Anatomy of Gastrointestinal System

Anatomy of Gastrointestinal System

• The gastrointestinal (GI) system includes the gastrointestinal tract and accessory organs.
• The gastrointestinal tract consists of: • Mouth • Pharynx • Esophagus • Stomach • Small intestine • Large intestine
• The accessory organs that are not part of the tract but secrete substances into it via connecting ducts include: • Salivary glands • Liver • Gallbladder • Pancreas

Function of the Gastrointestinal System

• The overall function of the gastrointestinal system is to process ingested foods into molecular forms that are then transferred, along with salts and water, to the body's internal environment, where they can be distributed to cells by the circulatory system.

Small Intestine

• The small intestine is divided into three segments: • Duodenum • Jejunum • Ileum
• Most of the chyme entering from the stomach is digested and absorbed in the duodenum and jejunum.
• Two major glands secrete substances that flow via ducts into the duodenum: • Pancreas • Liver

Functions of the Pancreas

• The pancreas has both endocrine and exocrine functions.
• The exocrine portion of the pancreas secretes: • Digestive enzymes • A fluid rich in bicarbonate ions that neutralize acid from the stomach

Functions of the Liver

• The liver has a variety of functions, including the exocrine function of secreting bile.
• Bile contains: • Bicarbonate ions • Cholesterol • Phospholipids • Bile pigments • A number of organic wastes • Bile salts
• The bicarbonate ions help neutralize acid from the stomach.
• The bile salts solubilize dietary fat and increase the rates at which they are digested and absorbed.

Bile Secretion and Storage

• Bile is secreted by the liver into small ducts that join to form the common hepatic duct.
• Between meals, secreted bile is stored in the gallbladder.
• The gallbladder concentrates the organic molecules in bile by absorbing salts and water.
• During a meal, the smooth muscles in the gallbladder wall contract, causing a concentrated bile solution to be injected into the duodenum via the common bile duct.

Enterohepatic Circulation

• Up to 95% of the cholesterol-based bile salts are "recycled" by reabsorption along the intestine.
• This recycling pathway from the intestine to the liver and back to the intestine is known as the enterohepatic circulation.

Absorption in the Small Intestine

• In the small intestine, monosaccharides and amino acids are absorbed by specific transporter-mediated processes in the plasma membranes of the intestinal epithelial cells.
• Fatty acids enter these cells by diffusion.
• Most mineral ions are actively absorbed by transporters.
• Water diffuses passively down osmotic gradients.

Motility of the Small Intestine

• The motility of the small intestine functions to: • Mix the luminal contents with the various secretions. • Bring the contents into contact with the epithelial surface where absorption takes place. • Slowly advance the luminal material toward the large intestine.

Structure of the Small Intestine

• The lumen of the small intestine has finger-like projections called villi.
• The surface of each villus is covered with a layer of epithelial cells whose surface membranes form small projections called microvilli.
• The combination of folded mucosa, villi, and microvilli increases the small intestine's surface area about 600-fold over that of a flat-surfaced tube having the same length and diameter.

Epithelial Cell Replacement

• Epithelial surfaces in the gastrointestinal tract are continuously being replaced by new epithelial cells.
• In the small intestine, new cells arise by cell division from cells at the base of the villi.
• These cells differentiate as they migrate to the top of the villus, replacing older cells that disintegrate and are discharged into the intestinal lumen.
• The entire epithelium of the small intestine is replaced approximately every five days.

Lymphatic and Blood Circulation

• The center of each intestinal villus is occupied by a single blind-ended lymphatic vessel termed a lacteal and by a capillary network.
• Most of the fat absorbed in the small intestine enters the lacteals.
• Material absorbed by the lacteals reaches the general circulation by eventually emptying from the lymphatic system into the thoracic duct.
• Other absorbed nutrients enter the blood capillaries.
• The venous drainage from the small intestine, as well as from the large intestine, pancreas, and portions of the stomach, passes first via the hepatic portal vein to the liver.

