Expired air

Breathing (sometimes called ventilation) is the process that moves air in and out of the lungs, to allow the diffusion of oxygen into the blood and of carbon dioxide out of the blood. In the respiratory system of most vertebrates (excluding fish) breathing consists of inhalation (breathing in) and exhalation (breathing out). A complete breath is sometimes called a respiratory cycle. The respiratory rate or breathing rate is the number of breaths taken in one minute and is one of the four primary vital signs.

At the end of each exhalation the adult human lungs still contain 2.5 – 3.0 liters of air, termed the functional residual capacity (FRC). Breathing replaces only about 15% of this volume of gas with moistened ambient air with each breath. This ensures that the composition of the FRC changes very little during the breathing cycle, and remains significantly different from the composition of the ambient air. The partial pressures of the gases in the blood flowing through the alveolar capillaries equilibrate with the partial pressures of the gases in the FRC, ensuring that the partial pressures of carbon dioxide, and oxygen of the arterial blood, and therefore its pH, remain constant. The equilibration of the gases in the alveolar blood with those in the alveolar air (i.e. the gas exchange between the two) occurs by passive diffusion.

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