Chapter II - Digestive System

STOMACH

In anatomy, the stomach is an organ in the alimentary canal used to digest food. Generally, the stomach's primary function is not the absorption of nutrients from digested food; this task is usually performed by the intestine. The main job of the stomach is to break down large food molecules into smaller ones, so that they can be absorbed into the blood more easily. Latin names for the stomach include Ventriculus and Gaster; many medical terms related to the stomach start in "gastro-" or "gastric". The stomach is a highly acidic environment (maintained by the secretion of hydrochloric acid) with peptidase digestive enzymes. In fact, the stomach's interior can secrete 2 to 3 liters of gastric fluid per day.

Anatomy of the human stomach
The stomach lies between the esophagus and the first part of the small intestine (the duodenum). It is on the left side of the abdominal cavity, the fundus of the stomach lying against the diaphragm. Lying beneath the stomach is the pancreas, and the greater omentum hangs from the greater curvature.

It is divided into five sections, each of which have different cells and functions. The gastric juice, which is in the stomach, has a pH of 1-3. The pH (power of hydrogen) scale goes from 1 to 14; the lower the pH, the more acidic a substance is. Sometimes gastric juice erodes the stomach wall or its layer of mucus, causing an ulcer.

Diseases
Curling ulcer
Cushing ulcer
Stomach cancer
Gastritis
Linitis plastica
Peptic ulcer
Zollinger-Ellison syndrome

PANCREAS
The pancreas is a retroperitoneal organ that serves two functions:
Exocrine - it produces pancreatic juice containing digestive enzymes
Endocrine - it produces several important hormones, namely insulin

Anatomy
The pancreas is a retroperitoneal organ located posterior to the stomach on the posterior abdominal wall.

In humans the pancreas is a small, elongated organ in the abdomen. It is described as having a head, body and tail. The pancreatic head abuts the second part of the duodenum while the tail extends towards the spleen. The pancreatic duct runs the length of the pancreas and empties into the second part of the duodenum at the ampulla of Vater. The common bile duct commonly joins the pancreatic duct at or near this point.

It is supplied arterially by the pancreaticoduodenal arteries, themselves branches of the superior mesenteric artery. Venous drainage is via the pancreaticoduodenal veins, which end up in the portal vein. The splenic vein passes posterior to the pancreas but is said to not drain the pancreas itself. The portal vein is formed by the union of the superior mesenteric vein and splenic vein posterior to the body of the pancreas.

Function
The pancreas produces enzymes that break down all categories of digestible foods (exocrine pancreas) and secretes hormones that affect carbohydrate metabolism (endocrine pancreas).

Diseases of the pancreas
Benign tumors
Carcinoma of pancreas
Cystic fibrosis
Diabetes
Pancreatitis
Acute pancreatitis
Chronic pancreatitis
Pancreatic pseudocyst

GALLBLADDER
The gallbladder (or cholecyst) is a pear-shaped organ that stores bile (or "gall") until the body needs it for digestion. It is connected to the liver and the duodenum by the biliary tract.

Function
The gallbladder stores bile, which is released when food containing fat enters the digestive tract, stimulating the secretion of cholecystokinin (CCK).

Role in disease

Cholestasis
Cholestasis is the blockage in the supply of bile into the digestive tract. It can be "intrahepatic" (the obstruction is in the liver) or "extrahepatic" (outside the liver). It can lead to jaundice, and is identified by the presence of elevated bilirubin level that is mainly conjugated.

Cholelithiasis
Up to 25% of all people have gallstones (cholelithiasis), composed of cholesterol, lecithin and bile acids. These can cause colicky shooting abdominal pain, usually in relation with the meal, as the gallbladder contracts and gallstones pass through the bile duct. Surgery (cholecystectomy, removal of the gallbladder) is the most common treatment for gallstones. It can be performed laparoscopically, and it is in fact one of the most common procedures done through the laparoscope.

People traditionally considered at an increased risk of cholelithiasis are people who are 5 F's:

Female
Fat (obesity)
Fair (Caucasian, but this is disputed by recent studies)
Forty (middle-aged)
Fertile (the risk is increased in pregnancy)

Cholecystitis
Acute or chronic inflammation of the gall bladder causes abdominal pain. 90% of cases of acute cholecystitis are caused by the presence of gallstones.

Choledocholithiasis
When gallstones obstruct the common bile duct, the patient develops jaundice and liver cell damage. It is a medical emergency, requiring endoscopic or surgical treatment.

Gallstone ileus
A rare clinical entity is ileus (bowel obstruction) by a large gallstone, or gallstone ileus. This condition develops in patients with longstanding gallstone disease, in which the gallbladder forms a fistula with the digestive tract. Large stones pass into the bowel, and generally block the gut at the level of Treitz' ligament or the ileocecal valve, two narrow points in the digestive tract. Therapy is surgical.

