and tail • Connected to the duodenum via the main pancreatic duct and accessory duct Anatomy of the Pancreas
  • 14.
    THE PANCREAS… • Pancreascontains two types of secretary glands: 1. Endocrine cells (islets of Langerhans) * secrete hormones 2. Exocrine cells (acinar cells) * secrete a mixture of fluid rich in NaHCO3 and digestive enzymes called pancreatic juice. Involved in digestion processes by producing: a. Digestive enzymes: necessary to digest CHO, fat, and protein b. Bicarbonates : to neutralize the gastric juice c. Water and electrolytes (Na+, K+ etc): Fig. Acinus of the Pancreas
  • 15.
    Digestive enzymes 1. Proteolyticenzymes: enzyme that helps to digest proteins. * Trypsin: activated by enterokinase, a duodenal enzyme that converts trypsinogen to trypsin * Chymotrypsin: protein-digesting enzyme in pancreatic juice, activated by trypsin * Carboxypolypeptidase: activated by trypsin * Ribonuclease: to digest nucleic acids * Deoxyribonuclease, Elastase, collagenase 2. Pancreatic enzymes involved in CHO digestion: * Pancreatic amylase 3. Pancreatic enzymes involved in fat digestion: * Pancreatic lipase, Cholesterol esterase, Phospholipase
  • 16.
    Hormonal regulation ofpancreatic secretion • Fats and amino acids stimulate the duodenum to releases CCK and Secretin. • These hormones flow through the blood and reach the pancreas. • In the Pancreas two processes take place a . CCK stimulates acinar cells to release digestive enzymes b. Secretin stimulates duct epithelium to release H2O & HCO3- • Both flow through common bile duct to duodenum to do their specific job.
  • 18.
    LIVER AND GALLBLADDER Liver •The liver is the heaviest gland in the body and the second largest organ in the body after the skin. • Weighs 1.36kg. • Located below diaphragm in the abdomen • Right lobe larger • Gallbladder on right lobe • Size causes right kidney to be lower than left
  • 19.
    Liver and Gallbladder… •The liver is divisible into left and right lobes, separated by the falciform ligament. • The gallbladder is a sac located in a depression on the posterior surface of the liver.
  • 20.
    Liver: Associated Structures… Bileleaves the liver via: • Bile ducts, which fuse into the common hepatic duct • The common hepatic duct, which fuses with the cystic duct • These two ducts form the common bile duct
  • 21.
    Liver: Microscopic Anatomy •Hexagonal-shaped liver lobules are the structural and functional units of the liver. – Composed of hepatocyte (liver cell) – Portal triads are found at each of the six corners of each liver lobule Portal triads consist of: 1. Bile duct 2. Hepatic artery – supplies O2 rich blood to the liver 3. Hepatic portal vein – carries venous blood with nutrients from digestive viscera
  • 23.
    Liver: microscopic structures… •Liver sinusoids – enlarged, leaky capillaries located between hepatic plates Cells in the liver • Kupffer cells – hepatic macrophages found in liver sinusoids. • Hepatocytes’ functions include: – Production of bile – Processing blood-borne nutrients – Storage of fat-soluble vitamins – Detoxification
  • 24.
    Functions of theliver 1. On Carbohydrate Metabolism:  It is the site of glycogenesis, gluconeogenesis and glycogenolysis • Turn proteins into glucose • Turn triglycerides into glucose • Turn excess glucose into glycogen & store in the liver • Turn glycogen back into glucose as needed 2. On Lipid Metabolism  It is the site of β-oxidation, formation of phospholipids, lipoproteins, synthesis of cholesterol, and conversion of CHO into fat
  • 25.
    Functions of Liver… 3.Protein Metabolism – Deamination of amino acids = removes amine group (NH2) from amino acids so can use what is left as energy source. – Converts resulting toxic ammonia (NH3) into urea for excretion by the kidney. – Synthesizes plasma proteins utilized in the clotting mechanism (fibrinogen) and immune system (gamma globulin for antibody production) – Convert one amino acid into another
  • 26.
    Other Functions ofthe liver 4. Inactivation of drugs & hormones (Sulfonamide, penicillin, thyroid .H, steroids) 5. Removes the waste product; bilirubin 6. Releases bile salts that help digestion of fat by emulsification. 7. Stores: fat soluble vitamins (A, B12, D, E, K), iron, copper and blood (a major blood reservoir) 8. Filtration of blood: phagocytizes worn out blood cells & bacteria. Removes blood clots and toxins from portal circulation 9. Activates vitamin D 10. Synthesis of blood clotting factors (F-I, II, VII, IX, X)
  • 27.
    Bile Production • Bileis an alkaline fluid (pH 8) • secreted in the liver and functions in emulsification of fat in the duodenum. • Emulsification means changing greater fat globules into smaller fat-droplets called micelles
  • 28.
