The gastrointestinal tract ( GI tract ), also called the digestive tract , alimentary canal , or gut , is the system of organs that takes in food, digests it to extract energy and nutrients, and expels the remaining waste. The major functions of the GI tract are digestion and excretion . In a normal human adult male, the GI tract is approximately 6.5 meters long (20 feet) and consists of the upper and lower GI tracts. The tract may also be divided into foregut, midgut, and hindgut, reflecting the embryological origin of each segment of the tract.
The GI tract can be divided into 4 concentric layers: mucosa the innermost layer of the GI tract, surrounding the lumen, or space within the tube. This layer comes in direct contact with the food (or bolus), and is responsible for absorption and secretion, important processes in digestion. submucosa consists of a dense irregular layer of connective tissue with large blood vessels, lymphatics and nerves branching into the mucosa and muscularis muscularis externa consists of a circular inner muscular layer and a longitudinal outer muscular layer. The circular muscle layer prevents the food from going backwards and the longitudinal layer shortens the tract. The coordinated contractions of these layers is called peristalsis and propels the bolus, or balled-up food, through the GI tract. adventitia (or serosa) consists of several layers of epithelia.
Upper gastrointestinal tract The upper GI tract consists of the mouth, pharynx, esophagus, and stomach. The upper GI tract roughly corresponds to the derivatives of the foregut , with the exception of the first part of the duodenum
The primary function of teeth is to tear, scrape, and chew food (=mechanical digestion). The mouth contains the buccal mucosa , which contains the openings of the salivary glands; the tongue; and the teeth .
Summary of salivary gland features Parotid gland (#1) – secretes a watery fluid rich in amylase. Sublingual gland (#3) – secretions are thick and stringy which binds food particles and lubricates during swallowing. Submandibular gland (#2) – secretes watery fluid rich in amylase but more viscous. Salivary Glands – moistens food particles, helps bind them together and begins the chemical digestion of food. Saliva contains enzymes that initiate the breakdown of carbohydrates.
Behind the mouth lies the pharynx (refer to our discussion of the respiratory system), which leads to a hollow muscular tube, the esophagus. The esophagus extends through the chest and pierces the diaphragm to reach the stomach. Food is passed through the esophagus by using the process of peristalsis . Specifically, it connects the pharynx with the stomach, where the second stage of digestion is initiated (the first stage of digestion is in the mouth, with teeth and tongue masticating food and mixing it with saliva).
The esophagus is lined with a mucous membrane , and is more deeply lined with muscle that acts with peristaltic action to move swallowed food down to the stomach.
The stomach is a bean-shaped hollow muscular organ involved in the second phase of digestion , following mastication. The stomach is usually a highly acidic environment due to gastric acid production which produces a pH range usually between 2 and 4 depending on food intake and other factors (a pH lower than 2 generally indicates a health problem). Such an environment is able to break down large molecules (such as from food) to smaller ones so that they can eventually be absorbed from the small intestine. The stomach can produce and secrete about 2 to 3 liters of gastric acid per day.
Two smooth muscle bands, or sphincters, keep the contents of the stomach contained. They are the Cardiac or esophageal sphincter dividing the tract above, and the Pyloric sphincter dividing the stomach from the small intestine. In humans, the stomach has a volume of about 50 mL when empty. After a meal, it generally expands to hold about 1 litre of food, but it can actually expand to hold as much as 4 litres. The human stomach has more nerve endings than the human brain . Chyme is the liquid substance found in the stomach before passing through the pyloric valve and entering the duodenum.
The movement and the flow of chemicals into the stomach are controlled by both the autonomic nervous system and by various digestive system hormones: Pepsin is a necessity in protein digestion . Other functions include absorbing water, some ions, and some lipid soluble compounds such as alcohol, aspirin, and caffeine. Gastrin causes an increase in the secretion of HCl and pepsinogen. The stomach is divided into four sections, each of which has different cells and functions. The sections are: Cardia where the contents of the esophagus empty into the stomach. Fundus formed by the upper curvature of the organ. Body or corpus the main, central region. Pylorus the lower section of the organ that facilitates emptying the contents into the small intestine.
Most cases of stomach ulcers, gastritis, and stomach cancer are caused by Helicobacter pylori infection.
