Anatomy and Physiology of the liver

• The liver is the largest internal organ in the body, and weighs about 3 pounds in an adult. The liver is located in the right upper quadrant of the abdomen, just below the diaphragm. A thick capsule of connective tissue called Glisson's capsule covers the entire surface of the liver. The liver is divided into a large right lobe and a smaller left lobe. The falciform ligament divides the two lobes of the liver.


• Each lobe is further divided into lobules that are approximately 2 mm high and 1 mm in circumference.


• These hepatic lobules are the functioning units of the liver. Each of the approximately 1 million lobules consists of a hexagonal row of hepatic cells called hepatocytes. The hepatocytes secrete bile into the bile channels and also perform a variety of metabolic functions. Between each row of hepatocytes are small cavities called sinusoids. Each sinusoid is lined with Kupffer cells, phagocytic cells that remove amino acids, nutrients, sugar, old red blood cells, bacteria and debris from the blood that flows through the sinusoids. The main functions of the sinusoids are to destroy old or defective red blood cells, to remove bacteria and foreign particles from the blood, and to detoxify toxins and other harmful substances. Approximately 1500 ml of blood enters the liver each minute, making it one of the most vascular organs in the body. Seventy-five percent of the blood flowing to the liver comes through the portal vein; the remaining 25% is oxygenated blood that is carried by the hepatic artery.

hepatic portal system

• The hepatic portal system begins in the capillaries of the digestive organs and ends in the portal vein. Consequently, portal blood contains substances absorbed by the stomach and intestines. Portal blood is passed through the hepatic lobules where nutrients and toxins are absorbed, excreted or converted.


• The consequences of portal hypertension are due to portal systemic anastomosis formed by the body as an attempt to bypass the obstructed liver circulation. These collateral vessels form along the falciform ligament, diaphragm, spleen, stomach and peritoneum. The collaterals find their way to the renal vein where blood drained from the digestive organs is let into the systemic circulation.

The liver is responsible for important functions, including:

• Bile production and excretion


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  Excretion of bilirubin, cholesterol, hormones, and drugs


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  Metabolism of fats, proteins, and carbohydrates


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  Enzyme activation


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  Storage of glycogen, vitamins, and minerals


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  Synthesis of plasma proteins, such as albumin and globulin, and clotting factors
  Blood detoxification and purification




• The liver synthesizes and transports bile pigments and bile salts that are needed for fat digestion. Bile is a combination of water, bile acids, bile pigments, cholesterol, bilirubin, phospholipids, potassium, sodium, and chloride. Primary bile acids are produced from cholesterol. When bile acids are converted or "conjugated" in the liver, they become bile salts.


• Bilirubin is the main bile pigment that is formed from the breakdown of heme in red blood cells. The broken-down heme travels to the liver, where is it secreted into the bile by the liver. Bilirubin production and excretion follow a specific pathway. When the reticuloendothelial system breaks down old red blood cells, bilirubin is one of the waste products. This "free bilirubin" is a lipid soluble form that must be made water-soluble to be excreted. The conjugation process in the liver converts the bilirubin from a fat-soluble to a water-soluble form. The liver also plays a major role in excreting cholesterol, hormones, and drugs from the body.

The liver plays an important role in metabolizing nutrients such as carbohydrates, proteins, and fats. The liver helps metabolize carbohydrates in three ways:

• Through the process of glycogenesis, glucose, fructose, and galactose are converted to glycogen and stored in the liver.


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  Through the process of glycogenolysis, the liver breaks down stored glycogen to maintain blood glucose levels when there is a decrease in carbohydrate intake.


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  Through the process of gluconeogenesis, the liver synthesizes glucose from proteins or fats to maintain blood glucose levels.




• The liver synthesizes about 50 grams of protein each day, primarily in the form of albumin. Liver cells also chemically convert amino acids to produce ketoacids and ammonia, from which urea is formed and excreted in the urine. Digested fat is converted in the intestine to triglycerides, cholesterol, phospholipids, and lipoproteins. These substances are converted in the liver into glycerol and fatty acids, through a process known as ketogenesis.


• Prothrombin and fibrinogen, substances needed to help blood coagulate, are both produced by the liver. The liver also produces the anticoagulant heparin and releases vasopressor substances after hemorrhage.


• Liver cells protect the body from toxic injury by detoxifying potentially harmful substances. By making toxic substances more water soluble, they can be excreted from the body in the urine. The liver also has an important role in vitamin storage. High concentrations of riboflavin or Vitamin B1 are found in the liver. 95% of the body's vitamin A stores are concentrated in the liver. The liver also contains small amounts of Vitamin C, most of the body's Vitamin D stores, and Vitamins E and K.