Liver – Alimentary Canal
The liver is in the upper right quadrant of the abdominal cavity, just inferior to the diaphragm. It is partially
surrounded by the ribs, and extends from the level of the fifth intercostal space to the lower margin of the
ribs. The reddish-brown liver is well supplied with blood vessels.
A fibrous capsule encloses the liver, and connective tissue divides the organ into a large right lobe and a smaller left lobe. Each lobe is separated into many tiny hepatic lobules (he˘-pat′ik lob′ulz), the liver’s functional units . A lobule consists of many hepatic cells radiating outward from a central vein. Vascular channels called hepatic sinusoids separate platelike groups of these cells from each other. Blood from the digestive tract, which is carried in the hepatic portal vein , brings newly absorbed nutrients into the sinusoids and nourishes the hepatic cells.
Large phagocytic macrophages called Kupffer cells (koop′fer selz) are fixed to the inner linings of the hepatic sinusoids. They remove bacteria or other foreign particles that enter the blood in the portal vein through the intestinal wall. Blood passes from these sinusoids into the central veins of the hepatic lobules and exits the liver via the hepatic vein.
Within the hepatic lobules are many fine bile canaliculi, which carry secretions from hepatic cells to bile ductules. The ductules of neighboring lobules unite to form larger bile ducts which then converge
to become the hepatic ducts. These ducts merge, in turn, to form the common hepatic duct.
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 hormones such as insulin and glucagon lower the blood glucose level by polymerizing glucose to glycogen, and raise the blood glucose level by breaking down glycogen to glucose or by converting non-carbohydrates into glucose.
The liver’s effects on lipid metabolism include oxidizing fatty acids at an especially high rate; synthesizing lipoproteins, phospholipids, and cholesterol; and converting parts of carbohydrate and protein molecules
into fat molecules (see p. 423). The blood transports fats synthesized in the liver to adipose tissue for storage.
The most vital liver functions concern protein metabolism. They include deaminating amino acids; forming urea (see p. 431); synthesizing plasma proteins, such as clotting factors (see chapter 12, pp. 328–330); 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 (see chapter 12, pp. 323–324) and phagocytize foreign antigens. The liver also removes toxic substances such as alcohol and certain other drugs from blood (detoxification) and secretes bile.
Many of these liver functions are not directly related to the digestive system and are, as indicated above, discussed in other chapters. Bile secretion, however, is important to digestion and is explained next in this chapter. Table 15.3 summarizes the major functions of the liver. Clinical Application 15.2 discusses viral infections of the liver.
Composition of Bile
Bile (bı¯l) 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 substances that have a digestive function. Bile pigments are breakdown products of hemoglobin from red blood cells and are normally excreted in the bile (see chapter 12, pp. 323–324).
The gallbladder (gawl′blad-er) is a pear-shaped sac in a depression on the liver’s inferior surface. It connects to the cystic duct (sis′tik dukt), which in turn joins the common hepatic duct (figs. 15.1 and 15.19). The gallbladder is lined with epithelial cells and has a strong, muscular layer in its wall. The gallbladder stores bile between meals, reabsorbs water to concentrate bile, and contracts to release bile into the small intestine.
The common hepatic and cystic ducts join to form the bile duct. It leads to the duodenum (the proximal part of the small intestine) (figs. 15.14 and 15.19), where the hepatopancreatic sphincter guards its exit. This sphincter normally remains contracted, so as bile collects in the bile duct it backs up into the cystic duct. When this happens, bile flows into the gallbladder, where it is stored.