The salivary glands, the pancreas, liver, and gallbladder are accessory organs of digestion. Ducts transport pancreatic juices from the pancreas and bile from the liver and gallbladder to the duodenum. The small intestine helps regulate the release of these juices by secreting hormones.
Pancreas
The pancreas lies deep in the peritoneal cavity, resting on the posterior abdominal wall. It is an elongated and somewhat flattened organ that has both an endocrine function and an exocrine function. The exocrine function of the pancreas is discussed here. Most of the pancreatic cells produce pancreatic juice, which contains sodium bicarbonate (NaHC03), a chemical that neutralizes the acidic pH of the chyme, and digestive enzymes for carbohydrates, proteins, and fats. Pancreatic amylase digests starch, trypsin and chymotrypsin digest protein, and lipase digests fat. In other words, the pancreas secretes enzymes for the digestion of all major types of food. These enzymes travel by way of the pancreatic duct to the duodenum of the small intestine. Any blockage that prevents pancreatic juice from entering the duodenum may result in a serious condition called acute pancreatitis. In this disorder, pancreatic juice backs up into the pancreatic duct, causing digestion of the pancreatic tissue itself.
The pancreas produces pancreatic juice which contains bicarbonate and digestive enzymes for carbohydrates. proteins. and fats.
Liver
The liver, which is the largest gland in the body, lies mainly in the right upper quadrant of the peritoneal cavity, under the diaphragm. There are two main lobes, the right lobe and the smaller left lobe, which crosses the midline and lies above the stomach. These two lobes are separated by the falciform ligament, which secures the liver to the anterior abdominal wall and the diaphragm. The liver contains approximately 100,000 lobules that serve as the structural and functional units of the liver. Triads consisting of the following structures are located between the lobules: (1) a branch of the hepatic artery, which brings oxygenated blood to the liver; (2) a branch of the hepatic portal vein, which brings nutrients from the intestines; and (3) a bile duct, which takes bile away from the liver. The central veins within the lobules enter the hepatic vein. The liver lies between the hepatic portal vein and the hepatic vein.
In some ways, the liver acts as the gatekeeper to the blood. As blood from the intestines passes through the liver, the liver removes poisonous substances and works to keep the contents of the blood constant. It also removes and stores iron and the fat-soluble vitamins A, D, E, and K. The liver makes the plasma proteins from amino acids, and these have important functions within the blood itself.
The liver under the influence of the pancreatic hormones insulin and glucagon maintains the blood glucose level at about 100 mg/100 ml + 0.1 %, even though a person eats intermittently. Any excess glucose in the hepatic portal vein is removed and stored by the liver as glycogen. Between eating periods, glycogen is broken down to glucose, which enters the hepatic vein; in this way, the glucose content of the blood remains constant.
If, by chance, the supply of glycogen or glucose is depleted, the liver converts amino acids to glucose molecules. In this process, ammonia is given off and is converted to urea, the common nitrogenous waste product of humans. After its formation in the liver, urea is transported to th kidneys for excretion.
The liver produces bile. Bile is a yellowish green fluid because it contains the pigments bilirubin and biliverdin, which result from the breakdown of hemoglobin-the pigment found in red blood cells. Bile also contains bile salts that emulsify fats once bile reaches the duodenum of the small intestine. When fats are emulsified, they break down into droplets that can be acted upon by a digestive enzyme called lipase from the pancreas. Emulsification is a process that can be witnessed by adding oil to water in a test tube. The oil has no tendency to mix with the water, but if a liquid detergent is added and the contents of the tube are shaken, the oil breaks down and disperses into the water.
The following are significant functions of the liver:
- Storage of glucose as glycogen after eating and the breakdown of glycogen to glucose between meals to maintain a constant blood glucose level.
- Production of urea from the breakdown of amino acids.
- Destruction of old red blood cells and conversion of hemoglobin to the breakdown products in bile (bilirubin and biliverdin).
- Production of bile, which is stored in the gallbladder before entering the small intestine, where it emulsifies fats.
- Production of the plasma proteins.
- Detoxification of the blood by removing poisonous substances and metabolizing them.
- In addition, the liver is involved in the storage of iron and the fat-soluble vitamins A, D, E, and K.
Serious Liver Disorders Jaundice, hepatitis, and cirrhosis are three life-threatening diseases that affect the entire liver and hinder its ability to repair itself. A person with jaundice has a yellowish tint to the whites of the eyes and lightly pigmented skin. Bilirubin has been deposited in the skin, due to an abnormally large amount in the blood. In hemolytic jaundice, red blood cells are broken down in abnormally large amounts; in obstructive jaundice, a bile duct is blocked, or the liver cells are damaged. Obstructive jaundice often occurs when gallstones block the common bile duct.
Jaundice can also result from hepatitis, inflammation of the liver, as discussed in the Medical Focus reading on this page.
Cirrhosis (sah-ro' sis) is a chronic disease of the liver in which the organ first becomes fatty. Liver tissue is then replaced by inactive fibrous scar tissue. In alcoholics, who often develop cirrhosis of the liver, the condition most likely is caused by the excessive amounts of alcohol (a toxin) the liver is forced to break down.
