The alimentary tract

The major blood vessels enter the liver on its inferior surface in a centrally placed groove called the porta hepatis, which anatomically separates the quadrate and caudate lobes. The liver has two sources of blood supply: fully oxygenated blood from the hepatic artery, which is a major branch of the celiac axis (the main artery that crosses the abdomen) after its emergence from the abdominal aorta; and partially oxygenated blood from the large portal vein, which in turn receives all venous blood from the spleen, pancreas, gallbladder, lower esophagus, and the remainder of the gastrointestinal tract, including the stomach, small intestine, large intestine, and upper portion of the rectum. The portal vein is formed by the juncture of the splenic vein with the superior mesenteric vein. At the porta hepatis the portal vein divides into two large branches, each going to one of the major lobes of the liver. The porta hepatis is also the exit point for the hepatic ducts. These channels are the final pathway for a network of smaller bile ductules interspersed throughout the liver that serve to carry newly formed bile from liver cells to the small intestine via the biliary tract.

Biliary tract

Anatomy

The biliary tract begins with the appearance of two large ducts, the right and left hepatic ducts, at the porta hepatis, a groove that separates two lobes on the right side of the liver. Just below the porta hepatis, these 1- to 2-cm (about half-inch) ducts join to form the hepatic duct, which proceeds for another 2 to 3 cm and is joined by the cystic duct, leading from the gallbladder. The resulting common bile duct progresses downward through the head of the pancreas. There it is usually joined by the main pancreatic duct (duct of Wirsung) at a slightly dilated area called the hepatopancreatic ampulla (ampulla of Vater), which lies in the wall of the inner curve of the descending duodenum, and terminates in the lumen of the duodenum at a 2- to 3-cm elevation called the duodenal papilla (papilla of Vater).

The common bile duct averages about 10 cm in length, and flow of bile from its lower end into the intestine is controlled by the muscular action of the hepatopancreatic sphincter (sphincter of Oddi), located in the duodenal papilla. The cystic duct varies from 2 to 3 cm in length and terminates in the gallbladder, a saccular structure with a capacity of about 50 ml (about 1.5 fluid ounces). Throughout its length, the cystic duct is lined by a spiral mucosal elevation, called the valvula spiralis (valve of Heister). Normally, the gallbladder lies partially embedded on the undersurface of the right lobe of the liver.

Pancreas

The pancreas is a long, narrow gland that is situated transversely across the upper abdomen, behind the stomach and the spleen. The midportion of the pancreas lies against the vertebral column, the abdominal aorta, and the inferior vena cava.

The pancreas is both an exocrine (ductal) and endocrine (ductless) gland. The exocrine tissue, called acinar tissue, produces important digestive enzyme precursors that are transmitted into the small intestine, while the endocrine tissue (contained in the islets of Langerhans) produces at least two hormones (insulin and glucagon) that are important in the regulation of carbohydrate metabolism. Two other hormones produced by the pancreas, vasoactive intestinal polypeptide and somatostatin, are pivotal elements in the control of intestinal secretion and motility.

Individual acinar cells have the shape of a truncated pyramid, arranged in groups around a central ductal lumen. These central ducts empty into progressively larger intercalated and collecting ducts that eventually join the pancreatic duct (duct of Wirsung). The pancreatic duct in turn enters the hepatopancreatic ampulla (ampulla of Vater) of the duodenum, where, in about 80 percent of instances, it is joined by the common bile duct. Occasionally the junction with the common bile duct is proximal to the ampulla, and in a few cases the pancreatic duct and the common bile duct join the duodenum separately.

General features of digestion and absorption

There are four means by which digestive products are absorbed: active transport, passive diffusion, facilitated diffusion, and endocytosis.

Active transport involves the movement of a substance across the membrane of the absorbing cell against an electrical or chemical gradient. It is carrier-mediated; that is, the substance is temporarily bound to another substance that transports it across the cell membrane, where it is released. This process requires energy and is at risk of competitive inhibition by other substances; that is, other substances with a similar molecular structure can compete for the binding site on the carrier. Passive diffusion requires neither energy nor a carrier; the substance merely passes along a simple concentration gradient from an area of high concentration of the substance to an area of low concentration until a state of equilibrium exists on either side of the membrane. Facilitated diffusion also requires no energy, but it involves a carrier, or protein molecule located on the outside of the cell membrane that binds the substance and carries it into the cell. The carrier may be competitively inhibited. Endocytosis takes place when the material to be absorbed, on reaching the cell membrane, is engulfed into the cell interior.

Absorption of food by the small intestine occurs principally in the middle section, or jejunum; however, the duodenum, although the shortest portion of the small intestine, has an extremely important role. The duodenum receives not only chyme saturated with gastric acid but pancreatic and liver secretions as well. It is in the duodenum that the intestinal contents are rendered isotonic with the blood plasma; i.e., the pressures and volumes of the intestinal contents are the same as those of the blood plasma, so that the cells on either side of the barrier will neither gain nor lose water.

Bicarbonate secreted by the pancreas neutralizes the acid secreted by the stomach. This brings the intestinal contents to the optimal pH, allowing the various digestive enzymes to act on their substrates at peak efficiency. A number of important gastrointestinal hormones regulate gastric emptying, gastric secretion, pancreatic secretion, and contraction of the gallbladder. These hormones, along with neural impulses from the autonomic nervous system, provide for autoregulatory mechanisms for normal digestive processes.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Ministry of the public health care

Karaganda State Medical University

Department of the foreign languages

 

 

 

 

Report

The theme: “The alimentary tract”

 

 

 

Made by: Zhubaeva Aida

2076 General Medicine

 

 

 

 

 

 

 

 

Karaganda 2013