Medical brain notes 44

  Physiologic Anatomy of the Liver The liver is the largest organ in the body, contributing about 2 per cent of the total body weight, or about 1.5 kg in the average adult human. The basic functional unit of the liver is the  liver lobule,  which is a cylindrical structure several millimeters in length and 0.8 to 2 millimeters in diameter. The human liver contains 50,000 to 100,000 individual lobules.         The liver lobule, shown in cut-away format in Figure 70–1, is constructed around a  central vein  that empties into the hepatic veins and then into the vena cava. The lobule itself is composed principally of many liver  cellular plates  (two of which are shown in Figure 70–1) that radiate from the central vein like spokes in a wheel. Each hepatic plate is usually two cells thick, and between the adjacent cells lie small  bilecanaliculi  that empty into  bile ducts  in the fibrous septa separat...

  Blood Flows Through the Liver from the Portal Vein and Hepatic Artery The Liver Has High Blood Flow and Low Vascular Resistance. About 1050 milliliters of blood flows from the portal vein into the liver sinusoids each minute, and an addi-tional 300 milliliters flows into the sinusoids from the hepatic artery, the total averaging about 1350 ml/min. This amounts to 27 per cent of the resting cardiac output. The pressure in the portal vein leading into the liver averages about 9 mm Hg, and the pressure in the hepatic vein leading from the liver into the vena cava normally averages almost exactly 0 mm Hg. This small pressure difference, only 9 mm Hg, shows that the resist-ance to blood flow through the hepatic sinusoids is nor-mally very low, especially when one considers that about 1350 milliliters of blood flows by this route each minute. Cirrhosis of the Liver Greatly Increases Resistance to Blood Flow. When liver parenchymal cells are destroyed, they are replaced with fibrous tis...

  Metabolic Functions of the Liver The liver is a large, chemically reactant pool of cells that have a high rate of metabolism, sharing substrates and energy from one metabolic system to another, process-ing and synthesizing multiple substances that are trans-ported to other areas of the body, and performing myriad other metabolic functions. For these reasons, a major share of the entire discipline of biochemistry is devoted to the metabolic reactions in the liver. But here, let us summarize those metabolic functions that are especially important in understanding the integrated physiology of the body. Carbohydrate Metabolism In carbohydrate metabolism, the liver performs the fol-lowing functions: 1. Storage of large amounts of glycogen 2. Conversion of galactose and fructose to glucose 3. Gluconeogenesis 4. Formation of many chemical compounds from intermediate products of carbohydrate metabolism The liver is especially important for maintaining a normal blood glucose concentration...

  Measurement of Bilirubin in the Bile as a Clinical Diagnostic Tool The formation of bile by the liver and the function of the bile salts in the digestive and absorptive processes of the intestinal. In addition, many substances are excreted in the bile and then eliminated in the feces. One of these is the greenish yellow pigment bilirubin.  This is a major end product of hemoglobin degradation. However, it also provides  an exceedinglyvaluable tool for diagnosing both hemolytic blood  diseases and various types of liver diseases.  Therefore,while referring to Figure 70–2, let us explain this. Briefly, when the red blood cells have lived out their life span (on average, 120 days) and have become too fragile to exist in the circulatory system, their cell membranes rupture, and the released hemoglobin is phagocytized by tissue macrophages (also called the reticuloendothelial system) throughout the body. The hemoglobin is first split into  globin  and...

  Jaundice-Excess Bilirubin in the Extracellular Fluid Jaundice  refers to a yellowish tint to the body tissues,including a yellowness of the skin as well as the deep tissues. The usual cause of jaundice is large quantities of bilirubin in the extracellular fluids, either free bilirubin or conjugated bilirubin. The normal plasma concentra-tion of bilirubin, which is almost entirely the free form, averages 0.5 mg/dl of plasma. In certain abnormal con-ditions, this can rise to as high as 40 mg/dl, and much of it can become the conjugated type. The skin usually begins to appear jaundiced when the concentration rises to about three times normal—that is, above 1.5 mg/dl. The common causes of jaundice are (1) increased destruction of red blood cells, with rapid release of bilirubin into the blood, and (2) obstruction of the bile ducts or damage to the liver cells so that even the usual amounts of bilirubin cannot be excreted into the gas-trointestinal tract. These two types of jaund...