The respiratory system enables gas exchange by ventilating the alveoli and maintaining concentration gradients that allow oxygen and carbon dioxide to diffuse between alveolar air and the blood.
Pulmonary ventilation moves air into and out of the lungs. During inspiration, contraction of the diaphragm and external intercostal muscles increases thoracic volume, reducing pressure inside the lungs and drawing air inward. Expiration reverses this process, removing air containing a higher concentration of carbon dioxide.
At the alveoli, external respiration occurs. Oxygen has a higher partial pressure in alveolar air than in deoxygenated blood, so it diffuses across the alveolar and capillary walls into the blood. Most oxygen then binds reversibly to haemoglobin in red blood cells, helping to maintain the diffusion gradient.
Carbon dioxide has a higher partial pressure in deoxygenated blood than in alveolar air. It therefore diffuses in the opposite direction, from the blood into the alveoli, before being removed during expiration.
Several adaptations make this diffusion rapid:
| Adaptation | Effect on gas exchange |
|---|---|
| Many alveoli | Provide a large surface area for diffusion |
| Alveolar and capillary walls are one cell thick | Create a short diffusion distance |
| Moist alveolar lining | Allows respiratory gases to dissolve before diffusing |
| Dense capillary network | Maintains blood flow and steep partial-pressure gradients |
| Continuous ventilation | Replenishes oxygen and removes carbon dioxide |
During exercise, increases in tidal volume and breathing frequency raise minute ventilation. Increased pulmonary blood flow also helps preserve steep oxygen and carbon dioxide gradients, supporting the greater rate of cellular respiration in working muscles.
A common misconception is that breathing and gas exchange are identical. Ventilation is the bulk movement of air, whereas gas exchange is diffusion across the alveolar-capillary membrane.
In an IB SEHS A.1.3 Transport response, distinguish ventilation from diffusion and link each alveolar adaptation explicitly to surface area, diffusion distance, or maintenance of a steep partial-pressure gradient.
