Haemoglobin and oxygen transport

Oxygen is transported in RBC through the blood system. In the capillaries it leaves the RBC and enters the plasma, where it is carried into the tissue fluid by the ultrafiltration of the plasma. In the red blood cells it is the molecule haemoglobin, a respiratory pigment, that is responsible for oxygen transport.

Haemoglobin is a conjugated protein found in large quantities in red blood cells. Each molecule consists of four polypeptides, two a-chais and two B-chains. Each polypeptide has a heam group attached, which contains iron (Fe2+). An oxygen molecule can associate with each haem to form oxyhaemoglobin. The equation shows that one molecule of oxyhaemoglobin can carry up to four molecules of oxygen:

The equation also shows that the reaction is reversible. In conditions where oxygen levels are high, oxyhaemoglobin is formed; if oxygen levels are low the oxyhaemoglobin dissociates (breaks down releasing oxygen).

Haemoglobin is usually deoxygenated of fully oxygenated with four oxygen molecules. It seldom transports one, two or three oxygen molecules around the body. When one oxygen molecule is taken up by a haemoglobin molecule, there is a conformational change (distortion) in the haemoglobin molecule, resulting in an easier (faster) uptake of the remaining three oxygen molecules (cooperative loading).

The amount of oxygen carried by all this haemoglobin is measured to the degree to which the blood is saturated - 50% saturated means that, on average, each haemoglobin molecule is carrying two oxygen molecules.

The amount of oxygen carried by the blood depends on the amount of oxygen available in the surroundings. The oxygen concentration in the environment is referred to as its partial pressure (pO2). The partial pressure of any gas is the proportion of total air pressure that is contributed to by that gas and is measured in kilopascals (kPa).

If haemoglobin molecules are exposed to a range of partial pressures of oxygen, their percentage saturation (with oxygen) can be plotted on a graph known as a oxygen dissociation curve.

A significant feature of the graph above is that is it S-shaped (sigmoidal). It shows that in high oxygen partial pressures, such as 14 kPa (in the lungs), oxyhaemoglobin is readily formed and the haemoglobin approaches full saturation - ie every haemoglobin molecule is fully saturated with oxygen. The haemoglobin remains saturated as the partial pressure falls (travels through ateries and arterioles). However, in low partial pressures, as found in respiring tissues, dissociation takes place and the oxygen is released and diffuses into the respiring tissue cells. The sigmoidal pattern…