    Secretion of Bile Bileis secreted by hepatocytes in the liver for two purposes 1. It facilitates fat digestion and absorption of in the SI 2. Serves as a means of excretion of waste products (bilirubin & cholesterol) Components of Bile • Bile contains the following constituents: 1. Bile salts (bile acids), ~11% 2. Bile pigment (bilirubin), ~1% 3. Others organic constituents like: Cholesterol, protein etc ( ~3%) 4. Electrolytes (Na+, K+, Ca2+, Cl-, and greater HC03- than plasma) ~1%. 5. H2O (~ 84%)
  • 29.
    Regulation of BileRelease • Acidic, fatty chyme causes the duodenum to release: – Cholecystokinin (CCK) and – secretin into the bloodstream • CCK and secretin transported in blood stimulate the liver to produce bile • Vagal stimulation causes weak contractions of the gall bladder
  • 30.
    The Gallbladder • Thin-walled,green muscular sac on the ventral surface of the liver • Stores and concentrates bile by absorbing its water and ions • Releases bile via the cystic duct, which flows into the bile duct
  • 31.
    Facts… • The averageadult consumes about 800 g of food and 1200 ml of water per day. • An additional 7000 ml of fluid from salivary glands, gastric glands, pancreas, liver, and intestinal glands is secreted into the tract each day. • 99 % is absorbed; only about 100 ml is normally lost in the feces. Fig: Average amounts of solids and fluid ingested, secreted, absorbed, and excreted from the gastrointestinal tract daily
  • 32.
    Characteristics, Length ofthe GIT - ~ 9 meters long. - Is open at both ends and highly susceptible in harboring microbial agents. - Voluntary segments of the GIT are mouth, tongue, upper 1/3 of the esophagus and the anal regions. - The GI-wall possesses several glands that secrete digestive juices and enzymes into the lumen.
  • 33.
    Functional structure, layersof the GIT Functions 1. Mucosa: protection, secretion & absorption of the luminal surfaces 2. Submucosa: blood vessels, glands, lymph, nerve plexuses are found.  Submucosa necessary for motility + secretion 3. Muscularis externa : Circular & Longitudinal smooth mus., mix and propel the chyme. 4. Serosa: outer most protective layer, consists of connective tissues.
  • 35.
    1. Mucosa: Functionsof the Mucosa The mucosa is divided into 3-sub- layers a. Epithelium:covers the mucosa and is columnar or stratified squamous cells. b. Lamina propria: is under the epithelium and contains capillaries for absorption & some lymph nodules for immunological defenses. C. Muscularies mucosa: Located underneath the lamina. • It is a thin layer of smooth muscle and its contraction and relaxation changes the degree or folding of the luminal diameter.
  • 36.
    (Cont… GIT) Functionof Submucosa 2. Submucosa - is located beneath the mucosa - Contains rich supply of  blood vessels,  nerves,  lymph nodes,  glands, and  nerve plexuses of the enteric nervous system called Submucosa (Meissiner’s) plexus.
  • 37.
    (Cont…GIT) Function ofMuscularis externa 3. Muscularies Externa (has 2- sublayers) and also contains myenteric nerve plexus. a. The inner circular layer their contractions decrease the diameter of the lumen. – At some regions, the circular SM thicken and forms a sphincter that prevents backflow of food contents. b. The outer longitudinal layer when it contracts, it decreases the length of the tract and cause shortening and lengthening of the tract. – The effect of both contractions helps to mix and propel the chyme (i.e., towards the anus).
  • 38.
    (Cont… GIT) Functionof the Serosa 4. Serosa – Is an outermost layer consisting of connective tissues. – It protects the underlying tissues and supplies blood vessels, lymph and nerves to the gut wall. – Its squamous epithelial cells secrete serous fluid that helps in moistening & lubricating the tubes outer surface. – This helps the abdominal cavity to slide freely against one another, there by decreasing friction.
  • 39.
    Regulation of GIT Neural& Hormonal Regulation of the GIT • Regulation of GI-activities is the role of Neural & Hormonal mechanisms: A. Neural regulation of GI-activities: includes that of a. Extrinsic (Autonomic N fibers) b. Intrinsic (Enteric fibers) B. Hormonal regulation: Includes secretion of different hormones like Gastrin, CCK, Secretin, GIP etc. C. Paracrine Regulation: • Paracrine secretions flow into the interstitial fluid and diffuse to neighboring cells where they exert their effects. E.g.: - Histamine (stimulates parietal cells to secrete HCl) - Somatostatin (inhibit secretion of gastrin by G-cells)
  • 40.
    Neural Innervations ofGIT A. Somatic innervations: The voluntary muscle fibers at the upper esophagus and lower end of the GIT are controlled by somatic nerves that are voluntarily in action. B. Autonomic innervations (extrinsic regulation): - have generally a modulatory role (cause strong smooth muscle contractions by peristalsis) C. Enteric NS innervations (intrinsic regulation)
  • 41.
