Respiratory organs and gas exchange

In calm breathing about 500 ml of air enters and leaves the lungs (tidal volume). The greatest amount of air that can be breathed out after a deep inhalation is the vital capacity; on average about 3500 ml (measured with a spirometer), more in trained people. There is also residual air that cannot be exhaled. Inhaled air contains about 21% oxygen and 0.04% carbon dioxide; exhaled air about 16% oxygen and 4% carbon dioxide. Gases move from a region of higher concentration to lower (diffusion): in the alveolus there is more oxygen and in blood less – oxygen enters blood and binds haemoglobin; carbon dioxide moves from blood into the alveolus. In tissues it is the reverse: oxygen goes from blood to cells and carbon dioxide from cells to blood. Carbon monoxide (CO) binds haemoglobin very firmly and prevents oxygen transport; it is colourless and odourless and builds up when stoves or gas appliances work badly. If carbon monoxide is suspected (headache, nausea, weakness), do not go into the room yourself: get outside at once, call adults, and phone 103 and the gas service (104); adults bring the poisoned person into fresh air only if it is safe. If breathing stops after drowning – call 103 at once and trained adults begin CPR.