Anatomy and Physiology
Human liver development begins during the third week of gestation and does not achieve mature architecture until about 15 years of age. It reaches its largest relative size, 10% of fetal weight, around the ninth week. It is about 5% of body weight in the healthy neonate. The liver is about 2% of body weight in the adult. It weighs around 1400g in an adult female and about 1800g in the male. The liver's main job is to filter the blood coming from the digestive tract, before passing it to the rest of the body. The liver also detoxifies chemicals and metabolizes drugs. As it does so, the liver secretes bile that ends up back in the intestines. The liver also makes proteins important for blood clotting and other functions. The liver is located in the right upper quadrant of the abdomen, just below the diaphragm. It is almost completely behind the rib cage but the lower edge may be palpated along the right costal margin during inspiration. The liver lies to the right of the stomach and overlies the gallbladder. It is connected to two large blood vessels, one called the hepatic artery and one called the portal vein. The hepatic artery carries blood from the aorta, whereas the portal vein carries blood containing digested nutrients from the entire gastrointestinal tract and also from the spleen and pancreas. These blood vessels subdivide into capillaries, which then lead to a lobule. Each lobule is made up of millions of hepatic cells which are the basic metabolic cells. Lobules are the functional units of the liver. The falciform ligament attaches the liver to the abdominal wall and diaphragm and divides the liver into a larger right lobe and a smaller left lobe. Lobules consist of hepatocytes and the spaces between them. Sinusoids are the spaces between the plates of hepatocytes. Sinusoids receive blood from the portal triads. About 25% of total cardiac output enters the sinusoids via terminal portal and arterial vessels. Seventy-five percent of the blood flowing into the liver comes through the portal vein; the remaining 25% is oxygenated blood that is carried by the hepatic artery. The blood mixes, passes through the sinusoids, bathes the hepatocytes and drains into the central vein. About 1.5 liters of blood exit the liver every minute. The liver is central to a multitude of physiologic functions, including: * Clearance of damaged red blood cells & bacteria by phagocytosis * Nutrient management
* Synthesis of plasma proteins such as albumin, globulin, protein C, insulin-like growth factor, clotting factors, etc. * Biotransformation of toxins, hormones and drugs
* Vitamin and mineral storage
Red blood cell (RBC) lifespan is about 120 days. Reticuloendothelial (macrophage) cells in the spleen, liver and bone marrow are primarily responsible for clearing pathogens and debris. Kupffer cells are reticuloendothelial cells resident in the liver sinusoids that scavange damaged RBCs and bacteria as they pass through. Hundreds of millions of RBCs are removed by the reticuloendothelial system every minute. Kupffer cells, like other reticuloendothelial macrophages, lyse RBCs into heme and globin. Globin is further catabolized into polypeptide components for reuse. Heme is broken into biliverdin and iron. Biliverdin is converted to bilirubin. Iron is transported by transferrin to the liver and spleen for storage and to the bone for hematopoiesis. Hepatocytes cleave bilirubin from albumin and absorb the bilirubin. In the hepatocyte cytoplasm, bilirubin is conjugated to glucouronic acid. Bilirubin uridine diphosphate glucuronyl transferase (UDPGT) catalyzes the bonding of glucuronic acid and bilirubin to produce water-soluble bilirubin. Water soluble conjugated bilirubin is secreted into canaliculi along with water, electrolytes, bicarbonate, bile acids, salts, cholesterol and phospholipids. This combination is called bile and serves as a detergent to keep bile soluble in the...
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