The primary function of the gastrointestinal (GI) system is to supply to the body such constituents as water, electrolytes, and nutrients which are necessary for life. In this article, we will discuss Gastrointestinal System Functional Anatomy.
(A). The different parts of the GI tract are:
— Small intestines
— Large intestines (colon)
— Rectum and anus
The accessory parts are:
— Liver and biliary system
— Salivary glands
(B). The functions can be subdivided into:
— Ingestion (eating) with movement of food down the GI tract
— Secretion of digestive juices and digestion
— Absorption of the products of digestion
1. Gastrointestinal System Functional Anatomy
1. The upper part of the GI tract mouth and esophagus is lined by squamous epithelium.
2. The stomach has special epithelium which is resistant to acid. Distributed in the stomach epithelium are special cells which secrete HCI and pepsin (a digestive enzyme). Secretion of acid is brought about by a special chemical histamine which acts on a receptor called H2 receptor.
3. In the duodenum, 2 important ducts or canals are the bile duct and the pancreatic duct. The bile duct brings bile which is necessary for digestion of fats. The pancreatic duct brings pancreatic juice which is necessary for digestion of carbohydrates, proteins, and fats.
4. The small intestines have a special type of lining. There are cells which secrete enzymes for digestion. The mucous lining is folded into small projections—villi. The Villi increase the surface area available for absorption of the products of digestion.
5. The wall of the GI tract is made up of smooth (plain) muscle sheets.
6. The smooth muscle remains in partial contraction or tone.
7. Contraction is so co-ordinated that the contents are propelled forwards peristalsis.
8. The nerve supply of the GI tract is via the autonomic nervous system. It is not under voluntary control.
9. The mouth and esophagus are under voluntary control making swallowing a voluntary act.
10. The anus is also under voluntary control making defecation also a voluntary act.
2. Sphincters in the Gastrointestinal Tract
Situated between the lower end of the esophagus and the upper end of the stomach, the cardiac sphincter prevents reflux (backflow) of stomach contents into the esophagus. Most importantly it prevents reflux of the acid from the stomach into the esophagus which could lead to erosion and ulceration of the esophagus. The cardiac sphincter opens only when food is swallowed.
Situated between the lower end of the stomach and the duodenum, the pyloric sphincter holds up food in the stomach and allows it to pass little by little into the duodenum by partially opening from time to time. Fatty foods can be held up for 6-8 hours but carbohydrates usually pass down in 1/2 to I hour. Drugs are available, which open the pyloric sphincter and allow rapid transit of stomach contents into the duodenum.
Situated between the end of small intestines and the cecum of the large intestines or colon, this sphincter allows the passage of small intestinal contents into the large intestines little by little by partially opening from time to time.
Situated at the end of the anus, the anal sphincter holds up feces in the rectum. This sphincter is under voluntary control and relaxes voluntarily at the time of defecation. The anal sphincter may become weak in old persons or in persons with nerve damage.
3. Digestion of Food
In the mouth during chewing, food is mixed with saliva and made into a semi-solid paste suitable for swallowing. Saliva contains an enzyme ptyalin which initiates the digestion of carbohydrates. Food then is subjected to the action of acid and pepsin in the stomach Acid activates pepsin. Pepsin partially digests carbohydrates. Pepsin and acid are secreted into the stomach by special cells in the wall of the stomach. The mucous lining of the stomach is made up of cells which are resistant to the action of acid and pepsin. Damage to the lining of the stomach causes the formation of a peptic ulcer which produces pain and may also produce perforation and bleeding. Both these complications may endanger life. The cause of peptic ulcer is not known, but it may be produced or aggravated by alcohol, smoking, and certain drugs like aspirin.
Fortunately, modern drugs are now available which block the formation of acid and cause healing of the peptic ulcer. The partially digested food then passes into the duodenum. In the duodenum, pancreatic juice (3000 ml/day) pours into the duodenum via the pancreatic duct. Pancreatic juice is extremely potent and breaks down proteins, fats, and carbohydrates. Bile flows into the duodenum via the bile duct. Bile contains bile salts which help in digestion of fats. The environment in the duodenum is powerfully alkaline. Sodium bicarbonate in the pancreatic juice neutralizes the acid and produces an alkaline environment.