    A. Autonomic Innervations(extrinsic regulation): • Functions of the parasympathetics & sympathetics nerve fibers. a. Parasympathetic fibers (cholinergic, Ach): - is excitatory and causes strong contractions on longitudinal muscles. - At sphincters (at circular SM): PS-fibers cause dilation. b. Sympathetic fibers (adrenergic, NA & adrenaline): - is generally inhibitory in action and causes dilatation of the alimentary tract. - At sphincters, it causes constriction of the circular SM
  • 42.
    Neural control ofthe GIT, cont,d B. Enteric NS: shows local, intrinsic regulation: - Can act independently to cause motility & secretion of the gut - Are very numerous in No (approx. > 100 mill. neurons) and are sometimes called little brain. Types of NTs secreted from these neurons: Several e.g., Ach, NE, Dopamine, CCK, Substance P, Vasoactive intestinal polypeptide (VIP), Somatostatin, etc. Components of ENS includes: 1. Myenteric (Auerbach’s) plexus: - mainly control motility of the GIT 2. Submucosa (Meissner’s) plexus: - mainly control glandular secretions
  • 43.
    Receptors of theGI Tract • Receptors initiate GI-reflexes that: – Activate or inhibit digestive glands – Mix lumen contents and move them along • Mechanoreceptors: ~ respond to distension, spastic contraction • Chemoreceptors: ~ respond to osmolarity, irritation, pH, presence of fat and protein food and end products of digested food • Thermoreceptors: respond to warm food/drinks Diffusion of gasses and other food contents are affected by temperature. When T- Increases rate of diffusion also increases • Pain receptors: respond to tissue injury in the GIT.
  • 44.
    Nervous Control ofthe GI Tract: Intrinsic and extrinsic control I. Intrinsic control - Involves the enteric nerve plexuses present with in the GIT. Nerve plexuses near the GI tract initiate short reflexes: • Submucosal plexus (plexus of Meissner) – controls GI secretary activities • Myenteric plexus (plexus of auerbach) – controls motility of the gut
  • 45.
    cont,d II. Extrinsic control InvolveCNS centers and extrinsic autonomic nerves • They control GI motility and secretion • They also modify the action of enteric nerves ~ Sympathetic NS ↓GI function, ↓Motility ↓Secretions ~ Parasympathetic NS ↑GI function, ↑Motility,↑Secretions
  • 46.
    GIT Reflexes: What dou understand from this Fig.? Figure 23.4
  • 47.
    GIT reflexes 1. Reflexesthat occur entirely within the enteric nervous system. These include reflexes that control  GI-secretion, peristalsis and mixing-contractions, local inhibitory effects. (Short reflexes)
  • 48.
    Reflexes: cont,d 2. Reflexesthat arise from the gut go to the sympathetic ganglia and then back to the GI-tract. (intermediate reflexes) Examples: a) The gastro-colic reflex: signals send from the stomach to cause evacuation of the colon (LI) b) The entero-gastric reflexes: signals from the colon & small intestine to inhibit stomach motility and secretion. c) The colono-ileal reflex: reflexes from the colon to inhibit emptying of ileal contents into the colon.
  • 49.
    Reflexes: cont,d 3. Reflexesfrom the gut to the spinal card or brain stem and then back to GIT: (long reflexes) • Example: defecation reflex that travel to – the spinal cord and back again to produce the powerful colonic, rectal, abdominal contraction required for defecation (the defecation reflexes).
  • 50.
    Hormonal control ofGI function • Entero-endocrine cells produce several GIT- hormones, capable of regulation motility and secretary activities. 1. Cholecytokinin (CCK, 33aas) • secreted by the mucosa of the jejunum in response to the presence of fatty food in the intestine. • has a very potent effect on gallbladder contractility for expelling bile into the intestine in order to facilitate fat digestion and absorption. • Increase the release of pancreatic lipase from acinar cells. • CCK inhibits stomach motility in order to give adequate time for fat digestion. 2. Secretin (27aas) • Secreted by the mucosa of the duodenum in response to acidic gastric juice pumped from the stomach. • It increases NaHCO3 secretion by the pancreas, a buffer that used to neutralize the acid..
  • 51.
    Functional movement ofthe GIT • Two basic types of movements occur in the GIT: 1. Propulsive movements: • which cause food to move forward along the tract at an appropriate rate for digestion and absorption. * Peristalsis: is the basic propulsive movement of the GIT that appears in the form of contractile rings around the gut and propels to the anal ward direction. • Peristalsis is an intrinsic property of the smooth muscles in the GIT that generate action potential rhythmically (basic electrical rhythm, BER). Peristalsis and Segmentation
  • 52.
    Peristalsis Factors that initiateperistalsis • Distension (overstretching) • Irritation of epithelial lining • Nervous mechanisms: ANS:- Parasympathetic nerve stimulation ENS:- myenteric plexus • Hormonal mechanisms: Gastrin, CCK, Insulin, Motilin • Drugs: Ach-like drugs: carbacoline, pilocarpin Factors that inhibit peristalsis • Congenital absence of myenteric plexus Drugs: cholinergic antagonists • Nervous mechanism: stimulation of SNS • Hormonal mechanisms: secretin, GIP, somatostatin, VIP, sub-P
  • 53.
