Transporting oxygen and carbon dioxide

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Haemoglobin can pick up four molecules of oxygen.

The oxygen concentration in the red blood cells when oxygen enters the cell is relatively low. So oxygen diffuses into the red blood cells from the air in the lungs.

In the body tissues the oxygen levels are relatively low, while the oxygen concentration in the red blood cells are much higher than in the surrounding tissue.

As a result, oxygen diffuses out of the red blood cells down its concentration gradient, and haemoglobin gives up their oxygen.

When resting only 25%of the oxygen carried by the haemoglobin is released to cells, compared to when being more active as 75% of the oxygen is released to cells.

OXYGEN

CARBON DIOXIDE

Waste carbon dioxide from respiring cells diffuses into the blood along a concentration gradient.

When carbon dioxide is dissolved in the blood it reacts slowly to form carbonic acid, H2CO3, which separates to form the ions H+ and HCO3-

CO2+H2O-->H2CO3-->HCO3-+H+

About 5% of the carbon dioxide is carried in the plasma.

Further 10-20% combines with haemoglobin molecules to form carbaminohaemoglobin.

Most of the carbon dioxide is transported in the cytoplasm of the red blood cells as hydrogen carbonate ions.

The enzyme carbonic anhydrase controls the rate of reaction between carbon dioxide and water in the blood.

In the body tissues the concentration of carbon dioxide is high, so carbonic anhydrase catalyses the formation of carbonic acid.

In the lungs the carbon dioxide concentration is low so carbonic anhydrase catalyses the reverse reaction and free carbon dioxide diffuses out of the blood and into the lungs.

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