Explain the oxygen dissociation curve. Explain the factors required for the transport of $O_2$ by it.

(N/A) Haemoglobin is a red-coloured, iron-containing pigment present in the $RBCs$. $O_2$ can bind with haemoglobin in a reversible manner to form oxyhaemoglobin.
Each haemoglobin molecule can carry a maximum of four molecules of $O_2$. The binding of $O_2$ with haemoglobin is primarily related to the partial pressure of $O_2$ $(pO_2)$.
Partial pressure of $CO_2$ $(pCO_2)$, hydrogen ion concentration $(H^+)$, and temperature are other factors that can interfere with this binding.
A sigmoid curve is obtained when the percentage saturation of haemoglobin with $O_2$ is plotted against the $pO_2$. This curve is called the oxygen dissociation curve.
This curve is highly useful in studying the effect of factors like $pCO_2$ and $H^+$ concentration on the binding of $O_2$ with haemoglobin.
In the alveoli, where there is high $pO_2$, low $pCO_2$, lower $H^+$ concentration, and lower temperature, the conditions are favourable for the formation of oxyhaemoglobin.
In tissues, where low $pO_2$, high $pCO_2$, high $H^+$ concentration, and higher temperature exist, the conditions are favourable for the dissociation of oxygen from the oxyhaemoglobin.
This clearly indicates that $O_2$ gets bound to haemoglobin at the lung surface and gets dissociated at the tissues. Every $100 \text{ ml}$ of oxygenated blood can deliver around $5 \text{ ml}$ of $O_2$ to the tissues under normal physiological conditions.