CHAPTER 18 Digestive System • Functions of the digestive system. • Structure and innervation of the digestive system. • Swallowing, peristalsis, Esophagus and LES. • Structure and secretions of the stomach. • Mechanism of HCl secretion, GI protection, & Ulcers. • Intestine, liver, gallbladder, and pancreas. • Regulation of digestive system. • Digestion and absorption of major food firstname.lastname@example.org
Functions of the Digestive System• The major functions of digestive system are to digest food and absorb the final products into blood.• Question) How the digestive system achieves the above goal?• Answer) by the following activities: 1- Motility = Movements (contractions) 2- Secretion = Exocrine + Endocrine 3- Digestion & absorption• The digestive system also must get ride of the undigested waste materials this is achieved through the process of defecation.
Layers of the G. I. T.1- Mucosa. - Direct contact with food, - directly involved in secretion and absorption, - contains lymph nodes, goblet cells, and other secretory cells, - muscularis mucosa, thin muscular layer changes the surface area of mucosa.2- Submucosa. - Tissue below mucosa, which is supportive to - mucosa, contains blood vessels, glands, nerve plexus. Submucosal plexus3- Mascularis. - Responsible for the motility of the GIT, - has inner circular and outer longitudinal smooth muscle, - also contains nerve plexuses. Myenteric plexus4- Serosa. - Outermost layer serves supportive and protective functions.
Innervation of the G. I. T.1- Autonomic nerve system (ANS) supply. a) Sympathetic Decrease GI activity b) Parasympathetic Increase GI activity2- Enteric nervous system (ENS).“intrinsic nervous system of the gut” a) Myenteric plexus. Its stimulation causes increase in motor activity(motility) of the gut. b) Submucosal plexus. Mainly controls secretion and blood flow of the GIT,also serves many sensory functions.
Swallowing - Swallowing reflex is initiated when pressure receptors in walls of pharynx are stimulated by food or drink. - Swallowing is co-ordinated by the swallowing center in brain stem (medulla oblongata). During swallowing both nasopharynx and epiglottis close to prevent food movingUpper Esophageal into the nose and trachea,Sphincter respectively.
SwallowingThe food passes down the esophagusby peristalsis and enters into thestomach via the Lower EsophagealSphincter (L.E.S.) LES
Peristalsis - Aboral propulsion of contractions at a velocity of 0.5 to 2 cm per sec. (much faster in the proximal compared to distal part of intestine). -Peristalsis waves are weak and die out after about 10 cm therefore giving a slow rate of movement of chyme (1 cm/min).Mouth Anus Direction of Bolus Aboral propulsion propulsion Circular contract Longitudinal relax Circular relax Longitudinal contract Presence of bolus causes local distention small intestine, and this activates myenteric plexus between circular and longitudinal muscle layers of intestine.
Structure and Function of the Stomach Fundus Structure: divided into: Body 1- Fundus 2- Body Storage 3- Antrum Mixing & Emptying Fuction: 1- Storage of food Retropulsion 2- Mixing of food 3- Emptying of food into small intestine AntrumThe process of mixing food in the stomach is called “retropulsion”
Control of Intragastric PressureEmpty Stomach Distension R Food Entering Stomach R Laplace law: Both surface tension and radius 2 x Surface Tension increase proportionally therefore Pressure = Radius the pressure stays constant - To keep the intragastric pressure constant, after food enters the stomach the peristalsis is inhibited for about 1 hour.
Structure and Function of the StomachGastric glands have several types of cells that secrete different materials whichcollectively are called gastric juice: 1-Mucus = lubrication & protection1- Gobblet cells- secret mucus. 2- HCl = Help digestion2- Parietal (oxyntic) cells – sceret HCl3- Chief cells – sceret enzyme (pepsinogen) 3- Pepsiongen = Digestion4- G cells – secret hormone gastrin 4- Gastrin = Stimulate HCl secretion5- D Cells – secret somatostatin 5- Somatostatin = Inhibit gastrin secretion6- Enterochromaffin-like (ECL) cells 6- Histamine = Stimulate HCl secretion –secret histamine.
How the Mucosa of GI is Protected against DigestionAcid (HCl) and enzyme (pepsin) help in digestion of food substances. By this actionthe food is broken into smaller molecules which can be absorbed into blood.The stomach protects itself from digestion by: 1- The chief cells of the gastric mucosa are extremely impermeable to HCl. 2- A layer of alkaline (HCO3-) mucus covers the mucosa. 3- The cells of mucosa are tightly fused to each other so that the HCl does not leak to the submucosa. 4- Rapid rate of cell division in the mucosa (entire epithelium is replaced every 3 days). 5- The enzyme pepsin is secreted in inactive form (pepsinogen).
