Gas exchange or respiration takes place at a respiratory surface - a boundary between the external environment and the interior of the body. For unicellular organisms the respiratory surface is simply the cell membrane, but for large organisms it usually is carried out in respiratory systems.

This name can cause problems - in biology the word "respiration" can mean cellular respiration or metabolism (ATP generation inside cells), however sometimes (such as here) it can also refer to breathing (which is how the word is most often used by non-biologists).

Gases cross the respiratory surface by diffusion, so from Fick's law we can predict that respiratory surfaces must have:

• a large surface area
• a thin permeable surface
• a moist exchange surface

Many also have a mechanism to maximise the diffusion gradient by replenishing the source and/or sink.

Control of respiration is due to rhythmical breathing generated by the phrenic nerve to stimulate contraction and relaxation of the diaphragm during inspiration and expiration. Ventilation is controlled by partial pressures of oxygen and carbon dioxide and the concentration of hydrogen ions. The control of respiration can vary in certain circumstances such as during exercise.

Gas exchange in humans and mammals

In humans and mammals, respiratory gas exchange or ventilation is carried out by mechanisms of the lungs. The actual exchange of gases occurs in the alveoli.

Convection occurs over the majority of the transport pathway. Diffusion occurs only over very short distances.

Gas exchange occurs only at pulmonary and systemic capillary beds.

Blood always contains both oxygen and carbon dioxide.

Trace gases present in breath at levels lower then a part per million are ammonia, acetone, isoprene. These can be measured using SIFT-MS Selected Ion Flow Tube Mass Spectrometry.

Transport of oxygen, carbon dioxide, and hydrogen ions

Blood carries oxygen, carbon dioxide and hydrogen ions between tissues and the lungs.

The largest fraction of carbon dioxide transported is in the form of bicarbonate.