Transport of oxygen

Blood carries O₂ and CO₂. About 97 per cent of O₂ travels bound to haemoglobin in RBCs; the remaining 3 per cent is dissolved in plasma.

Haemoglobin is a red, iron-containing pigment in RBCs. O₂ binds it reversibly to form oxyhaemoglobin, and each haemoglobin molecule can carry at most four molecules of O₂.

Binding of O₂ depends mainly on pO₂; pCO₂, hydrogen ion concentration and temperature also affect it.

Plotting percentage saturation of haemoglobin against pO₂ gives a sigmoid curve, the oxygen dissociation curve, which is very useful for studying how pCO₂, H⁺ concentration and other factors affect O₂ binding.

In the alveoli, high pO₂, low pCO₂, lower H⁺ concentration and lower temperature all favour formation of oxyhaemoglobin.

In the tissues, low pO₂, high pCO₂, higher H⁺ concentration and higher temperature favour dissociation of O₂ from oxyhaemoglobin.

Under normal physiological conditions, every 100 mL of oxygenated blood delivers around 5 mL of O₂ to the tissues.