Summary

Cells use oxygen to carry out metabolism and generate energy, and in the process they produce carbon dioxide, a harmful waste that must be removed. Animals have evolved mechanisms to bring oxygen to their cells and to take carbon dioxide away.

In humans this is done by a well-developed respiratory system of two lungs and their associated air passages. Respiration involves several steps:

• Breathing — atmospheric air is drawn in (inspiration) and alveolar air is pushed out (expiration).
• Exchange at the alveoli — oxygen and carbon dioxide are exchanged between deoxygenated blood and the alveolar air.
• Transport — these gases are carried throughout the body by the blood.
• Exchange at the tissues — oxygen and carbon dioxide are exchanged between oxygenated blood and the tissues.
• Cellular respiration — the cells finally use the oxygen.

Inspiration and expiration work by creating pressure differences between the atmosphere and the alveoli, using specialised muscles — the intercostal muscles and the diaphragm. The volumes of air moved during breathing can be measured with a spirometer, and these measurements have clinical value.

Gas exchange at both the alveoli and the tissues happens by diffusion. The rate of diffusion depends on the partial pressure gradients of oxygen and carbon dioxide, the solubility of the gases, and the thickness of the surface across which diffusion occurs. In the body these factors favour the movement of oxygen from the alveoli into the deoxygenated blood and from the oxygenated blood into the tissues, while favouring the movement of carbon dioxide in the opposite direction, from the tissues to the alveoli.

Oxygen is transported mainly as oxyhaemoglobin. Where oxygen pressure is high, as in the alveoli, oxygen binds to haemoglobin; where oxygen pressure is low and carbon dioxide and hydrogen ion levels are high, as in the tissues, it is easily released. …