Exchange Of Gases
Why Oxygen Moves Into Blood and Carbon Dioxide Moves Out
Picture a crowded room: when a door to an empty hallway opens, people naturally drift toward the emptier space, not because anyone pushes them, but because there is more room there. Gases behave the same way — they move from where they are concentrated to where they are less concentrated, a movement called diffusion. Air freshly breathed into the lungs is rich in oxygen; the blood arriving at the lungs from the body is poor in oxygen and rich in carbon dioxide. So oxygen diffuses from air into blood, and carbon dioxide diffuses from blood into air.
In a mixture like air, each gas behaves as though it alone occupies the space, driven by its own partial pressure — the pressure that gas alone would exert if it filled the whole volume. The rate at which a gas diffuses depends on the difference in its partial pressure across the barrier, how soluble it is, and how large and thin the exchange surface is.
The Alveolar–Capillary Barrier
Exchange happens across an extremely thin membrane where an alveolus and a capillary lie side by side, each wall only one cell thick and fused together — a total barrier of about 0.5 micrometres. It consists of a thin fluid lining inside the alveolus, the alveolar epithelial cell, a fused basement membrane, and the capillary endothelial cell.
The Pressure Gradients In Action
Typical partial pressures at sea level, in mm Hg:
| Gas | Alveolar air | Deoxygenated blood arriving at lungs | Gradient |
|---|
| Oxygen | 104 | 40 | 64 |
| Carbon dioxide | 40 | 45 | 5 |
Oxygen has a large gradient of 64 mm Hg and moves quickly into blood. Carbon dioxide has a much smaller gradient of only 5 mm Hg, yet diffuses just as effectively, because it is roughly 20–25 times more soluble than oxygen in the fluid lining the alveolus — solubility compensates for the smaller pressure difference.
Do not confuse partial pressure with percentage concentration. Oxygen is 21% of dry air, but by the time air reaches the humid alveoli and mixes with residual air, its partial pressure there drops to about 104 mm Hg.
The Journey of a Gas Molecule
An oxygen molecule dissolves in the fluid on the alveolar surface, crosses the epithelial cell, the fused basement membrane, and the capillary endothelial cell, enters the plasma, and binds to haemoglobin inside a red blood cell — a crossing that takes roughly a quarter of a second, well within the time blood spends in a pulmonary capillary. Carbon dioxide moves the opposite way: most of it travels in blood as bicarbonate inside red blood cells, and an enzyme called carbonic anhydrase converts bicarbonate back into carbon dioxide, which then diffuses into the alveolus to be exhaled.
A thicker barrier (as in fibrosis or oedema) slows diffusion; a smaller surface area (as in emphysema) reduces total exchange; a smaller partial-pressure gradient (as at high altitude) reduces oxygen uptake.
In short: exchange of gases is the diffusion of oxygen and carbon dioxide across the alveolar–capillary membrane, driven by their partial-pressure gradients — and the lung's huge surface area, extreme thinness, and rich blood supply all exist to make this simple diffusion as fast and complete as possible.
If you landed here after searching "Exchange Of Gases class 11" or "Exchange Of Gases diagram and explanation", you are in the right place — this is a core part of the Breathing and Exchange of Gases portion of the NCERT/CBSE Class 11 Biology syllabus. The same topic is frequently asked as a short-answer or assertion-reason question in NEET and other competitive medical entrance exams.