반응형
Accumulation of fatty acids (fatty infiltration of liver)
• Usually asymptomatic among heavy drinkers
• May cause hepatomegaly on exam
• Abnormal LFTs (AST>ALT)
• Often reversible with cessation of alcohol
• ↑ risk of cirrhosis
NTRODUCTION TO LIVERThe liver is one of the largest, most important, and least appreciated organs in the body. The bulk of the liver consists of hepatocytes, which are epithelial cells with a unique configuration.The liver is essentially an exocrine gland, secreting bile into the intestine. But, the liver is also -- and significantly so -- an endocrine gland and a blood filter. The liver has a diversity of functions not typically associated with glands. The liver is a metabolic factory, synthesizing and breaking down a variety of substances. It's functions include all of the following:
- formation and secretion of bile.
- storage of glycogen, buffer for blood glucose.
- synthesis of urea.
- metabolism of cholesterol and fat.
- synthesis and endocrine secretion of many plasma proteins, including clotting factors.
- detoxification of many drugs and other poisons.
- cleansing of bacteria from blood.
- processing of several steroid hormones and vitamin D.
- volume reservoir for blood.
- catabolism of hemoglobin from worn-out red blood cells.
Much of the liver's organization is conditioned by its central role in removing unwanted materials from blood and otherwise maintaining the blood's normal composition.
- The hepatic portal vein brings to the liver all of the blood which has previously passed through the intestine and spleen.
- The hepatic artery brings fresh, oxygenated blood from the aorta.
Portal venous blood from the intestine and spleen and arterial blood from the aorta mix together in hepatic sinusoids before leaving the liver in the hepatic vein.The liver receives over 25% of the total resting cardiac output and is responsible for over 20% of the body's resting oxygen consumption.The liver is organized into lobules which take the shape of polygonal prisms Each lobule is typically hexagonal in cross section and is centered on a branch of the hepatic vein (called, logically enough, the central vein).The fenestrated endothelium lining the sinusoids lies immediately adjacent to the cords, with no basement membrane and practically no intervening connective tissue, so that each hepatocyte is bathed on two faces by blood plasma.Although the liver is an exocrine gland (with bile duct leading to the intestine), the cells of the liver are NOT organized into secretory acini or tubules. Instead, hepatocytes form linear cords, within which a network of bile canaliculi provides passage through intercellular channels to the nearest branches of the bile duct.This remarkable tissue arrangement appears to optimize the liver's several roles as exocrine gland, endocrine gland, and blood filter.
Dual vascular supply.
All of the blood which passes through the intestine and spleen is delivered to the liver by the hepatic portal vein.This portal blood carries not only nutrients but also various contaminants (drugs, toxins from food, bacteria, byproducts of blood-cell recycling) which have been absorbed through the intestinal mucosa or produced in the spleen.The liver also receives arterial blood, carrying oxygen, from the hepatic artery.Blood from both portal vein and hepatic artery mixes together in the hepatic sinusoids and then passes out of the liver through the hepatic vein. The parenchyma of each lobule can be divided into arbitrary zones based on oxygen supply (see WebPath), with the central zone (closest the the central vein) poorest in oxygen.This blood flow pattern can yield visible differences in hepatocyte appearance. For example, glycogen is depleted first from the periphery during times of fasting and deposited first in the periphery during times of feasting. This leads to a pattern of glycogen which can reflect recent nutritional history.
Both the hepatic portal vein and the hepatic artery branch in parallel along the corners of hepatic lobules, in regions called portal areas.
Organization of Liver Lobules
The liver is organized into lobules (portal lobules, hepatic lobules) which take the shape of irregular polygonal prisms. Some texts refer instead to liver acini; see below.At the corners between adjacent lobules are the so-called portal areas(portal canals, portal triads). These are regions of connective tissue which include branches of the bile duct, the portal vein, and the hepatic artery.Lobules appear quite clearly in pig liver, which has an envelope of fibrous connective tissue around each lobule. (This tough connective tissue is one reason why pig liver, unlike calf liver or chicken liver, is not a popular menu item.)
The lobular organization of the human liver is not immediately evident under the microscope. Lobules do not have distinct boundaries, and they are seldom cut neatly in cross section.
To visualize lobules, first locate several portal areas, small patches of connective tissue each containing a duct, a large vein, and a small artery which mark the corners where lobules come together. The portal areas represent the stroma of the liver.
Then look for central veins -- conspicuous spaces, with no associated connective tissue, located roughly midway between portal areas. These central veins mark the centers of lobules.
Some texts refer to liver acini rather than liver lobules. These are both arbitrary ways to conceptualize the same tissue.
- As described above, a liver lobule encompasses the liver tissue that is served by single branch of a central vein (which is a branch of the hepatic vein). The lobule is typically hexagonal in cross section, with a central vein at its center and portal areas at its peripheral corners.
- In contrast, a liver acinus encompasses the liver tissue that is served by a single terminal branch of the hepatic artery. These small vessels extend out from portal areas, along the boundaries between adjacent lobules. An acinus is typically diamond-shaped in cross section, with a hepatic arteriole crossing the center and with central veins at the two opposite corners. The acinus includes triangular portions of two adjacent lobules.
- There is no fundamental difference between these two concepts; they simply focus attention on different aspects of liver vasculature.
|
(The image should be animated, if you watch patiently.)
There are other ways of dividing the parenchyma of the liver into units. Two common ways are divisions into portal lobules and liver acini. Portal lobules emphasize the afferent blood supply and bile drainage by the vessels of the portal triads. The secretory function of the liver is emphasized by the term liver acinus. Acini are smaller units than portal or "classical" liver lobules and relate structural units to terminal branches formed by the vessels of the portal triad but not the portal triad itself. [In the animation of the acinus, the green arrows represent bile drainage.] Representations of portal lobules and liver acini vary in different textbooks.
|
반응형