The liver, biliary system and pancreas

19 The liver, biliary system and pancreas



Learning objectives


After reading this chapter you should understand:


• the structure and function of the liver

• the structure and function of the biliary system

• the structure and function of the pancreas

Due to their anatomical proximity and their related functions in digestion, the liver, biliary system and pancreas are usually considered together. This chapter looks at the structure of the liver and its function, particularly in the production of bile. The way the biliary system concentrates and subsequently releases bile is described and, finally, the structure and secretions of the pancreas are considered. The function of the pancreatic enzymes is the subject of Chapter 21.



The liver


The liver is the largest gland in the body. It is situated in the upper right part of the abdominal cavity, occupying almost all of the right hypochondrium and fitting under the diaphragm. It has two main lobes, the right lobe being much larger than the left (Fig. 19.1). The right lobe lies over the right colic flexure and the right kidney and the left lobe over the stomach.


image

Fig. 19.1 General appearance of the liver.



Structure of the liver


The liver consists of a large number of hepatic lobules which appear hexagonal in shape (Fig. 19.2). Each is approximately 1 mm in diameter and has a small central intralobular vein (a tributary of the hepatic veins). Around the edges of the lobules are the portal canals, each containing a branch of the portal vein (interlobular vein), a branch of the hepatic artery and a small bile duct. These three structures together are known as the portal triad.


image

Fig. 19.2 (a) A liver lobule and (b) the arrangement of blood vessels and bile ducts.


The lobules are composed of liver cells, which are large cells with one or two nuclei and fine granular cytoplasm. The liver cells are arranged in sheets, one cell thick, called hepatic laminae. These laminae are arranged irregularly to form walls with bridges of liver cells connecting adjacent laminae. Between the laminae are spaces containing small veins with many anastomoses between them and small bile ducts called canaliculi.


In the liver the portal vein brings blood rich in foodstuffs from the alimentary tract and the hepatic artery brings blood rich in oxygen from the arterial system. These divide into smaller vessels and provide a capillary network among the liver cells forming the hepatic laminae. This capillary network then drains into the small veins in the centre of each lobule, which supply the hepatic vein. These vessels carry away the blood from both the portal capillaries and the deoxygenated blood, which has been brought to the liver by the hepatic arteries as oxygenated blood.



Functions of the liver


The functions of the liver can be divided into three sections: metabolic; storage; and secretory:


1. Metabolic functions:
• Stored fat is broken down to provide energy; this process is called desaturation.

• Excess amino acids are broken down and converted to urea.

• Drugs and poisons are detoxified.

• Vitamin A is synthesized from carotene.

• The liver is the main heat-producing organ of the body.

• The plasma proteins are synthesized.

• Worn-out tissue cells are broken down to form uric acid and urea.

• Excess carbohydrate is converted to fat for storage in the fat depots.

• Prothrombin and fibrinogen are synthesized from amino acids.

• Antibodies and antitoxins are manufactured.

• Heparin is manufactured.

2. Storage functions:
• vitamins A and D

• antianaemic factor

• iron from the diet and from worn-out blood cells
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Jul 18, 2016 | Posted by in NURSING | Comments Off on The liver, biliary system and pancreas

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