Your liver is the largest organ inside your body, weighing about 1.4 kg (3 pounds) in an average adult. The liver is in the right upper quadrant of the abdominal cavity, just inferior to the diaphragm in the right superior part of the abdominal cavity and under your right ribs just beneath your right lung – filling much of the right hypochondriac and epigastric regions and extending into the left hypochondriac region. The liver is partially surrounded by the ribs, and extends from the level of the fifth intercostal space to the lower margin of the right rib cage, which protects this highly vascular organ from blows that could rupture it. The liver is shaped like a wedge, the wide base of which faces right and the narrow apex of which lies just inferior to the level of the left nipple. The reddish-brown liver is well supplied with blood vessels.
A fibrous capsule encloses the liver, and ligaments divide the organ into a large right lobe and a smaller left lobe (Figure 2).
The liver also has two minor lobes, the quadrate lobe and the caudate lobe. Each lobe is separated into many tiny hepatic lobules, the liver’s functional units (Figure 3). A lobule consists of many hepatic cells radiating outward from a central vein. Blood-filled channels called hepatic sinusoids separate platelike groups of these cells from each other. Blood from the digestive tract, carried in the hepatic portal vein, brings newly absorbed nutrients into the sinusoids and nourishes the hepatic cells.
Large phagocytic macrophages called Kupffer cells are fixed to the inner linings of the hepatic sinusoids. They remove bacteria or other foreign particles that enter the blood through the intestinal wall, and are brought to the liver via the hepatic portal vein. Blood passes from these sinusoids into the central veins of the hepatic lobules and exits the liver via the hepatic veins.
Within the hepatic lobules are many fine bile canaliculi, which carry secretions from hepatic cells to bile ductules. The ductules of neighboring lobules converge to ultimately form the hepatic ducts. These ducts merge, in turn, to form the common hepatic duct.
Figure 1. Location of the human liver
Figure 2. Liver anatomy
Figure 3. Liver lobule
Note: (a) Cross section of a hepatic lobule. (b) Enlarged longitudinal section of a hepatic lobule. (c) Light micrograph of hepatic lobules in cross section.
Amazingly versatile, your liver performs over 500 functions. Its digestive function is to produce bile, a green alkaline liquid that is stored in the gallbladder and secreted into the duodenum. Bile salts emulsify fats in the small intestine; that is, they break up fatty nutrients into tiny particles, just as dish detergent breaks up a pool of fat drippings in a roasting pan. These smaller particles are more accessible to digestive enzymes from the pancreas. The liver also performs many metabolic functions and you cannot live without your liver:
- Picks up glucose from nutrient-rich blood returning from the alimentary canal and stores this carbohydrate as glycogen for subsequent use by the body.
- Processes fats and amino acids and stores certain vitamins.
- Detoxifies many poisons and drugs in the blood.
- Makes the blood proteins.
- It breaks down and stores many of the nutrients absorbed from the intestine that your body needs to function. Some nutrients must be changed (metabolized) in the liver before they can be used for energy or to build and repair body tissues.
- It makes most of the clotting factors that keep you from bleeding too much when you are cut or injured.
- It secretes bile into the intestines to help absorb nutrients (especially fats).
- It breaks down alcohol, drugs, and toxic wastes in the blood, which then pass from the body through urine and stool.
Almost all of these functions are carried out by a type of cell called a hepatocyte or simply a liver cell.
The liver carries on many important metabolic activities. The liver plays a key role in carbohydrate metabolism by helping maintain concentration of blood glucose within the normal range. Liver cells responding to the hormone insulin lower the blood glucose level by polymerizing glucose to glycogen. Liver cells responding to the hormone glucagon raise the blood glucose level by breaking down glycogen to glucose or by converting noncarbohydrates into glucose.
The liver’s effects on lipid metabolism include oxidizing (breaking down) fatty acids at an especially high rate; synthesizing lipoproteins, phospholipids, and cholesterol; and converting excess portions of carbohydrate molecules into fat molecules. The blood transports fats synthesized in the liver to adipose tissue for storage.
Other liver functions concern protein metabolism. They include deaminating amino acids; forming urea; synthesizing plasma proteins such as clotting factors; and converting certain amino acids into other amino acids.
The liver also stores many substances, including glycogen, iron, and vitamins A, D, and B12. In addition, macrophages in the liver help destroy damaged red blood cells and phagocytize foreign antigens. The liver also removes toxic substances such as alcohol and certain drugs from blood (detoxification).
Bile is a yellowish-green liquid continuously secreted from hepatic cells. In addition to water, bile contains bile salts, bile pigments (bilirubin and biliverdin), cholesterol, and electrolytes. Of these, bile salts are the most abundant and are the only bile components that have a digestive function.
Bile pigments are breakdown products of hemoglobin from red blood cells and are normally secreted in the bile.
Jaundice, a yellowing of the skin and mucous membranes due to accumulation of bile pigment, has several causes. In obstructive jaundice bile ducts are blocked, perhaps by gallstones or tumors. In hepatocellular jaundice the liver is diseased, as in cirrhosis or hepatitis. In hemolytic jaundice red blood cells are destroyed too rapidly, as happens with an incompatible blood transfusion or a blood infection.
Regulation of Bile Release
Normally bile does not enter the duodenum until cholecystokinin stimulates the gallbladder to contract. The intestinal mucosa releases this hormone in response to proteins and fats in the small intestine. The hepatopancreatic sphincter usually remains contracted until a peristaltic wave in the duodenal wall approaches it. Then the sphincter relaxes, and bile is squirted into the duodenum.
Functions of Bile Salts
Bile salts aid digestive enzymes. Bile salts affect fat globules (clumped molecules of fats) much like a soap or detergent would affect them. That is, bile salts break fat globules into smaller droplets that are more soluble in water. This action, called emulsification, greatly increases the total surface area of the fatty substance. The resulting fat droplets disperse in water. Fat-splitting enzymes (lipases) can then digest the fat molecules more effectively. Bile salts also enhance absorption of fatty acids, cholesterol, and the fat-soluble vitamins A, D, E, and K.
Low levels of bile salts result in poor lipid absorption and vitamin deficiencies.
The gallbladder is a pear-shaped sac in a depression on the liver’s inferior surface. The gallbladder is lined with epithelial cells and has a strong layer of smooth muscle in its wall. The gallbladder stores bile between meals, reabsorbs water to concentrate bile, and contracts to release bile into the small intestine. It connects to the cystic duct, which in turn joins the common hepatic duct.
The common hepatic duct and cystic duct join to form the bile duct (common bile duct). It leads to the duodenum where the hepatopancreatic sphincter guards its exit. Because this sphincter normally remains contracted, bile collects in the bile duct. It backs up into the cystic duct and flows into the gallbladder, where it is stored.
A liver function important to digestion is bile secretion. Table 1 summarizes the major functions of the liver.
Table 1. Major Functions of the Liver
|Carbohydrate metabolism||Polymerizes glucose to glycogen; breaks down glycogen to glucose; converts noncarbohydrates to glucose|
|Lipid metabolism||Oxidizes fatty acids; synthesizes lipoproteins, phospholipids, and cholesterol; converts excess portions of carbohydrate molecules into fats|
|Protein metabolism||Deaminates amino acids; forms urea; synthesizes plasma proteins; converts certain amino acids into other amino acids|
|Storage||Stores glycogen, iron, and vitamin A, vitamin D and vitamin B12|
|Blood filtering||Removes damaged red blood cells and foreign substances by phagocytosis|
|Detoxification||Removes toxins from blood|
|Secretion||Produces and secretes bile|