Biology – The gas exchange system | e-Consult

Gas exchange in the lungs is a crucial process for supplying the body with oxygen and removing carbon dioxide. It occurs in the alveoli, tiny air sacs within the lungs, and the capillaries that densely surround them. The process relies on the principle of diffusion, which is the passive movement of molecules from an area of high concentration to an area of low concentration.

Oxygen (O₂) diffuses from the alveoli, where the partial pressure of oxygen is high, into the blood in the capillaries, where the partial pressure of oxygen is low. This happens because the concentration gradient is steeper in the alveoli. The oxygen then binds to haemoglobin within red blood cells, forming oxyhaemoglobin. This binding significantly increases the blood's oxygen-carrying capacity.

Carbon dioxide (CO₂) diffuses in the opposite direction – from the blood in the capillaries, where the partial pressure of carbon dioxide is high (due to metabolic activity in body tissues), into the alveoli, where the partial pressure of carbon dioxide is low. CO₂ is transported in the blood in three main ways: as dissolved CO₂, as bicarbonate ions (HCO₃^-), and bound to haemoglobin. The conversion of CO₂ to bicarbonate ions is facilitated by the enzyme carbonic anhydrase, which is present in red blood cells.

Several factors facilitate efficient gas exchange:

• Large surface area: The alveoli provide a vast surface area for gas exchange.
• Thin walls: Both the alveolar walls and the capillary walls are only one cell thick, minimizing the diffusion distance.
• Moist surfaces: The alveolar and capillary surfaces are moist, allowing gases to dissolve and diffuse more easily.
• Partial pressure gradients: The differences in partial pressures of oxygen and carbon dioxide drive the diffusion process.
• Ventilation: The movement of air into and out of the lungs maintains the partial pressure gradients.