How Gas Exchange Works

The Miracle of Oxygen and Carbon Dioxide Exchange

Gas exchange is the primary function of the respiratory system. It occurs in the alveoli, where oxygen from inhaled air passes into the bloodstream and carbon dioxide — a waste product of cellular metabolism — moves from the blood into the lungs to be exhaled.

The Process Step by Step

1. Inhalation: The diaphragm contracts and moves downward. The intercostal muscles expand the ribcage. This creates negative pressure in the thoracic cavity, drawing air into the lungs.

2. Diffusion in the Alveoli: Oxygen molecules cross the thin alveolar walls and adjacent capillary walls (together only about 0.5 micrometres thick) and bind to haemoglobin in red blood cells. Simultaneously, carbon dioxide diffuses from the blood into the alveolar space.

3. Exhalation: The diaphragm and intercostal muscles relax, the thoracic cavity decreases in volume, and air rich in carbon dioxide is pushed out of the lungs.

Why This Matters

Efficient gas exchange depends on healthy alveoli, adequate blood flow to the lungs, and the proper thickness of the respiratory membrane. Conditions like emphysema, pulmonary fibrosis, and pulmonary oedema can impair this process, reducing the amount of oxygen delivered to your tissues.