Cellular Biology

The Structure and Function of the Liver

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The liver is the largest internal organ of the body, weighing about 1.36Kg, located in the right upper quadrant of the abdomen. It consists of two major lobes (left and right) and two minor lobes (caudate and quadrate).

The liver is involved in the normal physiological functions of many organs and systems of the body such as the cardiovascular and immune systems.

Many important substances are secreted into the gastro-intestinal tract by it, as well as storing and processing of nutrients, synthesizing new molecules, and detoxifying harmful chemicals.

The liver is the only solid organ which can regenerate itself.

The main functional cell in the liver is called a hepatocyte, and it performs many metabolic and secretory functions.

The cells are polygonal in shape and arranged in one to two cell thick plates separated by fine vascular sinusoids through which blood flows.

Sinusoids are low pressure vascular channels that receive blood from arteries supplying the liver. They are highly permeable, and allow substances to enter and leave the blood stream.

The close association of liver cells and the circulation allows absorption of nutrients from digestion as well as secretion of many products into the blood.

Blood flow into the liver sinusoids comes from terminal branches of both the hepatic portal vein and hepatic artery.

The liver is closely associated with the gallbladder which is situated in a depression on the inferior surface of the liver. It is a pear-shaped sac and is filled with stored bile.
The gallbladder's function is to store and concentrate the bile produced by the liver until it is needed in the small intestine.

Functions of the Liver:

+ Bile production - The liver produces and secretes about 600-1000 ml of bile each day. Bile helps to neutralize and dilute stomach acid and emulsify fats to aid in digestion.

+ Storage - Hepatocytes can remove sugar from the blood and store it as a substance called glycogen. Glycogen forms an energy reserve that can be quickly mobilized to meet a sudden need for glucose by the body.
Fats, vitamins (A, B12, D, E,and K), copper, and iron are stored as well, but this storage function is usually short term.

+ Nutrient Inter-conversion - the liver can convert some nutrients into others, this occurs when ingested nutrients are not always in the proportion needed by the tissues.
For example, a person who is on a diet that is excessively high in protein and inadequate amounts of carbohydrates and lipids. The hepatocytes will then break down proteins and form lipids and glucose.
Vitamin D obtained from sun exposure, food, and supplements, is biologically inert, and is converted by the liver and kidneys, to obtain the active form of the vitamin, which functions in calcium maintenance.

+ Detoxification - Many ingested substances are harmful to the body's cells, and by-products of metabolism if accumulated become toxic.
The liver detoxifies many substances by altering their structure to make them less toxic or make their elimination easier.

+ Phagocytosis - is the cellular process of engulfing solid particles such as bacteria and cell debris.
Hepatic phagocytic cells, known as Kupffer cells, lie along the sinusoid walls and phagocytize non-functioning blood cells, bacteria, waste products and other debris that enters the liver through the circulation.

+ Synthesis - The liver produces blood plasma proteins, such as albumins, fibrinogen, globulins, and clotting factors, which are released into the circulation.

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