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What are the principle roles of the liver?

- First destination of most nutrients and xenobiotics absorbed by the GI tract
- Bile production
- Elimination of unwanted molecules (metabolism, or excretion into the biliary tree and faecal excretion)
- Secretion of plasma proteins (e.g. albumin)
- Storage of important molecules (e.g. fuels, iron, vitamins)
- Regulation of metabolism (carbohydrate and lipid metabolism, which leads to storage, and amino acid metabolism, which leads to the urea cycle).


What is the urea cycle?

This is the pathway which makes urea.
It occurs in the liver, and it is the way in which we get rid of excess nitrogen.
The kidneys excrete the urea.


What can be used to separate plasma proteins by size?



What are the main groups of plasma proteins?

Albumin, alpha globulins, beta globulins and gamma globulins (which include immunoglobulins).


What are the main functions of plasma proteins?

- Maintenance of oncotic or colloid osmotic pressure
- Transport of hydrophobic substances (steroid hormones, free fatty acids, bilirubin, cholesterol)
- pH buffering (amino acid side chains can carry net charges)
- Enzymatic (e.g. blood clotting)
- Immunity


What is the most abundant plasma protein?
What percentage of all plasma proteins does it make up?
How many grams per litre should be present in the blood?

It makes up 50% of all plasma proteins.
There should be 35-45 g/L


What is the main determinant of plasma osmotic pressure?



Which hormone stimulates the production of albumin?



What can cause low levels of albumin?

Liver disease
Starvation/ low protein diet


How does albumin bind to the substances which it transports?

It has hydrophobic clefts in globular domains where the substances it transports can bind.


Describe the affinity of albumin for the substances it transports

It has a low affinity, but a high capacity because of its high concentration


What sort of substances does albumin transport?
Give examples.

It transports important, endogenous lipophilic substances
- Fatty acids
- Bilirubin
- thyroid hormones

It also transports exogenous substances e.g.
- drugs like aspirin


Why is iron important?
How is it transported?
How is it stored?

It is a component of heamoglobin, myoglobin and cytochromes.
It is transported as the ferric ion (Fe3+) bound to transferrin
It is stored in cells bound to ferritin


How is copper transported?
In what disease is there a deficiency of copper?

Bound to ceruloplasmin
Wilson's disease


Which hormones are hydrophobic?

The steroid hormones (which are derived from cholesterol)
T3/T4 thyroid hormones


How is thyroxine transported in the blood?

Bound to thyroid-binding globulin


How is cortisol transported in the blood?

Bound to cortisol-binding globulin


What is the effect have transporters on hormones?

It extends the biological half life of the hormones and increases their plasma concentration.
If they were not bound to transporters they would rapidly be eliminated by the liver or the kidney.


What is the origin and function of chylomicrons?

They originate in the small intestine and transport exogenous fat to the liver


What is the origin and function of VLDL?

They originate in the liver and transport endogenous fat to peripheral cells


What is the origin and function of ILD?

The origin is VLDL and it is a precursor of LDL


What is the origin and function of LDL?

Originate from liver/VLDL/IDL. The function is to transport cholesterol to the peripheral tissues.


What is the origin and function of HDL?

Originate from the intestine and the liver.
The function is to transport cholesterol from peripheral tissues to the liver.


Which lipoprotein transports dietary fat?



Which lipoprotein transports endogenous/synthesised fat?



Describe reverse cholesterol transport

This is the function of HDL.
Excess cholesterol is removed from cells and is esterified with fatty acids. It is then transported back to the liver and excreted as bile salts via the biliary system and faeces.


What is the only organ capable of metabolising and excreting cholesterol?

The liver


What is stored in the liver?

Vitamin A- stored as retinol palmitate
Vitamin D
Vitamin B12
Iron (from the breakdown of haemoglobin), stored bound to ferritin.


What is the function of cholesterol in mammalian cell membranes?

It increases rigidity


Cholesterol is the precursor for which 3 important classes of biologically active compounds?

Bile acids
Steroid hormones
Vitamin D

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