    2. Mixing movements: • which keep the intestinal contents thoroughly mixed at all times. • Mixing contractions are beneficial to mix the food contents with gastric juice (chyme). - Pacemaker cells are normally located at Fundus region and fire spontaneously causing the so called basic electrical rhythm (BER). - BER are slow waves that result in tonic/energized and weak contractions of the stomach walls. • When slow waves progress to the antrum, the peristalitic contractions becomes strong and cause mixing of the stomach contents. • The contractions also close the pyloric sphincter, which aids in the repulsion of the chyme back to the stomach. • This process helps in mixing the chyme.
  • 54.
    Clinical Paralytic ileus: • Paralysisof intestinal motility that is caused by increased in sympathetic stimulation and damage to the smooth muscles Hunger pangs (no food in stomach) can sometimes produce strong peristalitic contractions. • Such contractions fuse together and cause pain sensations.
  • 55.
    Mastication (Chewing) • Chewingis a process of mechanical breakdown of food. • Salivary secretion containing amylase involves chemical digestion and lubrication of chewing. • Teeth, tongue, jaws and lips are involved in chewing. • The movt of these organs are controlled by such centers located in the brainstem, HT, amygdala and cerebral cortex. • Mastication muscle are supplied mainly by the motor branch of the trigeminal nerve. • Chewing center is located in the pons
  • 56.
    Deglutition (Swallowing) • Itis the propulsion of food from mouth to the esophagus that is controlled by the swallowing center in the medulla. • Involves the coordinated activity of the tongue, soft palate, pharynx, esophagus and many separate muscle groups. Has 3 stages: 1. Voluntary stage of swallowing: Buccal/oral phase – bolus is forced into the oropharynx 2. Involuntary stage A. Pharyngeal stage of swallowing: – contributes the passage of food through the pharynx to the esophagus. – Controlled by the medulla and lower pons. B. The esophageal stage of swallowing: – promote the passage of food to the stomach.
  • 57.
    Secretion of theesophagus • Mucous glands that surround the esophagus secrete mucus that provide lubrication and protection from gastric digestion (HCl) that may arise from reflux of stomach contents.
  • 58.
    Lower esophageal (Gastroesophageal)sphincter • It is a thickened circular smooth muscle at the junction b/n the esophagus and the stomach. • Function: prevents the reflux of gastric contents into the esophagus.
  • 59.
    Common Disorders ofthe Esophagus 1. Gastro-esophageal reflux disease (heart burn). • It occurs when the cardiac sphincter remains open (incompetence of the lower esophageal sphincter) . 2. Achalasia: • The lower esophageal sphincter remains contracted (closed). This effect results in accumulation of food in the esophagus causing several disturbances (bad smell) and dilation of esophageal walls. (lower esophagus)
  • 60.
    Digestive Processes Mechanical Digestion: ~The physical breakdown of food. Ex: Grinding of the teeth or churning of the stomach. Chemical Digestion: ~ The catabolic breakdown of food using enzymes & acids. Absorption: ~ The passage of digested food from the lumen of the GI Tract into the blood or lymph.
  • 61.
    The Mouth • Themouth (oral cavity) is responsible for mechanical digestion of solid food by mastication. • Mastication helps mix food with saliva. Parts of the oral cavity: - The cheeks, - The lips - The tongue - The hard and soft palate - The teeth - Saliva is released into the oral cavity
  • 62.
    Salivary Glands andSaliva • Saliva is produced in and secreted from salivary glands. • The basic secretory units of salivary glands are clusters of cells called an acini. • These cells secrete a fluid that contains; – water, – electrolytes, – mucus and – enzymes • All of which flow out of the acinus into collecting ducts.
  • 63.
    Types of SalivaryGland cells • The secretary unit in salivary glands is the acini. There are two types • Serous acini: secret watery secretion rich in the enzyme pthyalin. • The gland is called serous salivary gland • Mucous acini: secret thick and sticky secretion rich in mucin, a glycoprotein, which disolves in water gives mucus. • The gland is known as mucus salivary gland.
  • 64.
    Functions of Saliva a.Digestion: CHO-digestion begins in saliva . • The enzyme ptyline breaks starch- to-maltose. • Lingual lipase begins fat digestion in the mouth. b. Protection: has anti-microbial actions (contains Lysozyme & thiocyanate that kills microbes). c. Involved in speech: Clear & fluent articulation is possible in the presence of saliva. d. Secretes HCO3 - : good to maintains the pH to neutral range (6-to-7), the neutral pH is good for ptyline action. e. Lubrication: Mucin found in saliva facilitates moistening and swallowing of food. f. Endocrine function: Sex steroids and some peptides like VIP are found in saliva to plasma levels. – So, sex steroids in saliva help to diagnose hypogonadism/ sex gland secret little or no hormone.
