Breathing

Breathing (respiration or ventilation) is the rhythmic process of moving air into (inhalation) and out of (exhalation) the lungs to enable gas exchange with the internal environment, primarily to remove carbon dioxide and take in oxygen.

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).

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. During forceful inhalation accessory muscles may augment the pump-handle and bucket-handle movements of the ribs to further increase chest volume. At rest exhalation is largely passive as inhalatory muscles relax and the elastic recoil of the lungs and chest wall returns the chest to its resting position. At this resting point the lungs contain the functional residual capacity(about 2.5–3.0 L in an adult human).

During heavy breathing (hyperpnea), such as with exercise, exhalation also involves active contraction of the abdominal muscles, which pushes the diaphragm upward and reduces end-exhalatory lung volume. Even at maximum exhalation a normal mammal retains residual air in the lungs.

Diaphragmatic (or abdominal) breathing produces visible abdominal movement; use of accessory muscles with clavicular elevation is seen in labored breathing, for example during severe asthma or chronic obstructive pulmonary disease (COPD) exacerbations.

Air is ideally inhaled and exhaled through the nose. The nasal cavities — divided by the nasal septum and lined with convoluted conchae — expose inhaled air to a large mucosal surface so it is warmed and humidified and particulate matter is trapped by mucus before reaching the lower airways. Some of the heat and moisture are recovered during exhalation when air passes back over cooler, partially dried mucus.