Ventilation-perfusion coupling is the relationship between the amount of air reaching the air sacs of the lungs and the amount of blood reaching the lungs. It is common to express ventilation and perfusion together as a ratio.
The lungs contain small air sacs, or alveoli. The amount of oxygen that reaches the alveoli is the ventilation. Perfusion is the amount of blood that gets to the alveoli. For adults, there are approximately 200 milliliters of oxygen in one liter of blood and 210 milliliters in dry air. This makes the best ventilation/perfusion ratio about 1.05 under dry conditions and 1.0 under humid conditions.
In most people, the actual ratio differs vertically within the lungs. The top part of the lung, or apex, sits above the heart and naturally has a higher ventilation-perfusion ratio, while the lower part, or base, of the lung is below the heart and has a lower ratio. This is because ventilation and perfusion become increasingly efficient lower in the lungs but the increase is greater for perfusion.
If an area of the lung occurs receives blood but no oxygen, a pulmonary shunt occurs. Dead space refers to areas of the lungs that receive air but no blood flow and do not function in gas exchange.