  • 65.
    Types of Salivaryglands and their secretions a. Parotid 25%: Secrete mainly serous watery fluid rich in ptyline. b. Submandibular 70%: Produce both serous and mucous fluid. c. Sublingual ~5%: Secrete mainly thick mucous with little serous fluid
  • 66.
    Constituents of saliva A.H2O (99.5%): B. Solid ( 0.5%) – Inorganic (0.2%) Electrolytes : Na+, Cl-, K+, HCO3- Mg, Iodine, etc. – organic substances (0.3%) include: Enzymes: amylase, lingual lipases, lysozymes, thiocyanate, Glycoproteins, (albumin, globulin), IgA, mucus, etc) (Total secretion = about 1-1.5 L/day)
  • 67.
    Reflex control ofsalivation: (Nervous control) • Sight, smell, and test or thinking of food  Receptors in oral cavity or smell  Sensory fibers from the tongue to the nuclei in brain stem (MO), so called Salivatory nuclei  parasympathetic fibers act on salivary glands to increase copious salivary secretion and sympathetic fibers to increase thick and sticky secretion. • Salivation can also be controlled by higher centers like hypothalamus which has nerve connections with salivatory nuclei in the Medulla oblongata (MO). • Higher centers like appetite area in the hypothalamus are also involved in reflex control.
  • 68.
    Phases of Salivarysecretions Three phases of salivary secretions include 1. Cephalic (brain) phase: triggered by thought, smell, or sight of food (conditioned reflexes) 2. Oral phase: triggered by food that stimulate touch & test receptors in the mouth (unconditioned reflex) 3. Gastric phase: triggered by substances which stimulate the gastric mucosa (acids or sour tastes) in the stomach (unconditioned reflex) .
  • 69.
    Stomach • Chemical breakdownof proteins begins and food is converted to chyme • Cardiac region: surrounds the cardiac orifice • Fundus: dome-shaped region beneath the diaphragm • Corpus (Body): midportion of the stomach • Antrum (Pyloric antrum): the dilated part of the stomach before the pyloric region. • Pyloric region: made up of the antrum and canal which terminates at the pylorus • The pylorus is continuous with the duodenum through the pyloric sphincter
  • 71.
    Functions of thestomach  Storage of large quantities of food until it can be pumped into the duodenum.  Can accommodate large amount of food up to 1.5 liters .  Mixing of food with gastric secretion to form a semi-fluid chyme.  Slow emptying the food from the stomach into the small intestine at a rate suitable for proper digestion and absorption by the small intestine.  Secretary function: HCl, mucous, pepsin, gastrin, IF  Sterilization  Digestion  Absorption  Facilitates defecation
  • 72.
    Functional layers ofthe stomach Mucosa: protection, secretion & absorption from the luminal surface. Submucosa: contains blood vessels, lymph nodes and some nerve fibers. Between Submucosa and circular muscles is found a nerve plexus called meissners plexus which is involved in GIT- secretions. Muscularis externa: contains circular & Longitudinal smooth muscle layers. Between them is found Myeinteric plexus involved in motility. Contraction of the circular muscles decreases the diameter of the lumen and contraction of the longitudinal, decreases the length of the GIT tract. The Serosa: outer most protective layer, consists of connective tissues
  • 74.
    Microscopic Anatomy ofthe Stomach • Muscularis: Allows the stomach to churn, mix, and pump food physically. • Epithelial lining: contains Goblet cells that produce a coat of alkaline mucus • The mucous surface layer traps a bicarbonate- rich fluid beneath it • Gastric pits – contain gastric glands that secrete gastric juice, mucus, and gastrin.
  • 75.
    Glands of theStomach Fundus and Body • Gastric glands of the fundus and body have a variety of secretory cells – Mucous neck cells – secrete acid mucus – Parietal cells – secrete HCl and intrinsic factor – Chief cells – produce pepsinogen • Pepsinogen is activated to pepsin by: – HCl in the stomach – Pepsin itself via a positive feedback mechanism – Enteroendocrine cells – secrete gastrin, histamine, endorphins, serotonin, cholecystokinin (CCK), and somatostatin
  • 76.
    Stomach walls, cont,d -Rugae, increases surface area as the stomach fills with chyme. - Folds have tube like depressions called gastric pits that contain different gastric glands. - The secretions are released into the base of the gastric pit.
  • 77.
    Production of gastricjuices in stomach cells Cells Secretions Location 1. Parietal cells (oxyntic cells) HCl + IF the body and fundus 2. Chief cells Pepsinogen the body and fundus 3. Mucous cells: Mucous the body and fundus 4. G-cells: Gastrin at pyloric antrum 5. D-cells Somatostatin 6. Enterochromaffin-like cells - Histamine throughout the tubular glands *Rennin in children (curdles milk)
  • 78.