Activation of PepsinInactive enzyme Active enzyme
Small IntestineBoth Pilace circularisand villi increasesurface area
A VillusIntestine folds pilacae circularisand on these folds there are villiwhich have a layer of epithelialcells which their membrane isalso has folds called microvilli.These arrangement increase theSurface area of the small intestine.Microvilli
Intestinal MotilityTypes of contractions in small intestine:1- Segmentation: (mixing movement)
Intestinal Motility2- Peristalsis: (propulsive movement) Direction of Bolus propulsion Circular contract Longitudinal relax Circular relax Longitudinal contract
Structure and Motility of the Colon1- Mixing movements: Haustrations = occur in cecum, entire colon, and rectum. Are characterized by contractions of inner circular muscles.2- Propulsive movement: Mass peristalsis = strong peristalsis which directed aborally, occur several times a day in cecum and entire colon. Reverse peristalsis = orally directed which gives more time to the fecal matter to be expose of water absorption.
LIVER- Is the largest internal organ in the body.- Has many functions: 1- Detoxification of blood 2- Regulating metabolism 3- Protein synthesis 4- Storage site for vitamins and iron 5-Secretion of Bile 5- Secretion of bile.* Bile secretion is the principal digestive function of the liver.- Bile contains: - Bile salts LIVER - Urobilinogen - Cholestrol - Lecithin - Bilirubin - Electrolytes - Bicarbonate Small intestine
GallbladderThe bile which is secreted by the liver isstored and concentrated in gallbladder.When bile is needed for digestion, thewalls of gallbladder contract and thisejects the bile through the bile ductinto duodenum.
PancreasWeighs about 100g, and produces about 1L of juice/day.Secretions are both endocrine and exocrine. Digestive Metabolic function functionIslets of Langerhans Acini: secret pancreatic juiceSecret insulin and glucagon
Pacreatic Juice• Pancreatic juice contains: – Water and electrolytes (Na+, K+, Cl+, Ca++) – Bicarbonate (HCO3-) – Digestive enzymes: Sodium Bicarbonate For neutralization of acid • Amylase • Trypsin • Lipase • Ribonuclease Enzymes for digestion • Deoxyribonuclease • Etc. of food
Regulation of the Digestive System• The activities of the G.I. Tract is regulated by: – Neural mechanisms: • Automomic nervous system – Sympathetic Inhibit activity of GI tract – Parasympathetic Stimulate activity of GI tract • Enteric nervous system – myenteric Control contractions – Submucosal Control Secretions & Blood flow – Action of hormones (endocrine): • Hormones secreted from the glands located in the G.I. Tract act on effector cells of the G.I. Itself. Examples of such hormones are: secretin, gastrin, CCK (cholecystokinin) From small intestine stomach From
Regulation of Gastric Secretion• Contractions and secretions of the stomach to some extent are automatic. For example: the presence of food causes distension in the stomach wall and this will lead to contractions. Presence of some food materials in the stomach stimulate its glands to secret gastric juice.• However, when ANS or hormones are active their effects dominate the function of the stomach.• There are 3 phases in the control of gastric secretion: 1- Cephalic phase: sight, smell, taste, even thoughts of food can initiate this phase. 2- Gastric phase: when food enters the stomach this phase becomes active. 3- Intestinal phase: when food enters small intestine this phase becomes active.
Cephalic Phase Sight Smell Sound Cortex Thoughts Taste Vagal Appetite Hypothalamus Touch Mouth Temp. Nuclei center Etc. PancreasSmall intestine Vagus Parietal cells HCl Chief cells Enzyme ACh Mucus cells Mucus G cells Gastrin Gastric Gland- Cephalic phase is abolished by vagotomy.
Gastric Phase Vagal Nuclei Vago-Vagal Pancreas Vagus Small intestine Food entering stomach ACh Distension, protein ACh digestion products Local Parietal cells HCl Chief cells Enzyme Enteric Mucus cells Mucus G cells Gastrin Plexus Gastric Gland Blocker of ACh- Gastric secretion is abolished after administration of atropine
STRONG Intestinal Phase1- Inhibition: At low duodenal pH and presence of food Food entering small intestine Gastric gland Decrease acid Somatostatin Secretin S cell Secretion CCK Secretin is release in response Blood to acid and CCK is released in response Fat. WEAK2- Stimulation: Occurs when duodenal pH is relatively high. Gastric gland Food entering small intestine G cell Increased acid Secretion Gastrin Blood