Gas exchange happens in the alveoli, tiny air sacs wrapped in capillaries. Oxygen moves into the blood and carbon dioxide moves out.
Diffusion
Gases move from high to low pressure. Alveolar oxygen is higher than in the blood arriving from the body, so oxygen moves in; carbon dioxide moves the other way.
The membrane
The barrier is very thin and very large. Fluid (pulmonary edema), scarring (fibrosis) or lost alveoli (emphysema) slow diffusion.
Matching air and blood
Good exchange needs air and blood in the same place. Pneumonia fills alveoli with fluid (blood without air); a pulmonary embolism blocks blood flow (air without blood).
Carbon dioxide and breathing
Carbon dioxide is the main driver of breathing. Breathing less raises CO2 and makes the blood acidic.
At the bedside
Low oxygen saturation tells you gas exchange is failing. Check breathing rate, effort, lung sounds and, in sick patients, a blood gas.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).