The Blood

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  • Created by: Tasha.L
  • Created on: 24-04-16 15:50

The blood consists of roughly 45% cells and 55% plasma

Functions of the mammalian blood:

1. Transport of soluble organic compounds from the small intestine to other parts of the body.

2. Transport of soluble excretory products, such as carbon dioxide, from the tissues where they are produced to the organs of excretion, such as the lungs.

3. Transport of metabolic by-products, such as urea, from where they are produced to other parts of the body.

4. The transport of hormones from the glands to all parts of the body.

5. The distribution of heat from deeply seated organs.

6. Transport of oxygen from the lungs to the respiring tissues, and for the transport of carbon dioxide from the respiring tissues in the opposite direction.

7. Defence against disease, for example, the platelets clot the blood, and the white blood cells can engulf bacteria by phagocytosis.

8. Plasma protein activity helps to maintain osmotic pressure and a constant pH. 


Oxygen is transported in the red blood cells in haemoglobin. It combines with haemoglobin to form oxyhaemoglobin. This is a reversible reaction:

Oxygen + haemoglobin --> oxyhaemoglobin

4O2 + Hb --> Hb.4O2

Haemoglobin is a quaternary protein consisting of four polypeptide chains, two alphas and two betas. Haemoglobin had four haem groups, each of which contains and iron ion. One oxygen molecule can bind with one iron ion. Therefore, haemoglobin has the capacity to carry four oxygen molecules. 

The first oxygen molecule that attaches changes the shape of the haemoglobin molecule so that it becomes easier for the second molecule to attach. The second molecule also induces a shape change, making it easier four the third oxygen molecule to attach. This is called cooperative binding and it allows haemoglobin to pick up oxygen very rapidly in the lungs. However, when the third oxygen molecule binds, a shape change does not occur. Therefore, a high partial pressure of oxygen is required for the fourth oxygen molecule to bind. 


The oxygen dissociation curve is s-shaped. At low partial pressures of oxygen, the percentage saturation with oxygen is low. This is because the haem groups are in the centre of the haemoglobin molecule, making it difficult for the first oxygen molecule to reach a haem group and associate with it. However, as the partial pressure increases so does the percentage saturation. This is because the greater the number of oxygen molecules available, the more likely the…


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