Cambridge O Level Biology · Syllabus 5090 · Human Gas Exchange
Ventilation
What is Ventilation?
The movement of air into and out of the lungs, also called breathing. Ventilation is produced by muscular changes in the volume of the thoracic cavity: contraction of the external intercostal muscles and of the diaphragm increases that volume, and relaxation decreases it. Because the air already in the lungs then occupies a larger or a smaller space, the pressure inside the lungs falls below or rises above atmospheric pressure, and air moves down the resulting pressure difference. Ventilation does not itself exchange any gas; its function is to renew the air in the alveoli so that the concentration gradients on which gas exchange depends are maintained.
This definition is part of the Human Gas Exchange chapter in Cambridge O Level Biology.
Questions students ask about Ventilation
What is the difference between breathing and respiration?
Breathing, properly called ventilation, is the movement of air into and out of the lungs; it is a physical process driven by muscles changing the volume of the chest. Respiration is a chemical process that takes place inside living cells, in which energy is released from nutrient molecules such as glucose. Breathing supplies the oxygen that respiration needs and removes the carbon dioxide it produces, but no energy is released by breathing itself.
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.
What is the difference between breathing rate and breathing depth?
Breathing rate is how many breaths are taken in a given time, usually breaths per minute. Breathing depth is the volume of air moved in a single breath. Multiplying the two gives the ventilation rate, the total volume of air moved per minute. Counting breaths measures rate only, and says nothing about depth.