The liver is the largest gland in the body. It weighs about 1.4 kg (3 lb) and is dark reddish brown in color. The liver is located mostly in the right upper quadrant of the abdominopelvic cavity just inferior to the diaphragm, where it is protected by the inferior ribs.
The liver has many important and vital functions, though most are not associated with digestion.
(1) The liver produces and secretes bile, a substance that aids in the digestion and absorption of lipids, and heparin, a blood anticoagulant. It also produces and secretes plasma proteins.
(2) The liver plays a critical role in carbohydrate metabolism. When blood glucose levels are elevated, the liver can convert and store the excess as glycogen or triglycerides. When blood glucose levels are low, the liver can convert glycogen, glycerol, fatty acids, and amino acids into glucose.
(3) As part of lipid metabolism, the liver forms lipoproteins for the transport of fatty acids, triglycerides, and cholesterol. It also can synthesize cholesterol and use it to form bile salts.
(4) As a part of protein metabolism, the liver removes the amine groups from amino acids so that the remainder of the molecules can be used in cellular respiration or in forming glucose and triglycerides.
(5) The liver is used for the storage of triglycerides, glycogen, iron, and vitamins A, D, E, K, and B12.
(6) It detoxifies the blood by modifying many drugs and toxic chemicals to form less toxic compounds.
(7) Using phagocytosis, the liver removes worn-out blood cells and any bacteria present.
The liver is encased in a dense irregular connective tissue capsule that, in turn, is covered by the peritoneum for additional support. A ligament of dense regular connective tissue, called the falciform ligament, joins the liver to the diaphragm and the anterior abdominal wall and separates the two main lobes: a larger right lobe and a smaller left lobe. Several blood vessels and the common hepatic duct enter or exit the liver from a small area on the posterior surface of the liver.
The liver receives blood from two sources. The hepatic artery proper brings oxygenated blood to the hepatocytes (liver cells). The hepatic portal vein brings deox- ygenated, nutrient-rich blood from the digestive tract. As blood flows through the liver, hepatocytes remove, modify, or add substances to the blood before it leaves the liver via the hepatic veins.
Microscopically, the liver consists of multitudes of hepatic lobules, which serve as the structural and functional units. Each lobule is a short, roughly hexagonal cylinder with a central vein running through its core from which thin sheets of hepatocytes radiate. Hepatic triads, located at the corners where several lobules meet, are composed of three vessels: a branch of the hepatic artery proper, a branch of the hepatic portal vein, and a small interlobular bile ductule carrying bile. Between the sheets of hepatocytes are hepatic sinusoids, blood-filled spaces that carry blood from the hepatic artery proper and hepatic portal vein to the central vein of the lobule. As blood flows through the hepatic sinusoids, an exchange of materials occurs between the blood and the hepato- cytes. Macrophages in the epithelium lining the sinusoids remove cellular debris and bacteria. The central veins of the lobules ultimately merge to form the hepatic veins.
As noted, the production of bile is the only digestive function of the liver. Bile is collected in tiny ducts that merge to form the interlobular ductules of the hepatic triads, which in turn unite to form the right and ieft hepatic ducts exiting the right and left lobes of the liver. The right and left hepatic ducts merge to form the common hepatic duct, which carries bile out of the liver. The common hepatic duct and the cystic duct, a short duct that extends from the gallbladder, merge to form the bile duct, which carries bile to the duodenum. The cystic duct carries bile to and from the gallbladder, a small, pear-shaped sac that stores bile temporarily between meals.
Hepatocytes continuously produce bile, a yellowish green liquid. Bile consists of water, bile salts, bile pigments, cholesterol, and minerals. Bile pigments, such as yellow-colored bilirubin, are waste products of hemoglobin breakdown that are excreted through bile. Jaundice is a medical condition in which too much bilirubin is circulating in the blood due to liver or kidney malfunction or excessive red blood cell destruction. The excess bilirubin ends up being deposited within the skin, cornea, and mucous membranes, causing yellow discoloration.
Bile salts are the only bile components that play a digestive role. When in contact with fatty substances, they break up large fat globules into very small droplets, a process called emulsification (e-mul-si-fi-ka’-shun). Emulsification greatly increases the surface area of the fats exposed to water and lipases. In this way, bile salts aid the digestion of fats. Bile salts also aid the absorption of fatty acids, cholesterol, and lipid-soluble vitamins by the small intestine.
Bile normally enters the duodenum only when chyme is present. When the intestine is empty, the hepatopancreatic sphincter at the base of the bile duct constricts, which forces bile to enter the gallbladder for temporary storage.
When lipid-rich chyme enters the duodenum, it stimulates the release of cholecystokinin from the i ntestinal mucosa. CCK is carried by the blood to the gallbladder, where it stimulates contraction of muscles in the gallbladder wall. The contractions eject bile from the gallbladder into the bile duct. CCK also relaxes the hepatopancreatic sphincter so bile is injected into the small intestine. Note that this hormonal control releases bile only when it is needed in the small intestine.