Liver Physiology And Anatomy

Liver is the biggest organ and the biggest gland, and among the most essential organs which functions as a hub for waste metabolites excretion and nutrients metabolism. Liver is positioned in the upper part of the right quadrant of the stomach cavity under the right hemidiaphragm. The organ is safeguarded by the rib cage and upholds its position via peritoneal reflection known as ligamentous attachment. These attachments though they are not true ligaments, they are avascular and are the equivalent of the liver visceral peritoneum or in connection to the Glisson capsule . The liver basic purposes it to control safety and flow of food substances taken in from the digestive system prior to the substances distribution to the general circulatory system. It contains two lobes characteristically described by functional anatomy and morphological anatomy. A total liver function loss could results to death within a short time period (minutes), illustrating the great importance of the liver in the body. This paper discusses the physiology and anatomy of the liver.  

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Liver weighs about 1500g and takes about 2.5% of body weight of human adults. The liver surface is dome shaped and smooth, such that it is associated with the diaphragm interior surface concavity. The liver mainly lies in the right upper abdomen quadrant where it is protected and hidden by the diaphragm and thoracic cage. A normal liver is position deep to the 7-11 ribs on the right side and transverses the midline to the nipple on the left side.  Liver is sectioned into four lobes which include quadrate, right, caudate, and left (2).  The left and right lobes are the biggest whiles the quadrate and caudate are smaller and positioned posteriorly. The liver is positioned by mesenteries posteriorly system and is also connected to the diaphragm via falciform ligament. The liver primary functional unit is the lobule. One lobule is approximately sesame seed size and with a shape close to that of hexagonal. The liver lobule basic structures include 3 hepatocytes plates that create the lobule bulk, portal triads at every hexagon corner, liver sinusoids which transverse the portal triads from the central vein, central vein, disse space, kupffer cells or hepatic macrophages and bile canaliculi created between adjacent hepatocytes walls.

The liver contain a number of functions in the body which are best categorized as bile secretion, storage of vitamins and minerals, bilirubin metabolism, metabolic detoxification, hematologic and vascular functions and nutrients metabolism. The liver also plays an essential in nutrients distribution, metabolism and toxic xenobiotics and metabolites detoxification. The liver is an organ that is metabolically active which uses carbohydrates for fatty acids and cholesterol synthesis, and stores glucose and free fatty acids as glycogen and triglycerides (TG) respectively. The liver plays an essential obligation in maintaining levels of blood glucose in fasting by synthesizing amino acid glucose and glucose release from glycogen . The liver makes a great contribution in enhancing the intestinal digestion through secretion of between 700 to 1200 milliliters of bile every day. The bile which is produced by hepatocytes and released into the canaliculi is needed for the absorption and emulsification of fats in the intestine.  After enabling absorption and emulsification of fat, the largest volume of bile salt is absorbed actively in the ileum terminal and transported back to the liver for re-secretion through the portal circulation.

The liver is also involved in the destruction of aged red blood cell to create bilirubin. The macrophages of the system of the mononuclear phagocyte takes up the old red blood cells and destroy them, mostly in the liver and spleen. Hemoglobin is then separated to globin and heme components, where the globin is degraded further to its amino acid constituent which, is then recycled to create new protein. The heme part is converted by iron enzymatic cleavage to biliverdin, where it is attached to transferrin found in the plasma and it is then either utilized in by bone marrow to create fresh red blood cells or stored in the liver. The liver can as well store blood in huge volume due to its widespread vascular network. The quantity stored at any single duration relies on the pressure associations in the veins and arteries. This plays a great part in the liver hematologic and vascular functions such that in hemorrhage event, the liver can maintain circulatory volume of the system by releasing the stored blood. The liver sinusoids kupffer cells remove foreign particles and bacteria from the portal blood, an activity that helps a lot in destroying bacteria in the intestines and hence inhibiting infections . Liver is also enhanced with natural immune cells that comprises of γδT cell at 3-5% frequency in all lymphocytes of the liver. These γδT cells act as a bridge between adaptive and innate immunity since they demonstrates a reorganized T-cell receptor (TCR) which acknowledges specific antigens and can as well release pro-inflammatory cytokines rapidly such as interleukin (IL)-17A after stimulation.   By creating IL-17A to promote adaptive immunity and recruit neutrophils, IL-17A generating γδT cells contain a significant obligation in defense of host over viral, fungal and bacterial infections that include autoimmune, tumor surveillance and stress diseases.  The liver in addition has hemostatic functions where it synthesizes clotting factors, fibrinogen, and prothrombin.

The main liver metabolic functions are lipid and glucose metabolism and drug detozification and xenobiotics. In carbohydrate metabolism, the liver converts fructose and glacatose to glucose through gluconeogenesis. In lipid metabolism, fatty acids are esterificated through glycerol and acetyl-CoA. Fatty acids new lipogenesis from acetyl-CoA is controlled by insulin through sterol controlling element-binding protein-1cc activation that manages the lipogenic enzymes transcription that includes fatty acid synthase. Unlike muscle cells that synthesize protein for personal use, protein synthesized by hepatocytes is meant for the entire body. Thus most of the circulating proteins are created by hepatocytes. The proteins include coagulation factors, immune-associated proteins, and cargo proteins. During protein metabolism, amino groups are transferred by aminotransferases to a recipient molecule from a donor molecule. The α-ketoglutarate and alanine are converted to glutamate and pyruvate through the facilitation of aspartate aminotransferase. Liver is also involved in metabolic detoxification. In this particular role, liver modifies endogenous and exogenous chemicals such as drugs hormones and foreign molecules to make them less biologically active and less toxic. This metabolic detoxification reduces renal tubular and intestinal reabsorption of possibly toxic substances and enables their renal and intestinal excretion. In this manner hormone, alcohol, steroids, barbiturates, and ampheramines are detoxified or metabolized, preventing adverse effects and excessive accumulation. Even though metabolic detoxification is normally protective, the metabolic detoxification products can become toxin after sometime. For instance the alcohol metabolism toxins are hydrogen and acetaldehyde. Excessive alcohol intake over a lengthy period of time makes its end products to destroy the hepatocytes. The alcohol impairs livers functional ability through; destruction of cellular mitochondria by acetaldehyde and promotion of accumulation of fats by hydrogen. Liver is also some other minor roles that include storage of minerals such as copper and iron when taken in excess and release the when needed by the body. It also vitamins that include vitamin A, D, K, E and B12, where by, D and B12 are stored for a number of months while vitamin A is stored for years. Vitamin E and K are stored as ferritin in the liver, which is a complex iron-protein, where it is released from production of red blood cells when needed.

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Liver is also regarded as the chief hematopoietic organ in specific fetal development stages and remains to be a hematopoietic organ after the child birth. This is because liver can generate all linages of leukocytes from hematopoietic resident stem cells. The liver portal tract has a number of different hematopoietic origin cells. In addition, the liver has cells engaged in innate and adaptive immunity. Compared to other organs, liver is specifically rich in the natural immune system cells with the main types of these cells being NK T cells and kupffer cells. NK cells are among the chief INFγ producers in lipopolysaccharide (LPS) response, which relies partly on the NK cells activation by IL-12 initiated by stimulated kupffer cells.

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