Most human cells require a continual supply of oxygen for aerobic respiration and produce carbon dioxide. The gas exchange system makes that possible in an organism too large for diffusion alone.
Air travels down the trachea, which divides into two bronchi, then into smaller bronchioles, ending in millions of tiny air sacs called alveoli.
Gas exchange happens in the alveoli, and they are well adapted for it: there are a great many of them, giving a large surface area; their walls are one cell thick, so the diffusion distance is short; they have a rich blood supply, which keeps the concentration gradient steep; and they are moist, so gases dissolve.
Oxygen diffuses from the alveoli into the blood, and carbon dioxide diffuses the other way.
Breathing is a pressure effect. To breathe in, the ribs move up and out and the diaphragm flattens, so the volume of the chest increases and the pressure inside falls below atmospheric pressure, so air flows in.
To breathe out, the ribs move down and in and the diaphragm domes upwards, so the volume decreases, the pressure rises, and air flows out.
Lung volume can be measured by breathing out into an inverted container of water and reading off the volume of water displaced.
Exercise makes the breathing rate and depth increase, so more oxygen reaches the muscles and more carbon dioxide is removed.
Asthma narrows the airways and produces extra mucus, making it harder to move air; an inhaler relaxes the airway muscles.
Smoking damages the cilia that clear mucus, destroys alveoli walls and so reduces the surface area for exchange, and the carbon monoxide in smoke reduces how much oxygen the blood can carry.
Plants exchange gases too, through pores in the underside of the leaf called stomata, each controlled by a pair of guard cells.
Stomata allow gases to diffuse between the leaf and the atmosphere, and close to reduce water loss. During photosynthesis there is usually a net movement of carbon dioxide into the leaf and of oxygen out of it. Water vapour also escapes through them, which is transpiration.
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