<ul><li>Lower gastrointestinal tract </li></ul><ul><li>The lower GI tract comprises the intestines and anus. </li></ul><ul><li>1. Bowel or intestine: </li></ul><ul><ul><li>small intestine , which has three parts: </li></ul></ul><ul><ul><ul><li>A. duodenum </li></ul></ul></ul><ul><ul><ul><li>B. jejunum </li></ul></ul></ul><ul><ul><ul><li>C. ileum </li></ul></ul></ul><ul><ul><li>large intestine , which has three parts: </li></ul></ul><ul><ul><ul><li>A.cecum (the vermiform appendix is attached to the cecum). </li></ul></ul></ul><ul><ul><ul><li>B.colon (ascending colon, transverse colon, descending colon and sigmoid flexure) </li></ul></ul></ul><ul><ul><ul><li>C. rectum </li></ul></ul></ul><ul><li>2. anus </li></ul>
The intestinal tract can be broadly divided into two different parts, the small and large intestine. Grayish-purple in color, the small intestine is the first and longest , measuring twenty feet on average in an adult man.
The lumen is the cavity where digested material passes through and from where nutrients are absorbed . Along the whole length of the small intestine are goblet cells . These secrete mucus which lubricates the passage of food along and protects it from digestive enzymes. Villi are extensions of the mucosa and increase the overall surface area of the intestine. Micro villi are present on the epithelium of a villus and further increase the surface area over which absorption can take place.
The small intestine is divided into three structural parts: A. duodenum 25 cm (9.84 in) in length B. jejunum 2.5 m (8.2 ft) C. ileum 3.5 m (11.5 ft) Although the small intestine is much longer than the large intestine (typically 4-5 times longer), it is referred to as such due to its comparatively smaller diameter . On average, the diameter of the small intestine of an adult human measures approximately 2.5-3cm , and the large intestine measures about 7.6 cm in diameter. Absorption of the majority of nutrients takes place in the jejunum , with the following notable exceptions: Iron is absorbed in the duodenum Vitamin B12 is absorbed in the terminal ileum . Water and lipids are absorbed by passive diffusion throughout
The digestion of proteins into peptides and amino acids principally occurs in the stomach but some also occurs in the small intestine. The small intestine is where the most chemical digestion takes place: lipids (fats) are degraded into fatty acids and glycerol. Lipid digestion is the sole responsibility of the small intestine. Pancreatic lipase breaks down triglycerides into free fatty acids and monoglycerides. carbohydrates are degraded into simple sugars (e.g., glucose). In the small intestine pancreatic amylase breaks down carbohydrates into oligosaccharides. Chemical digestion in the duodenum
The duodenum is largely responsible for the breakdown of food in the small intestine . The pH in the jejunum is usually between 7 and 8 (neutral or slightly alkaline). The jejunum and the ileum are suspended by mesentery which gives the bowel great mobility within the abdomen. It also contains muscles to help move the food along.
The function of the ileum is to absorb the products of digestion. The wall itself is made up of folds, each of which has many tiny finger-like projections known as villi , on its surface. In turn, the epithelial cells which line these villi possess even larger numbers of microvilli . Therefore the ileum has an extremely large surface area both for the absorption (attachment) of enzyme molecules and for the absorption of products of digestion.
<ul><li>Large Intestine: Parts and location </li></ul><ul><li>Parts of the large intestine are: </li></ul><ul><li>Cecum - the first part of the large intestine </li></ul><ul><li>Vermiform appendix - small appendage on the cecum containing lymphatic tissue </li></ul><ul><ul><li>Locations along the colon are: </li></ul></ul><ul><ul><ul><li>The ascending colon </li></ul></ul></ul><ul><ul><ul><li>The transverse colon </li></ul></ul></ul><ul><ul><ul><li>The descending colon </li></ul></ul></ul><ul><ul><ul><li>The sigmoid colon - the s shaped region of the large intestine </li></ul></ul></ul><ul><li>Rectum </li></ul><ul><li>Anal canal - last segment of the large intestine </li></ul><ul><li>Internal anal sphincter - involuntary, smooth muscle </li></ul><ul><li>External anal sphincter - voluntary, skeletal muscle </li></ul><ul><li>Anus </li></ul>The large intestine , or the organ which is now more commonly referred to as the colon , is the last part of the digestive system : the final stage of the alimentary canal. Its function is to absorb the remaining water from indigestible food matter, store these unusable food matter (wastes) and then eliminates the wastes from the body.
The large intestine hosts several kinds of bacteria (i.e. ecoli) that deal with molecules the human body is not able to breakdown itself. This is an example of symbiosis. These bacteria also account for the production of gases inside our intestine (this gas is released as flatulence when removed through the anus). The large intestine houses over 700 species of bacteria that perform a variety of functions including synthesis of vitamin B complex vitamins and vitamin K.
The large intestine takes 12 to 24 hours to finish up the remaining processes of the digestive system. Food is not broken down any further in this stage of digestion. The large intestine absorbs vitamins that are created by the bacteria. The most obvious difference between the small intestine and the large intestine is that the large intestine is wider and lacks villi. Medical literature shows that the appendix is rich in infection-fighting lymphoid cells, and therefore, has an important role in immunity. Appendicitis is the result of a blockage that traps infectious material in the lumen. The appendix can be removed with no damage or consequence to the patient
The cecum is a pouch connected to the ascending colon of the large intestine and the ileum. It is considered to be the beginning of the large intestine.
