H4.1 Blood supply to the liver.

,liver circulationThere are two main blood vessels supplying the liver.

a) Hepatic artery carrying oxygenated blood.

b) Hepatic portal vein which carries blood(deoxygenated) from the gut to the liver. The content of this blood vessel is highly variable due to absorption of digestion products in the gut. Note that the gut there is no mesenteric vein from the gut to the vena cava. In this way the liver can effect immediate control over level of nutrient in the blood.

There is one major blood vessel taking blood vessels away from the liver:

c) The hepatic vein drains blood from the liver to the vena cava and the general circulation.

 

 

A section through the liver does not reveal any complex architecture that can be seen without a microscope.

Rather the liver is divided into microscopic units called lobules:

 

Note that the structure of the lobule is not required for the syllabus but it is felt necessary to illustrate the structure so that the position of the sinusoid and its function can be appreciated.

liver lobule

 

Each lobule is surrounded by branches of the hepatic artery and hepatic portal vein.

Both these blood vessels connect to a small branch of the which runs centrally through the liver lobule.

Each set of hepatic artery and Hepatic portal vein is connected to the Hepatic vein via a blood vessel called a sinusoid. Which has a highly fenestrated endothelial lining, which is highly porous(see below).

Behind the sinusoid walls are the hepatocytes of the liver. Cuboidal shaped cells that carry out a wide range of homeostatic,detoxifying functions.

 

 

 

 

 

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liver lobulesinusoid

 

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lobule blood supply

 

The sinusoid walls are very thin without tight junctions such that the basement membrane acts as the filter to blood content.

The content of the blood is readily accessible to the hepatocytes which remove and add various molecules to blood.

As previously covered in topic12 this is the site of transamination, deamination and the ornithine cycle of amino acid metabolism.

This vertical section through a lobule illustrates the arrangement of sinusoids with the other blood vessels

Note that in the lumen of the sinusoid there are phagocytic cells that have a role in the breakdown of haemoglobin called Kupffer cells ( not illustrated).

 

 

 

 

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H4.2 Regulation of blood nutrients.

The reason for controlling internal regulations within the body are covered in section 5.6 and should be review.

Specifically within the liver there are certain reasons why regulation of nutrients should occur here:

In general however the nutrient content of blood is greatly affected by the frequency of feeding and the absorption of the products of digestion in the gut.

If this entered the general circulation it could have a wide range of damaging effects. Therefore blood does NOT travel to the vena cava directly but rather passes via the hepatic portal vein for modification. In this way the blood is regulated before it can enter the general circulation.

Specifically:

    • Glucose can affect the osmotic concentration of the blood. Hypertonic blood due to sugar absorption in the gut could be damaging particularly to the brain. Therefore rather than enter general circulation the blood sugar levels are regulated by the hepatocytes

    • Salts as above can affect the osmotic concentration of the blood and are controlled for the same reason.

    • Amino acids as with sugar can affect the osmotic concentration of blood but also amino acids cannot be stored like carbohydrate and therefore excess amino acids need to be regulated and detoxified.

    • Vitamins & Minerals although never required in great amounts the regulation of these elements of the diet is crucial for enzyme function, skeletal development, vision along with a variety of other essential functions. There are some mammals that concentrate very high and toxic levels of vitamins in the liver such as Thalarctos maritimus (polar bear).

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H4.3 Stored nutrients.

 

nutrients stored in liver

 

 

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H4.4 Plasma proteins and cholesterol synthesis.

 

 

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H4.5 Detoxification

Natural Substances:

In addition to the food molecules produced by plants there may also be natural compounds produced by the plant to deter herbivores. An example of this is the nicotine produced in the tobacco plant.

Animals also produce molecules which are toxic to predators.

The liver has a role in detoxification and breaking down these molecules.

The liver possess an enzyme system which has broad specificity for this purpose.

Artificial Substances:

man made molecules may be toxic to the

 

 

including the breakdown of poisons, venom, drugs and alcohol.

 

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H 4.6 Breakdown of haemoglobin and bile formation

bile formation

(a) Erythrocytes die after about 120 day and are the removed in the liver

(b) The Kupffer cells( phagocytic cells) found within the lumen of the sinusoid engulf the erythrocytes and release the haemoglobin to the hepatocytes.

(c) Haemoglobin is broken down in the Kupffer cell to form haem and globin groups.

(d) The protein globin group is hydrolysed by peptidases in the kupffer cell to amino acids which are either used in protein synthesis or they are metabolised.

(e) The haem group is broken down to remove the inorganic group, Iron which is then stored in the liver.

Bilirubin the other by-product of haem breakdown has no functional role and is modified by the hepatocytes before being secreted as part of Bile.

(f) Bilirubin is stored in the gall bladder. After secretion into the duodenum it is modified by bacteria to form Sterobilin that colours faeces.

(g)Iron is sent to the major bones for the process of haemopoiesis (blood cell formation).

 

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Alcohol consumption and the liver

NT = Necrotic tissue FC = Fatty change

alcohol hepititisalcohol dehyrogenase

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Chronic consumption of alcohol (ethanol) causes a pregressive disease within the liver.

Liver cells metabolise ethanol in an enzyme pathways as illustrated above.

Acetyaldehyde accumulates after excessive intake of alcohol and along with dehydration is responsible for 'hangover'.

Some people have particularly active alcohol dehydrogenase and the accumulation of acetaldehyde is responsible for 'allergy' like symptoms.

Fatty acids accumulate in liver cells and they also deposite around liver capillaries reducing blood flow and ultimately killing the cells (necrosis).

This leads to the deposition of collagen (fibrosis) and ultimately this will also lead to the condition called cirrhosis.

Whilst it is possible to recover from some of the early stage desribed the later stage (cirrhosis) can lead to liver failure.

 

 

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Click4Biology: Option H Liver Function

 

Functions of the liver

H4.1 Blood supply to the liver.

H4.2 Regulation of blood nutrients.

H4.3 Stored nutrients.

H4.4 Plasma proteins and cholesterol synthesis.

H4.5 Livers role in detoxification.

H4.6 Breakdown of haemoglobin and bile formation.

H4.7 Alcohol and the liver.