The liver has amazing regenerative powers and can recover if the rate of regeneration exceeds the rate of damage. During liver failure, however, there may not be enough time for the liver to heal itself. Liver transplantation is usually the preferred treatment, but artificial livers have been developed and tried in a few cases. One type consists of a cartridge containing cloned liver cells. As the patient's blood passes through cellulose acetate tubing, it is serviced in the same manner as with a normal liver. In the meantime, the patient's liver has a chance to recover.
The liver is a very critical organ, vvith numerous important fUnctions. and any malfunction is a matter of considerable Ooncem. The liver receives blood from the small intestine by way of the hepatic portal vein.
Gallbladder
The gallbladder is a pear-shaped, muscular sac attached to the ventral surface of the liver. The liver produces bile, which enters the many bile ducts associated with hepatic lobules. These bile ducts join to form the common bile duct that enters the duodenum. Any excess bile backs up through the cystic duct into the gallbladder, where it is stored.
Bile, which contains bile salts, bile pigments, cholesterol and electrolytes, becomes concentrated in the gallbladder as water is reabsorbed. Normally cholesterol stays in solution, but sometimes it may come out of solution and form crystals. This may happen if the liver secretes too much cholesterol and/or the bile becomes too concentrated in the gallbladder. The crystals can become larger and larger until they form gallstones. As mentioned, if the gallstones leave the gallbladder and block the common bile duct, obstructive jaundice may occur. Gallbladder disease can be very painful and accompanied by nausea and vomiting. In such cases, the gallbladder can be surgically removed. Today, it is possible to use a laser that results in only four tiny cuts, each a quarter to half inch long.
Liver produces bile which is stored in the gallbladder. Digestive enzymes and bile enter the duodenum via ducts.
Large Intestine
The large intestine, which includes the cecum, colon, rectum, and anal canal, is larger in diameter than the small intestine (6.5 centimeters compared to 2.5 centimeters). It begins in the lower right quadrant of the peritoneal cavity. The cecum (se'kum), which lies inferior to this point, has a small projection called the vermiform appendix (vermiform means wormlike). Superior to the cecum, the large intestine is termed the ascending colon. At the level of the liver, the large intestine bends sharply and becomes the transverse colon. At the left abdominal wall, the large intestine bends again to become the descending colon. In the pelvic region, the large intestine turns medially to form an S-shaped bend known as the sigmoid colon. The last 20 centimeters of the large intestine, the rectum, ends in the anal canal, which opens at the anus.
The large intestine absorbs water and electrolytes. It also prepares and stores nondigestible material (feces) for defecation at the anus. In addition to nondigestible remains, feces also contain bile pigments, which give them color, and large quantities of bacteria, particularly Escherichia coli.
The E. coli live off any substances that were not digested earlier. When they break down this material, they
emit odorous molecules that cause the characteristic fecal odor. Some of the vitamins (vitamin K and some B complex vitamins), amino acids, and other growth factors produced by these bacteria are absorbed by the intestinal lining. In this way, E. coli and other bacteria perform a service for the human body.
The large intestine does not digest food. It absorbs vvater and some electrolytes and stores nondigestible material prior to defecation.
Diarrhea and Constipation
Two common everyday complaints associated with the large intestine are diarrhea and constipation. In diarrhea, too little water has been absorbed; in constipation, too much water has been absorbed.
The major causes of diarrhea are infection of the lower tract and nervous stimulation. In the case of infection, such as food poisoning caused by eating contaminated food, the intestinal wall becomes irritated, and peristalsis increases. Lack of water absorption is a protective measure, and the diarrhea that results serves to rid the body of the infectious organisms. When a person is under stress, the nervous system sometimes stimulates the intestinal wall, resulting in diarrhea. Loss of water due to diarrhea may lead to dehydration, a serious condition in which the body tissues lose their normal water content.
When a person is constipated, the feces are dry and hard.
Diarrhea occurs vvhen too little vvater has been absorbed in the large intestine; constipation occurs vvhen too much vvater has been absorbed.
Disorders of the Large Intestine
The appendix is a fingerlike projection from the cecum of the large intestine. Unfortunately, the appendix can become infected, resulting in appendicitis, a very painful condition in which the fluid content of the appendix can increase to the point that it bursts. The appendix should be removed before it bursts to avoid a generalized infection of the peritoneal membrane of the abdominal cavity.
Diverticulosis (di"ver-tI-kyu-lo'sis) is characterized by the presence of diverticula, or saclike pouches, of the colon. Ordinarily, these pouches cause no problems. But about 15% of people with diverticulosis develop an inflammation known as diverticulitis. The symptoms of diverticulitis are similar to those of appendicitis-cramps or steady pain with local tenderness. Fever, loss of appetite, nausea, and vomiting may also occur. Today, high-fiber diets are recommended to prevent the development of these conditions and of cancer of the colon.
The colon is subject to the development of polyps, small growths that generally appear on epithelial tissue, such as the epithelial tissue that lines the digestive tract. Whether polyps are benign or cancerous, they can be removed individually, along with a portion of the colon, if necessary. If the last portion of the rectum and the anal canal must be removed, then the intestine is sometimes attached to the abdominal wall through a procedure known as a colostomy (kah-los'tah-me), and the digestive remains are collected in a plastic bag fastened around the opening. Recently, the use of metal staples has permitted surgeons to join the colon to a piece of rectum that formerly was considered too short.
