D3. Functions of the liver

Lesson one: Understand how the liver intercepts blood from the gut and stores nutrients, synthesise blood proteins, breaks down toxins. Also how red blood cells are recycled. What is jaundice?  What is good cholesterol?

Keywords:

  • Bile salts: alkaline salts which cause emulsification of facts, stored in the gall bladder and released by the bile duct
  • Haemoglobin: the iron containing compound in red blood cells which stores oxygen
  • High density lipoprotein cholesterol: so called ‘good cholesterol’

https://www.webmd.com/cholesterol-management/guide/understanding-numbers#1

How does the liver do it’s job

The liver is able to respond to the intake of nutrients and substances so rapidly because it intercepts blood flow from the small intestine via the hepatic portal vein (small intestine –) liver). The liver also receives blood from the hepatic artery, which branches off the aorta.

Liver structure is arrange to allow blood to pass through narrow channels where hepatocytes (liver cells) may interact with the contents. Hepatocytes are rich in mitochondria, reflecting the high level of metabolic activity in the liver. The liver is divided into sections called lobules.

Note the Kupffer cells (involved in blood recycling).

image credit: eclinpath

image credit: eclinpath

 

5 things that the liver does:

  1. Detoxification. Imagine a person inbibes and alcoholic beverage eg. a shot of rum. After the effects of the alcohol have been registered in the brain (alcohol lowers inhibitions, as it is an depressant and slows down transmission across synapse), the alcohol must be eliminated as it is a toxin. Hepatocytes absorb the alcohol by endocytosis, and then break it down into ethanal (less toxic) by use of the enzyme alcohol dehydrogenase. There is a limit to how quickly alcohol can be broken down by the liver, which is one reason why alcohol intake should be regulated

2. Nutrient storage. Vitamin A, Vitamin D, and glucose are stored in the liver when in excess and released when there is a deficit (glucose is stored in the form of glycogen). Eating animal liver is a source of vitamins. Some animal liver contains so much Vitamin A they are toxic (eg. polar bears).

3. Recycling Blood cells. Red blood cells only live for about 4 months. (they have no nucleus). Old red blood cells are absorbed by phagocytosis in Kupffer cells in the liver. The components are then recycled:

  • The non-iron part of haemoglobin becomes the yellow bilirubin, a component of bile
  • The iron part of haemoglobin is transported to bone marrow by the transport protein transferin
  • The rest is broken down into amino acids and fatty acids / phospholipids

If something goes wrong with the metabolism of bilirubin, levels of bilirubin build up in the blood, causing jaundice (yellowing of the eyes and skin, caused by hepatitis).

4. Regulating protein content of blood. The liver can produce blood proteins like albumin, if they are low. Proteins or amino acids cannot be stored. Excess amino acids are broken down in the liver by a process called deamination.

Deamination (removal of amine group), creates ammonia, which is changed into urea( CH4N2O). The remainder of the amino acid can be converted to pyruvate and respired.

5. Regulating lipids content of the blood. Excess cholesterol is converted into bile salts. If cholesterol levels are low the liver also synthesises cholesterol. Cholesterol is needed for synthesis of steroid hormones eg. testosterone and oestrogen, and also for synthesis of vitamin D. Good cholesterol Hight-density-lipoprotein cholesterol, helps to remove bad cholesterol (LDL or low-density-cholesterol) from the lining of arteries.

 

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