Carriage of oxygen

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Red blood cells contain the complex protein haemoglobin that combines with oxygen to form oxyhaemoglobin for the transport of oxygen in the blood. Haemoglobin becomes oxygenated.

Haemoglobin consits of 4 subunits, where each subunit consits of a polypeptide chain and a haem group. 
The haem group is one iron ion, Fe2+, that can attract and hold 1 oxygen molecule beacuse it has an affinity for oxygen.
Because there are 4 haem groups in a Hb molecule each Hb molecule can hold 4 oxygen molecules. So oxyhaemoglobin is HbO8. 

Oxygen will diffuse into the blood plasma in the lungs, it then diffuses into the red blood cells which takes oxygen out of the solution, thus mainttaining a steep diffusion gradient so that more oxygen can diffuse into the plasma and then red blood cells from the lungs. 

At aerobically repsiring tissues the oxyhaemoglobin, HbO, dissociates releasing oxygen into these tissues for respiration. 

PARTIAL PRESSURE: the proportion of total pressure provided by a particular gas as part of a mixture of gases.
OXYGEN TENSION: the amount of oxygen in the ar expressed at the pressure creates by the presence of oxygen, expressed in kPa.

The ability of haemoglobin to take up oxygen and release oxygen depends of the oxygen tension of the surrounding tissues.
The amount of oxygen is measured according to the relative amount of pressure that it contributes to a mixture of gases = partial pressure,

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