Oxygen Dissociation Curves
The oxygen content of blood fully saturated or oxygenated can be calculated. It is known as the oxygen capacity of blood and this varies for different species. In humans the oxygen carrying capacity is 20 ml oxygen per 100 ml blood. The relationship of oxygen carrying capacity to surrounding oxygen concentration can be shown graphically by oxygen dissociation curves.
Figure: Oxygen dissociation curve shows how haemoglobin's oxygen binding capacity depends on partial pressure of oxygen.
These curves are obtained by subjecting blood samples to different partial pressures of oxygen. The per cent oxyhaemoglobin saturation at different partial pressures of oxygen is plotted. The oxygen dissociation curve is S-Shaped or a sigmoid curve. From the graph we can see how haemoglobin acts as a carrier of oxygen. Total saturation occurs in the lungs where arterial pressure is above 95 mm Hg and the oxygen is unloaded at low Po2 found in tissues (about 40 mm Hg).