Oxyhaemoglobin is haemoglobin bound to oxygen. Oxygen is transported in the form of oxyhaemoglobin from the lungs to different tissues.
Oxyhaemoglobin is formed in the lungs when oxygen binds to haemoglobin, present in the RBCs. Oxygen binds to the haemoglobin reversibly. Haemoglobin is the respiratory pigment present in RBCs. It is required for the transport of oxygen from the lungs to various tissues.
The binding of oxygen to haemoglobin is dependent on the partial pressure of oxygen (pO2). In the alveoli, the partial pressure of oxygen is high which favours the formation of oxyhaemoglobin.
Other factors that favour oxyhaemoglobin formation are low pCO2, less hydrogen ion concentration (high pH) and lower temperature that exists in the alveoli and favour the binding of oxygen to haemoglobin.
In tissues, the condition is reversed. The low pO2, high pCO2, high hydrogen ion concentration (low pH) and a higher temperature present there, all favour the dissociation of oxyhaemoglobin. Thus, oxygen is released in tissues.
Haemoglobin is present in the tense (T) state and relaxed (R) state. The R state has more affinity with oxygen and thus favours oxyhaemoglobin formation. Each haemoglobin can bind to four oxygen molecules. The binding of one oxygen molecule brings about conformational changes in the haemoglobin molecule favouring the binding of other oxygen molecules. Thus, the binding of oxygen to haemoglobin is called cooperative binding.
When we plot the partial pressure of oxygen against the percentage saturation of haemoglobin, we get a sigmoid curve. It is known as the oxygen dissociation curve.
We can study the effect of various factors on the binding of oxygen to haemoglobin by any deviation in the curve. E.g. When pH is low or the concentration of H+ is high the oxygen dissociation curve shifts towards the right, i.e. dissociation of oxyhaemoglobin is favoured. The curve shifts towards the left when the pH is high or at a low H+ concentration.
To summarise, around 97 per cent of oxygen is transported as oxyhaemoglobin through RBCs in the blood. Various factors that affect the formation and dissociation of oxyhaemoglobin are as follows:
- Factors that increase oxygen affinity to haemoglobin and favour oxyhaemoglobin formation are high pO2, low pCO2, high pH or low H+ concentration, low temperature, low 2,3 BPG.
- Factors that decrease oxygen affinity to haemoglobin and favour oxyhaemoglobin dissociation are low pO2, high pCO2, low pH or high H+ concentration, high temperature, high 2,3 BPG.
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