4.2.1 Influencing the curve

Where the partial pressure of oxygen is high, such as in the pulmonary capillaries of the lungs, oxygen binds readily to haemoglobin. Here, the haemoglobin is almost 100% saturated, meaning that each molecule of haemoglobin contains four molecules of oxygen. As oxygenated haemoglobin moves through the tissue capillaries, it encounters decreasing partial pressures of oxygen and the affinity, or strength of binding between haemoglobin and oxygen, decreases.

As a result, the percentage of haemoglobin saturated with oxygen falls as oxygen diffuses into the tissues. The partial pressure of oxygen, at which there is 50% saturation of haemoglobin, is called the P50, and under resting conditions is about 26.7 millimetres of mercury. The affinity of haemoglobin for oxygen increases and decreases to maintain homeostatic delivery of oxygen to cells depending on several biological factors.

For example, during exercise, muscle cells become more active and produce more carbon dioxide and heat which, in turn, increase the acidity and content of the haemoglobin protein 2,3-diphosphoglyceric acid, or DPG. The active cells will also be using more oxygen. So the partial pressure of oxygen in the cells will drop.

To meet the increased demands for oxygen, the affinity of haemoglobin for oxygen will decrease as the partial pressure of oxygen drops. This effectively shifts the P50 to the right, and means that haemoglobin becomes desaturated more quickly. Notice in this example how under conditions of increased exercise the same partial pressure of oxygen results in less haemoglobin saturation, as oxygen diffuses more easily into the active cells. Conversely, when levels of carbon dioxide are low, such as in the capillaries of the lungs, the affinity of haemoglobin for oxygen increases.

This effectively shifts the P50 to the left, and means that oxygen diffusing from the alveoli binds tightly to the haemoglobin entering the lungs. This helps to keep oxygen bound to haemoglobin as it leaves the lungs to be carried to the peripheral tissues. An easy way to remember how the oxygen–haemoglobin dissociation curve changes based on metabolic demand is to use the mnemonic CADET face right, meaning that increasing the levels of CO2, Acidity 2,3-DPG, exercise and temperature will shift the P50 to the right, and vice versa.