    HCl secretion byParietal cells 1. H+ ions that result from the dissociation of H2O in the cytoplasm of the parietal cells are continuously pumped actively through the membrane of the gland into the gland lumen (pit). 2. intracellular) CO2 and OH- combine to produce HCO3-. 3. Cl- ions are transported from the blood into the parietal cell and finally into the lumen (pit) of the gland by facilitated diffusion. 4. HCO3 - in exchange to Cl- is transported in reverse direction (from the cytoplasm into the blood, charge balance). 5. Finally, H+ and Cl- ions combine in the lumen of the gland (pit) and produce HCl • HCl is collected and stored in the pit until used for different physiological functions. Secretion of HCl by parietal cells
  • 79.
    Regulation and Mechanismof HCl Secretion • The four inputs to parietal cells that regulate acid secretion by controlling the transfer of the H-K ATPase pumps in cytoplasmic vesicle membranes to the plasma membrane.
  • 80.
    Pepsin Secretion • Severalprotein-digesting enzymes collectively known as pepsin • Pepsin is secreted by chief cells in the form of an inactive precursor called pepsinogen. • The acidity in the stomach’s lumen converts pepsinogen to pepsin, the fully active form • Thus the activation of pepsin is positive-feedback process. Fig: Conversion of pepsinogen to pepsin in the lumen of the stomach.
  • 82.
    Emptying of thestomach • The rate at which the stomach empties depends on the fluidity of the chyme and its contents. • Thus, liquids empty faster than solids (fast CHO> protein> fat). Factors that affect emptying include neural and hormonal 1. Neural factors that affect emptying: a. Parasympathetic: Causes increase rate of emptying by opening the pyloric sphincter. b. Sympathetics: Increases constriction of the pyloric sphincter through its adrenergic receptors, thus it has the effect of delaying emptying. c. Enteric nerves (Enterogastric reflex): When fat or protein chyme reaches the duodenumreceptors detect and send impulses to enteric nerves of the stomach cause the inhibition of stomach motility and secretion.
  • 83.
    (Cont…) Stomach emptying 2.Hormonal factors that slow emptying: - CCK (stimulated by fat and peptides) - GIP (gastric inhibitory polypeptide), stimulated by CHO and fat - Secretin : stimulated by high acid (HCl) • The hormones after being secreted pass through the blood and reach the stomach, where they inhibit stomach motility and secretion. • They also cause constriction of the pyloric sphincters thereby delaying emptying.
  • 84.
    Physiological advantage ofdelaying stomach contents 1. It gives ample time for nutrients (e.g., like fat) to remain longer in the stomach and be digested by gastric juices. 2. The delay prevents acids (HCl) not to be damped into the duodenum at higher rates to cause duodenal ulcers. 3. The delay also gives time for pancreatic secretions to reach duodenum and neutralize the acid.
  • 85.
    Stomach Lining • Thestomach is exposed to the harshest conditions in the digestive tract To keep stomach from digesting itself, the stomach has ; • A mucosal barrier with: – A thick coat of bicarbonate-rich mucus on the stomach wall – Epithelial cells that are joined by tight junctions – Gastric glands that have cells impermeable to HCl • Damaged epithelial cells are quickly replaced
  • 86.
    Absorption from thestomach An insignificant absorptive functions takes place in the stomach, these are: a. Alcohol b. Certain drugs (aspirin, morphine etc.) c. Small quantities of H2O. NB: Organic nutrients (glucose, amino acids, and FFA) etc are not usually absorbed from the stomach.
  • 87.
    Phases of gastricjuice secretion The cephalic (~15%) (neural): • Afferent impulses from taste, smell, sight, or thought of food are carried to MO that causes reflex stimulation of the stomach to secrete gastric juices. The gastric phase (~70%) • When food distends the stomach, mechanoreceptors and chemoreceptor are activated by products of food. • Sensory information from the receptors pass to the brain stem (MO) • Reflex Vagal stimulation on the stomach causes increased gastric secretions through Ach . 3. The intestinal phase (~5%) • This stage begins when chyme reaches the duodenum. • This phase is mostly inhibitory to gastric juice secretion. • Secretion of hormones like Secretin, CCK, GIP that reflexly inhibit gastric secretion in the stomach.
  • 88.
    Small Intestine: GrossAnatomy • Runs from pyloric sphincter to the ileocecal valve • Has three subdivisions: – duodenum, – jejunum, and – ileum • The bile duct and main pancreatic duct join the duodenum • The jejunum extends from the duodenum to the ileum • The ileum joins the large intestine at the ileocecal valve
  • 89.
    Small Intestine: grossanatomy • SI is specialized for completion of digestion and absorption of nutrients. a. Duodenum: ~25 cm, mainly secretary, mucose, hormones, enzymes) b. Jejunum : ~2.5m mainly absorptive c. Ileum : ~3.7m mainly absorptive
  • 90.
