Cambridge O Level Biology · Syllabus 5090 · Human Gas Exchange
Gas Exchange
What is Gas Exchange?
The movement of oxygen and carbon dioxide between an organism and its environment by diffusion across a specialised surface. In humans this occurs at the alveoli of the lungs: oxygen diffuses from the alveolar air, where its concentration is higher, through the alveolar wall and the capillary wall into the blood, while carbon dioxide diffuses in the opposite direction, from the blood where its concentration is higher into the alveolar air. Both gases move down their own concentration gradients, so the process is passive and requires no energy from respiration.
This definition is part of the Human Gas Exchange chapter in Cambridge O Level Biology.
Questions students ask about Gas Exchange
Where in the body does gas exchange happen?
At the alveoli, the microscopic air sacs at the ends of the finest bronchioles in the lungs. Nowhere else in the gas-exchange system does any gas move between the air and the blood. The nose, larynx, trachea, bronchi and bronchioles carry air but exchange none of it.
Why are the alveoli so good at gas exchange?
There are very many of them, so the total surface area is very large. The alveolar wall and the capillary wall are each one cell thick, so the distance a gas must travel is extremely short. The lining is moist, so gases dissolve before they diffuse. A dense network of capillaries carries absorbed oxygen away and delivers carbon dioxide, and continuous ventilation refreshes the air on the other side. The last two matter most, because they are what keep the concentration gradients steep.