Q.Write the partial pressures of oxygen and carbon dioxide in alveoli and deoxygenated blood.

Imagine a delivery van that grips its parcels tightly on the open highway but happily drops them off the moment it reaches a crowded, busy neighbourhood. Haemoglobin, the red oxygen-carrying pigment inside your red blood cells, behaves in much the same way — and the reason it lets go of oxygen exactly where the body needs it is captured by the Bohr effect.

The Bohr effect is the way haemoglobin's grip on oxygen loosens as its surroundings become richer in carbon dioxide, more acidic (higher in hydrogen ions), and warmer. Oxygen binds to haemoglobin reversibly, meaning the bond can form and break depending on local conditions. Whether it forms or breaks is decided chiefly by how much oxygen is around, but the amount of carbon dioxide, the acidity, and the temperature all pull on that decision too.

This matters because the two places that matter most — the lungs and the working tissues — offer almost opposite conditions:

• In the alveoli of the lungs, oxygen is plentiful while carbon dioxide, acidity and temperature are all low. Every one of these favours haemoglobin picking up oxygen and forming oxyhaemoglobin.
• In the busy tissues, cells are steadily producing carbon dioxide, raising the acidity, and running warmer. These conditions favour oxygen breaking away from haemoglobin, so it is released precisely where cells are demanding it. …