    Small intestine, structure •Structure of the wall is similar to others and consists of 4-layers (mucosa, Submucosa, Muscularis externa and Serosa). • Enteric and Vagal nerves abundantly innervate the Small intestine. • The inner lining is greatly folded to increase the surface area for absorption, thus – The microvillus + Villi + greater foldings (plicea circulares) increase the absorptive surface of Small intestine by ~ 600-fold).
  • 91.
    Small Intestine: microscopicanatomy • Structural modifications of the small intestine wall increase surface area • Plicae circulares: ~ deep circular folds of the mucosa and sub mucosa • Villi ~ finger like extensions of the mucosa • Microvilli ~ tiny projections of absorptive mucosal cells’ plasma membranes
  • 93.
    The Villi • Villi:Are numerous finger like projections that emerge from the mucous membrane. • Each Villi consist of a layer of absorptive epithelial cells that contain minute microvilli (brush border) at their surfaces. • The epithelial cells contain digestive enzymes like sucrase, maltase, lactase, and peptidases in the membranes of the microvilli.
  • 94.
    Movement in thesmall intestine ~ Two types of movements occur in the SI: 1. Mixing movement 2. Propulsive movement Mixing movements (segmentation contractions) ~ Ring like contractions appear at regular intervals ~ Segmentation contractions exert chopping action on intestinal chyme and mix it with digestive juice
  • 95.
    Propulsive movements • Areperistaltic waves that propel chyme anal wards • Initiated by intestinal distension. • Chyme is propelled in the SI until it reaches the terminal ileocecal sphincter
  • 96.
    Ileocecal Sphincter • Function: preventsback flow of fecal matter from the cecum to the ileum Factors regulating the sphincter • Pressure and chemical irritation of ileum – relax it and initiates peristalsis. • Pressure and chemical irritation of cecum – inhibit peristalsis of ileum and closes the sphincter.
  • 97.
    Digestive enzymes secretedin the SI 1. Peptidase: splits peptides into amino acids 2. Three enzymes hydrolyzing diasaccharides into monosaccharides: sucrase, maltase and lactase 3. Intestinal lipase: splits neutral fats into glycerol and fatty acids. Regulation of SI secretion • Local factors: tactile, distension, irritation, pH. • Hormonal: Secretin, CCK, VIP, Glucagon, GIP • Nervous: vagal stimulation increases intestinal secretion Sympathetic stimulation decreases intestinal secretion • Enteric reflexes: stimulation of submucosal plexus
  • 98.
    Digestion in theSmall Intestine • As chyme enters the duodenum: – Carbohydrates, proteins and fat digestion have taken place – Digestion continues in the small intestine – Chyme is released slowly into the duodenum – Because it is hypertonic and has low pH, mixing is required for proper digestion • Virtually all nutrient absorption takes place in the small intestine
  • 99.
    Digestion in theSmall Intestine 1. Digestion of Carbohydrates Involved Enzymes and their sight of Secretion • Mouth: salivary amylase • Esophagus & stomach: nothing happens • Duodenum: pancreatic amylase • Brush border enzymes (maltase, sucrase & lactase) act on disaccharides and produce monosaccharides: fructose, glucose & galactose
  • 100.
    Starch - Salivary amylase(20-40%) - Pancreatic amylase (50-80%) Maltose and 3-9 glucose polymers Lactose Sucrose -Maltase -Lactase -Sucrase -Dextrase Glucose + Glucose Glucose and Glucose and Galactose Fructose Digestion of Carbohydrates…
  • 101.
    2. Proteins Digestion: InvolvedEnzymes and their Sight of Secretion Enzymes used: pepsin in the stomach • Enzymes acting in the small intestine * Pancreatic enzymes – trypsin, chymotrypsin, and carboxypeptidase * Brush border enzymes – aminopeptidases, carboxypeptidases, and dipeptidases
  • 102.
    Digestion of Proteins,cont,d Stomach • HCl denatures or unfolds proteins • HCl activates pepsinogen into pepsin • Pepsin turns proteins into peptides Pancreas • Digestive enzymes: split peptide bonds between different amino acids. Small Intestine • Brush border enzymes * enzymes break peptide bonds that attach terminal amino acids to carboxyl ends of peptides (carboxypeptidases) * enzymes break peptide bonds that attach terminal amino acids to amino ends of peptides (aminopeptidases) * enzymes split dipeptides to amino acids (dipeptidase)
  • 103.
    3. Chemical Digestionof Fats • Happens mainly in the small intestine Two phases: 1. Emulsification – By bile salts – Emulsion droplets 2. Chemical digestion • Pancreatic lipase –Free fatty acids –Monoglycerides
  • 104.
    Dietary source offat • Neutral fats (triglycerides), Cholesterol and cholesterol esters Phospholipids Fat Emulsified fat FFA + Glycerides -Lingual lipase -Gastric lipase -Pancreas lipase Cholesterol Bile salt FFA + Glycerides Cholesterol esters Cholesterol Esterase Phospholipids-A2 Phospholipase FFA + Phopholipids Digestion of Lipids Bile salt
  • 105.