Ascending colon The ascending colon, on the right side of the abdomen, is about 12.5 cm long. Transverse colon The transverse colon hangs off the stomach, attached to it by a wide band of tissue called the greater omentum. More cancers form as the large intestine goes along and the contents become more solid (water is removed) in order to form feces. Descending colon The descending colon is the part of the colon from the splenic flexure to the beginning of the sigmoid colon. Sigmoid colon The sigmoid colon is the part of the large intestine after the descending colon and before the rectum. The name sigmoid means S-shaped. The walls of the sigmoid colon are muscular, and contract to increase the pressure inside the colon, causing the stool to move into the rectum . Sigmoidoscopy is a common diagnostic technique used to examine the sigmoid colon.
The rectum is the final straight portion of the large intestine, terminating in the anus. The human rectum is about 12 cm long. The rectum intestinum acts as a temporary storage facility for feces. As the rectal walls expand due to the materials filling it from within, stretch receptors from the nervous system located in the rectal walls stimulate the desire to defecate. If the urge is not acted upon, the material in the rectum is often returned to the colon where more water is absorbed. If defecation is delayed for a prolonged period, constipation and hardened feces results. The rectum shortens as material is forced into the anal canal and peristaltic waves propel the feces out of the rectum. The internal and external sphincter allow the feces to be passed by muscles pulling the anus up over the exiting feces. Anus- terminal end of the digestive system
The liver is an organ that plays a major role in metabolism and has a number of functions in the body, including glycogen storage, protein synthesis, and drug detoxification. This organ also is the largest gland in the human body. It produces bile , an alkaline compound which aids in digestion. The liver is the second largest organ (the largest organ being the skin). The liver lies on the right of the stomach and makes a kind of bed for the gallbladder (which stores bile).
The bile produced in the liver is collected in bile canaliculi, which merge to form bile ducts. The gallbladder stores about 50 ml of bile (or "gall") until the body needs it for digestion. The bile, produced in the liver, emulsifies fats and neutralizes acids in partly digested food. After being stored in the gallbladder, the bile becomes more concentrated than when it left the liver, increasing its potency and intensifying its effect on fats.
The pancreas is an organ in the digestive and endocrine system . It is both exocrine (secreting pancreatic juice containing digestive enzymes) and endocrine (producing several important hormones, including insulin and glucagon).
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In anatomy, the kidneys are bean-shaped excretory organs . Part of the urinary system, the kidneys filter wastes (such as urea) from the blood and excrete them, along with water, as urine. The medical field that studies the kidneys and diseases of the kidney is called nephrology . The basic functional unit of the kidney is the nephron , of which there are more than a million within the cortex and medulla of each normal adult human kidney. Nephrons regulate water and soluble matter (especially electrolytes) in the body by first filtering the blood under pressure, and then reabsorbing some necessary fluid and molecules back into the blood while secreting other, unneeded molecules.
Nephron – Glomerulus/Bowman’s Capsule A glomerulus is a capillary tuft surrounded by Bowman's capsule in nephrons of the kidney. The resistance of the arterioles results in high pressure in the glomerulus aiding the process of ultrafiltration where fluids and soluble materials in the blood are forced out of the capillaries and into Bowman's capsule. A glomerulus and its surrounding Bowman's capsule constitute a renal corpuscle , the basic filtration unit of the kidney.
The kidney is one of the major organs involved in whole-body homeostasis. Among its homeostatic functions are acid-base balance, regulation of electrolyte concentrations, control of blood volume, and regulation of blood pressure. The kidneys accomplish these homeostatic functions independently and through coordination with other organs, particularly those of the endocrine system. The kidney communicates with these organs through hormones secreted into the bloodstream. Congenital absence of one or both kidneys, known as unilateral or bilateral renal agenesis can occur.
The ureters are the ducts that carry urine from the kidneys to the urinary bladder. The ureters are muscular tubes that can propel urine along by the motions of peristalsis . In the adult, the ureters are usually 25-30cm long. The urinary bladder is a hollow, muscular, and distensible (or elastic) organ that sits on the pelvic floor. It is the organ that collects urine excreted by the kidneys prior to disposal by urination. Urine enters the bladder via the ureters. The urethra is a tube which connects the urinary bladder to the outside of the body. The urethra has an excretory function in both genders to pass urine to the outside, and also a reproductive function in the male, as a passage for sperm. The external urethral sphincter is a smooth muscle that allows voluntary control over urination.