LIVER
The liver is an organ in vertebrates, including humans. It plays a major role in metabolism and has a number of functions in the body including drug detoxification, glycogen storage, and plasma protein synthesis. It also produces bile, which is important for digestion. Medical terms related to the liver often start in hepato- or hepatic from the Greek word for liver, hepar.

Anatomy
The adult human liver normally weighs between 1.0 - 2.5 kilograms, and is a soft, pinkish-brown "boomerang shaped" organ. It is the second largest organ (the largest organ being the skin) Its anatomical postion in the body is : immediately under the diaphragm on the right side of the upper abdomen. The liver lies on the right of the stomach and makes a kind of bed for the gallbladder (which stores bile).

The liver is supplied by two major blood vessels: the hepatic artery and the portal vein. The hepatic artery normally comes off the celiac trunk. The portal vein brings venous blood from the spleen, pancreas, and small intestines, so that the liver can process the nutrients and byproducts of food digestion. The hepatic veins drain directly into the inferior vena cava.

The bile produced in the liver is collected in bile canaliculi, which merge to form bile ducts. These eventually drain into the right and left hepatic ducts, which in turn merge to form the common hepatic duct. The cystic duct (from the gallbladder) joins with the common hepatic duct to form the common bile duct. Bile can either drain directly into the duodenum via the common bile duct or be temporarily stored in the gallbladder via the cystic duct. The common bile duct and the pancreatic duct enter the duodenum together at the ampulla of Vater. The branchings of the bile ducts resemble those of a tree, and indeed the term "biliary tree" is commonly used in this setting.

The liver is one of the only internal human organs capable of natural regeneration of lost tissue; as little as 25% of remaining liver can regenerate into a whole liver again. This is predominantly due to the hepatocytes acting as unipotential stem cells. There is also some evidence of bipotential stem cells, called oval cells, which can differentiate into either hepatocytes or cholangiocytes (cells that line the bile ducts).

Physiology
The various functions of the liver are carried out by the liver cells or hepatocytes.

• The liver produces and excretes bile required for food digestion. Some of the bile drains directly into the duodenum, and some is stored in the gallbladder.
• The liver performs several roles in carbohydrate metabolism:
• Gluconeogenesis (the formation of glucose from certain amino acids, lactate or glycerol)
• Glycogenolysis (the formation of glucose from glycogen)
• Glycogenesis (the formation of glycogen from glucose)
• The breakdown of insulin and other hormones
• The liver also performs several roles in lipid metabolism:
• Cholesterol synthesis
• The production of triglycerides (fats).
• The liver produces coagulation factors I (fibrinogen), II (prothrombin), V, VII, IX, and XI, as well as protein C, protein S and antithrombin.
• The liver breaks down hemoglobin (bile pigments are its metabolites), toxic substances and most medicinal products. This sometimes results in toxication, when the metabolite is more toxic than its precursor.
• The liver converts ammonia to urea.
• The liver stores a multitude of substances, including glucose in the form of glycogen, vitamin B12, iron, and copper.
• In the first trimester fetus, the liver is the main site of red blood cell production. By the 42nd week of gestation, the bone marrow has almost completely taken over that task.
• Producing an artificial organ or device capable of emulating all functions of the liver is outside the reach of science in the foreseeable future. Some functions can be emulated by liver dialysis, an experimental treatment for liver failure.

Diseases of the liver
Many diseases of the liver are accompanied by jaundice caused by increased levels of bilirubin in the system. The bilirubin results from the breakup of the hemoglobin of dead red blood cells; normally, the liver removes bilirubin from the blood and excretes it through bile.

• Hepatitis, inflammation of the liver, caused mainly by various viruses but also by some poisons, autoimmunity or hereditary conditions.
• Cirrhosis is the formation of fibrous tissue in the liver, replacing dead liver cells. The death of the liver cells can for example be caused by viral hepatitis, alcoholism or contact with other liver-toxic chemicals
• Hemochromatosis, a hereditary disease causing the accumulation of iron in the body, eventually leading to liver damage
• Cancer of the liver (primary hepatocellular carcinoma or cholangiocarcinoma and metastatic cancers, usually from other parts of the gastrointestinal tract)
• Wilson's disease, a hereditary disease which causes the body to retain copper
• Primary sclerosing cholangitis, an inflammatory disease of the bile duct, autoimmune in nature.
• Primary biliary cirrhosis, autoimmune disease of small bile ducts
• Budd-Chiari syndrome, obstruction of the hepatic vein.

A number of liver function tests are available to test the proper function of the liver. These are enzymes that are most abundant in liver tissue, metabolites or products.