Food passes down from the duodenum into the small intestines. Glands in the cell of the small intestine produce powerful digestive enzymes and the partially digested food is completely digested. Carbohydrates are converted into glucose or galactose for absorption. Proteins are broken down into amino acids which are then absorbed. Fats are broken down into fatty acids and glycerol. The products of digestion and most drugs are absorbed in the upper part of the small intestines called jejunum. The more distal part of the small intestines called the ileum is relatively ineffective in absorbing the products of digestion.
Thereafter the unabsorbed food residues, water, and electrolytes pass downwards into the colon. Absorption from the colon is minimal. During its extremely slow passage in the colon, the water in the food residue is absorbed, so that the semi-liquid residue received from the small intestines is dried into solid feces. Feces is stored in the last part of the colon called the rectum and then defecated when convenient. The products of digestion are carried by the blood to the liver via the portal veins. The portal vein drains blood from the GI tract; therefore all absorbed products of digestion must pass through the liver.
During its passage through the liver, the absorbed products of digestion are detoxified and modified by the liver. The absorption of food is facilitated by the special mucous membrane lining of the small intestines. The mucous lining is formed into folds, each fold containing a blood vessel. The presence of millions of these folds called Villi increases the surface available for absorption. In many diseases, the folds become defective and the mucous lining of the small intestine becomes flat. Absorption is impaired. In the absence of efficient absorption, the patient becomes thin and wasted. This is called malabsorption syndrome and occurs as a consequence of severe anemia,
vitamin deficiency, and some virus infections.
4. The Liver
The liver is one of the most important organs in the body. It has an all-important role in (a) biotransformation (b) metabolism (c) detoxication (d) excretion.
The basic functional unit in the liver is the liver lobule.
1. The lobule is a collection of liver cells, arranged radially.
2. The lobule contains a central vein ( the branch of hepatic vein) which collects blood from the liver cells and drains into the circulation. On the outer side of lobule lie:
(a) A branch of hepatic artery which supplies oxygen to the liver cell.
(b) A branch of bile duct which collects bile (and excretory products) and discharges it into the duodenum via the bile duct.
(c) A branch of the portal vein. The portal vein drains blood from the intestines containing the products of digestion. The portal vein has no direct communication with the central vein and all the blood, in the portal vein must pass through the liver cells before it can reach the central vein.
3. The structure of the lobule ensures that all the blood, containing the products of digestion, must pass through the liver cells before it can reach the general circulation.
4. In addition to liver cells, the lobule also contains large cells which belong to the immune system. These are called Kupffer cells. Kupffer cells play no role in digestion.
7. Some Common GI Disorders
When peristalsis is ineffective in propelling the contents forward, the smooth muscles of the intestines go into spasm. This produces pain. Spasm may be intermittent, giving rise to intermittent pain, which is called colic. The commonest cause of colic is gas in the intestines.
Excessive peristalsis causes rapid propulsion of the contents of the intestines. Sufficient time is not available for complete digestion and absorption of water. As a result, there is increased the frequency with the passage of liquid or semi-liquid stools. This is called diarrhea.
Very slow passage of the contents of the intestines cause a greater absorption of water and produces hard stools. Constipation is normally seen in old persons and also when the residue in the intestines is very small (low roughage diet). Some drugs can produce constipation.
Nausea and Vomiting
When the stomach wants to throw out its content rapidly there is reverse peristalsis. The cardiac sphincter opens and the contents of the stomach are vomited out. Vomiting may be caused by irritation of the stomach by alcohol, food and many drugs.
Obstruction to the passage of contents of the GI tract may be produced by a foreign body such as a mass of roundworms or by surgical conditions in which the intestines get coiled and twisted (volvulus) or obstructed by a hernia. Unless quickly relieved by surgery, the patient may die.