Breathing

Breathing (respiring or ventilating) is the rhythmic process of inhaling air into the lungs providing oxygen, and exhaling the air removing carbon dioxide, in the gas exchange process.

All aerobic organisms require oxygen for cellular respiration, which extracts energy from food and produces carbon dioxide as a waste product. External respiration (breathing) brings air to the alveoli where gases move by diffusion; the circulatory system then transports oxygen and carbon dioxide between the lungs and the tissues.

In vertebrates with lungs, breathing consists of repeated cycles of inhalation and exhalation through a branched system of airways that conduct air from the nose or mouth to the alveoli. The number of respiratory cycles per minute — the respiratory or breathing rate — is a primary vital sign. Under normal conditions, depth and rate of breathing are controlled unconsciously by homeostatic mechanisms that maintain arterial partial pressures of carbon dioxide and oxygen. Keeping arterial CO₂ stable helps maintain extracellular fluid pH; hyperventilation and hypoventilation alter CO₂ and thus pH and produce distressing symptoms.

Breathing also supports speech, laughter and certain reflexes (yawning, coughing, sneezing) and can contribute to thermoregulation (for example, panting in animals that cannot sweat sufficiently).

Mechanics

The lungs do not inflate themselves; they expand only when the thoracic cavity volume increases. In mammals this expansion is produced mainly by contraction of the diaphragm and, to a lesser extent, by contraction of the intercostal muscles, which lift the rib cage.