Immune Functions

• The gastrointestinal tract also expresses a variety of immune functions, allowing it to produce antibodies and fight infectious organisms that are not destroyed by the acidity of the stomach.

Large Intestine

• Only a small volume of water, salts, and undigested material is passed on to the large intestine, since most substances are absorbed in the small intestine.
• The large intestine temporarily stores the undigested material (some of which is metabolized by bacteria) and concentrates it by absorbing salts and water.
• Contractions of the rectum, the final segment of the large intestine, and relaxation of associated sphincter muscles expel the feces (defecation).
• In the large intestine, active transport of sodium, coupled with osmotic absorption of water, are the primary activities.
• Microbes in the large intestine are active in the production of vitamin K.

Bile Production and Storage

• Bile is secreted by the liver into small ducts that join to form the common hepatic duct.
• Between meals, bile is stored in the gallbladder.
• The gallbladder concentrates the organic molecules in bile by absorbing salts and water.
• During a meal, the smooth muscles in the gallbladder wall contract, causing a concentrated bile solution to be injected into the duodenum via the common bile duct, an extension of the common hepatic duct.

Bile Salts and Recycling

• Up to 95% of cholesterol-based bile salts are "recycled" by reabsorption along the intestine.
• This recycling pathway from the intestine to the liver and back to the intestine is known as the enterohepatic circulation.
• Increasing dietary fiber is thought to reduce blood cholesterol content by trapping a greater percentage of the bile in bulkier, more fibrous feces.

Small Intestine Functions

• In the small intestine, monosaccharides and amino acids are absorbed by specific transporter-mediated processes in the plasma membranes of the intestinal epithelial cells.
• Fatty acids enter these cells by diffusion.
• Most mineral ions are actively absorbed by transporters, and water diffuses passively down osmotic gradients.
• The motility of the small intestine functions to:
• Mixes the luminal contents with the various secretions.
• Brings the contents into contact with the epithelial surface where absorption takes place.
• Slowly advances the luminal material toward the large intestine.

Blood Circulation and Liver Function

• The venous drainage from the small intestine, as well as from the large intestine, pancreas, and portions of the stomach, passes first via the hepatic portal vein to the liver.
• This is important because the liver contains enzymes that can metabolize (detoxify) harmful, lipophilic compounds that may have been ingested, thereby preventing them from entering the circulation.

Immune Function of the Gastrointestinal Tract

• The gastrointestinal tract also expresses a variety of immune functions, allowing it to produce antibodies and fight infectious organisms that are not destroyed by the acidity of the stomach.

Large Intestine Functions

• Only a small volume of water, salts, and undigested material is passed on to the large intestine, since most substances are absorbed in the small intestine.
• The large intestine temporarily stores the undigested material (some of which is metabolized by bacteria) and concentrates it by absorbing salts and water.
• Contractions of the rectum, the final segment of the large intestine, and relaxation of associated sphincter muscles expel the feces (defecation).

Large Intestine Activities

• Active transport of sodium, coupled with osmotic absorption of water, are the primary activities in the large intestine.
• Microbes in the large intestine are active in the production of vitamin K.

Digestion and Absorption

• Digestion is the process of breaking down large particles and high-molecular weight substances into small molecules.
• Digestion is accomplished by the action of hydrochloric acid in the stomach, bile from the liver, and a variety of digestive enzymes that are released by the system's exocrine glands.
• Absorption is the movement of molecules produced by digestion from the lumen of the gastrointestinal tract across a layer of epithelial cells into the blood or lymph.

Gastrointestinal Motility

• Contractions of smooth muscles in the gastrointestinal tract wall serve two functions:
• They mix the luminal contents with the various secretions.
• They move the contents through the tract from mouth to anus.
• These contractions are referred to as the motility (peristalsis) of the gastrointestinal tract.

Gastrointestinal System Functions

• The functions of the gastrointestinal system can be described in terms of four processes:
• Digestion
• Secretion
• Absorption
• Motility and the mechanisms controlling them