Good day everyone, today we are going to discuss fully on the liver:
After studying this section, you should be able to:
- describe the location of the liver in the abdominal cavity
- describe the structure of a liver lobule
- outline the functions of the liver.
The liver is the largest gland in the body, weighing between 1 and 2.3 kg. It is situated in the upper part of the abdominal cavity occupying the greater part of the right hypochondriac region, part of the epigastric region and extending into the left hypochondriac region. Its upper and anterior surfaces are smooth and curved to fit the under surface of the diaphragm; its poste- rior surface is irregular in outline.
Organs associated with the liver
Superiorly and anteriorly – diaphragm and anterior abdominal wall
Inferiorly stomach, bile ducts, duodenum, hepatic flexure of the colon, right kidney and adrenal gland Posteriorly – oesophagus, inferior vena cava, aorta, gall bladder, vertebral column and diaphragm Laterally – lower ribs and diaphragm.
The liver is enclosed in a thin inelastic capsule and incom- pletely covered by a layer of peritoneum. Folds of perito- neum form supporting ligaments that attach the livers to the inferior surface of the diaphragm. It is held in position partly by these ligaments and partly by the pressure of the organs in the abdominal cavity.
The liver has four lobes. The two most obvious are the large right lobe and the smaller, wedge-shaped, left lobe. The other two, the caudate and quadrate lobes, are areas on the posterior surface .
The portal fissure
This is the name given to the region on the posterior surface of the liver where various structures enter and leave the gland.
The portal vein enters, rrying blood from the stomach, spleen, pancreathe small and large intestines.
The hepatic artery ente carrying arterial blood. It is a branch from the coelia tery, which branches from the abdominal aorta.
Nerve fibres, sympathetic and parasympathetic, enter here. The right and left hepatic ducts leave, carrying bile from the liver to the gall bladder. Lymph vessels leave the liver, draining lymph to abdominal and thoracic nodes.
The hepatic artery and the portal vein take blood to the liver. Venous return is by a variable number of hepatic veins that leave the posterior surface and immediately enter the inferior vena cava just below the diaphragm.
Structure of a liver lobule
The lobes of the liver are made up of tiny functional units, called lobules, which are just visible to the naked eye . Liver lobules are hexagonal in outline and ace formed by cuboidal cells, the hepatocytes, arranged in pats of columns radiating from a central vein. Between 100 pairs of columns of cells are sinusoids (blood vessels with incomplete walls, containing a mixture of Hood from the tiny branches of the portal vein and hepatic artery. This arrangement allows the arterial blood and portal venous blood (with a high concentration of nutrients) to mix and come into close contact with the liver cells.
Blood drains from the sinusoids into central or centrilobular veins. These then merge with veins from other lobules, forming larger veins, until eventually they become the hepatic veins, which leave the liver and empty into the inferior vena cava.
Functions of the liver
The liver is an extremely active organ, which has important functions that are described below.
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The liver has an important role in maintaining plasma glucose levels. After a meal when levels rise, glucose is converted to glycogen for storage under the influence of the hormone insulin. Later, when glucose levels fall, the hormone glucagon stimulates conversion of glycogen into glucose again, keeping levels within the normal range.
Stored fat can be converted to a form in which it can be used by the tissues to provide energy.
Deamination of amino acids. This process:
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- removes the nitrogenous portion from amino acids that are not required for the formation of new protein; urea is formed from this nitrogenous portion and is excreted in urine
Transamination. Removes the nitrogenous portion of amino acids and attaches it to other carbohydrate molecules forming new non-essential amino acids.
Synthesis of plasma proteins. These are formed from amino acids and include albumins, globulins and blood clotting factors.
Breakdown of erythrocytes and defence against microbes
This is carried out by phagocytic hepatic macrophages (Kupffer cells) in the sinusoids although breakdown of red blood cells also takes place in the spleen and bone marrow.
Detoxification of drugs and toxic substances
These include ethanol (alcohol), waste products and microbial toxins. Some drugs are extensively inactivated by the liver and are not very effective when given by mouth (orally), e.g. glyceryl trinitrate. This is because after absorption from the alimentary tract, they travel in the blood to the liver where they are largely metabolised so that levels in the blood leaving the livers and which enters the systemic circulation are inadequate to achieve therapeutic effects. This is known as ‘first pass metabolism’.
Inactivation of hormones
These include insulin, glucagon, cortisol, aldosterone, thyroid and sex hormones.
Production of heat
The livers uses a considerable amount of energy, has a high metabolic rate and consequently produces a great deal of heat. It is the main heat-producing organ of the body.
Secretion of bile
The hepatocytes synthesise the constituents of bile from the mixed arterial and venous blood in the sinusoids These include bile salts, bile pigments and cholesterol (see below).
Stored substances include:
- fat-soluble vitamins: A, D, E, K
- iron, copper
- some water-soluble vitamins, e.g. vitamin B
Composition of bile
Between 500 and 1000 ml. of bile is secreted by the liver daily. Bile consists of:
- mineral salts
- bile pigments, mainly bilirubin
- bile salts