Liver

The liver is the largest internal organ, weighing about 1.5 kg (3.3 lb) in the adult. The blood flow to the liver is 1350 ml/min (27% of the cardiac output) on average and comes from two sources:

• Hepatic artery

• Hepatic portal vein

The hepatic artery supplies the liver with 300 ml/min of oxygenated blood from the aorta. The remaining 1050 ml/min of blood flow is delivered by the hepatic portal vein. This blood comes directly from the digestive tract. It is low in oxygen but contains a high concentration of nutrients absorbed from the intestines.

The liver performs many important functions, including:

• Storage of blood

• Filtration of blood

• Storage of vitamins and iron

• Formation of blood coagulation factors

• Metabolism and excretion of certain drugs, bilirubin, and hormones

• Metabolism of carbohydrates, proteins, and lipids

• Formation of bile

The liver is a large and distensible organ. As such, large quantities of blood may be stored in its blood vessels providing a blood reservoir function. Under normal physiological conditions, the hepatic veins and hepatic sinuses contain approximately 450 ml of blood, or almost 10% of blood volume. When needed, this blood may be mobilized to increase venous return and cardiac output.

Blood flowing from the intestines to the liver through the hepatic portal vein often contains bacteria. Filtration of this blood is a protective function provided by the liver. Large phagocytic macrophages, referred to as Kupffer cells, line the hepatic venous sinuses. As the blood flows through these sinuses, bacteria are rapidly taken up and digested by the Kupffer cells. This system is very efficient and removes more than 99% of the bacteria from the hepatic portal blood.

The liver serves as an important storage site for vitamins and iron. Sufficient quantities of several vitamins may be stored so as to prevent vitamin deficiency for some period of time:

Iron is stored in the liver in the form of ferritin. When the level of circulating iron becomes low, ferritin releases iron into the blood.

Several substances that contribute to the blood coagulation process are formed in the liver. These include fibrinogen, prothrombin, and several of the blood clotting factors (II, VII, IX, and X). Deficiency in any of these substances leads to impaired blood coagulation.

The liver is capable of detoxifying or excreting into the bile many drugs, such as sulfonamides (antibacterial drugs), penicillin, ampicillin, and erythromy-cin. Bilirubin, the major end-product of hemoglobin degradation, is also excreted in the bile. In addition, several hormones are metabolized by the liver, including thyroid hormone and all of the steroid hormones, such as estrogen, cortisol, and aldosterone.

In terms of nutrients, the liver is the most important metabolic organ in the body. It receives a large volume of nutrient-rich blood directly from the digestive tract, which provides an abundant amount of substrates for metabolism. Metabolic processes involving carbohydrates include:

• Storage of a significant amount of glycogen

• Conversion of galactose and fructose into glucose

• Gluconeogenesis

Metabolic processes involving proteins include the following:

• Deamination of amino acids

• Formation of urea (for removal of ammonia from body fluids)

• Formation of plasma proteins

• Conversion of amino acids into other amino acids and essential compounds

Most cells in the body metabolize lipids; however, some processes of fat metabolism occur mainly in the liver. These include:

• Oxidation of fatty acids to supply energy for other body functions

• Synthesis of cholesterol, phospholipids, and lipoproteins

• Synthesis of fat from proteins and carbohydrates

Another important product of liver metabolism is the bile necessary for digestion and absorption of dietary lipids. Bile is an aqueous, alkaline fluid consisting of a complex mixture of organic and inorganic components. The major organic constituents of bile are the bile salts, which account for approximately 50% of the solid components. Derived from cholesterol, bile salts are amphipathic molecules; in other words, these molecules have a hydrophilic region and a hydrophobic region. Inorganic ions are also present in the bile and include Na+, K+, Ca++, Cl-, and HCO3- ions. The total number of cations exceeds the total number of anions.

Bile is produced continuously by the liver; bile salts are secreted by the hepatocytes; and the water, sodium bicarbonate, and other inorganic salts are added by the cells of the bile ducts within the liver. The bile is then transported by way of the common bile duct to the duodenum. Bile facilitates fat digestion and absorption throughout the length of the small intestine. In the terminal region of the ileum, the final segment of the small intestine, the bile salts are actively reabsorbed into the blood, returned to the liver by way of the hepatic portal system, and resecreted into the bile. This recycling of the bile salts from the small intestine back to the liver is referred to as enterohepatic circulation.

Bile secretion by the liver is stimulated by:

• Parasympathetic stimulation

The return of the bile salts to the liver from the small intestine is the most potent stimulus of bile secretion. In fact, these bile salts may cycle two to five times during each meal. The intestinal hormone secretin, which is released in response to acid in the duodenum, enhances aqueous alkaline secretion by the liver. Secretin has no effect on the secretion of bile salts. During the cephalic phase of digestion, before food even reaches the stomach or intestine, parasympathetic stimulation, by way of the vagus nerve, promotes bile secretion from the liver.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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