The mammalian (hence human) gas exchange system is made of the trachea, from which the bronchi branch off, followed by the bronchioles into the lungs, and finally the alveoli, which are the functional unit of the lungs. Of course this is nonsense without an image:
Air enters the lungs via the trachea, bronchi and bronchioles into the tiny air sacs – the alveoli. The epihelium of the alveoli is extremely thin (just one-cell wide, in fact) to allow fast diffusion of oxygen into the red blood cells, and of carbon dioxide out of them. The capillaries surrounding alveoli are so narrow, that the red blood cells have to be squished in order to pass through. This shortens the diffusion pathway, which in turn increases the rate of diffusion.
What allows diffusion to take place, of course, is the concentration gradient formed between the air in the alveoli and the red blood cells. Red blood cells deprived of oxygen and loaded with carbon dioxide (the blue/purple ones) will release carbon dioxide into the fresh air, then take up oxygen from it afterwards.
Since lungs aren’t made of muscle, how is their movement brought about in ventilation (breathing)? Intercostal (between-ribs) muscles and the diaphragm are responsible. Their contraction is caused by nerve signals from the respiratory centre in the medulla (in the brain). This results in the intercostal muscles pulling the ribs up, while the diaphragm is flat, and the abdominal organs are pushed downwards. The thorax (chest cavity) increases in volume, so lowers its pressure below that of the atmosphere, resulting in air being drawn into the lungs. Exhaling, on the other hand, does not require muscular activity. Elastic recoil of the muscles, as well as the weight of the ribcage and abdominal organs, result in the pressure inside the lungs increasing, therefore pushing the air back outside.
In medicine, it’s important to have a calculation for the amount of air a person takes in or out over time.
Pulmonary ventilation = Tidal volume x Ventilation rate
Tidal volume is the volume of air inhaled or exhaled in one breath.
Ventilation rate is the number of breaths taken in one minute.
So, if someone was breathing 20 times a minute, a volume of 300 cm3 per breath, then what is their pulmonary ventilation?
By the above formula, 300 cm3 x 20 = 6000 cm3. Simplified, this is 6 litres per minute.