    Absorption in theSmall Intestine • Absorption ~ is the passage of the end products of digestion from the GI tract into blood or lymph Occurs by • diffusion, • facilitated diffusion, • osmosis, and • active transport.
  • 106.
    Small intestine, Absorption •Absorption & transport – Carbohydrates are degraded into their simpler forms; glucose, galactose, and fructose are absorbed by secondary active transport (diffusion together with Na+ ion). – Fats are degraded into FFA + glycerol and taken by lacteals that enter the thoracic duct and finally join the circulation. – Protein are absorbed as amino acids by active transport. – Electrolytes Na+, Ca2+, iron etc are absorbed by active transport.
  • 107.
    Where will theabsorbed nutrients go?
  • 108.
    Water Absorption • 95%of water is absorbed in the small intestines by osmosis • Water moves in both directions across intestinal mucosa • Net osmosis occurs whenever a concentration gradient is established by active transport of solutes into the mucosal cells • Water uptake is coupled with solute uptake, and as water moves into mucosal cells, substances follow along their concentration gradients.
  • 109.
  • 110.
    Large Intestine • Issubdivided into – the cecum, • appendix, – colon, – rectum, and • anal canal • The cecum: – Contains appendix
  • 111.
    Colon • Has distinctregions: – ascending colon, – transverse colon, – descending colon, and – sigmoid colon • The sigmoid colon joins the rectum • The anal canal, the last segment of the large intestine, opens to the exterior at the anus
  • 112.
    Valves and Sphinctersof the Rectum and Anus • Valves of the rectum stop feces from being passed with gas • The anus has two sphincters: – Internal anal sphincter composed of smooth muscle – External anal sphincter composed of skeletal muscle • These sphincters are closed except during defecation
  • 113.
    Large intestine (colon) Functionof the large intestine Other than digestion of bacteria, no further digestion takes place 1. Water absorption 2. Electrolyte (like NaCl) absorption 3. Mucous & HCO3 - secretion 4. Storage, transport, and evacuation of feces 5. Absorption of some drugs 6. Bacterial fermentation in the colon 7. Stimulates synthesis of some vitamins
  • 114.
    • The largeintestine takes about 16 hours to finish up the remaining processes of the digestive system. • Food is no longer broken down at this stage of digestion. • The colon absorbs vitamins which are created by the colonic bacteria • It also compacts feces, and stores fecal matter in the rectum until it can be discharged via the anus in defecation.
  • 115.
    Bacteria • The bacteriaof the large intestine consist of: – Bacteria surviving the small intestine that enter the cecum – Those entering via the anus • The large intestine houses over 700 species of bacteria that perform a variety of functions. • The large intestine absorbs some of the products formed by the bacteria inhabiting this region.
  • 116.
    Bacteria in thelarge intestine: – Ferment indigestible carbohydrates, undigested polysaccharides (fiber) are metabolized to short- chain fatty acids by bacteria in the large intestine and absorbed by passive diffusion. – Release irritating acids and gases (flatus). The bicarbonate that the large intestine secretes helps to neutralize the increased acidity – They produce large amounts of vitamins, especially vitamin K and biotin (a vitamin B), for absorption into the blood.
  • 117.
    Large intestine, structure •The difference of large intestine from small intestine is that the large intestine does not have villi, and thus, has lower surface area • The large intestine mucosa contains deep crypts and the epithelium linings contain abundant goblet cells. • Goblet cells secrete mucus in response to tactile stimuli and parasympathetic stimulation.
  • 118.
    Large intestine, Functionalstructures • The colon is about 1.3 m long and has the following parts: • Cecum,--> Ascending colon --> Transverse colon --> Descending colon --> Sigmoid colon Rectum --> and Anus • Ileocecal valve (sphincter) : connects ileum -to- cecum. It is mostly in a contracted state b/s: a. By remaining contracted, it prevents bacterial penetration back to the SI. b. It permits slow flow of chyme to the LI to increase transit time.
  • 119.
    Movement in thelarge intestine * Two types of movements 1. Mixing movements (Haustration) 2. Propulsive movements (mass movements) * Mass movement :is initiated by local distension, gastro-colic reflex * Poor motility of the transverse colon causes greater absorption and constipation * Excess motility of the sigmoid colon causes less absorption and diarrhoea or loose stool
  • 120.
    Defecation reflex • RectalStimulation distension of myenteric plexus Initiates peristalsis in descending, sigmoid colon and rectum Inhibition of internal anal sphincter (IAS) by • Parasympathetic stimulation • myenteric plexus  Peristaltic wave forces feces to the anus  Voluntary Relaxation of external anal sphincter ►Defecation
  • 122.
    Secretion of thelarge intestine Glands • Cryps of Lieberkuhn Secret H2O, electrolytes • Goblet cells and mucous  Regulated by local (tactile) factors