Exhalation

Exhalation (or expiration) is the flow of the breath out of an organism.

In animals, it is the movement of air from the lungs out of the airways, to the external environment during breathing.

This happens due to elastic properties of the lungs, as well as the internal intercostal muscles which lower the rib cage and decrease thoracic volume.

As the thoracic diaphragm relaxes during exhalation it causes the tissue it has depressed to rise superiorly and put pressure on the lungs to expel the air.

During forced exhalation, as when blowing out a candle, expiratory muscles including the abdominal muscles and internal intercostal muscles generate abdominal and thoracic pressure, which forces air out of the lungs.

Exhaled air is 4% carbon dioxide,^[1] a waste product of cellular respiration during the production of energy, which is stored as ATP.

Exhalation has a complementary relationship to inhalation which together make up the respiratory cycle of a breath.

Exhalation and gas exchange[edit]

The main reason for exhalation is to rid the body of carbon dioxide, which is the waste product of gas exchange in humans. Air is brought into the body through inhalation. During this process air is taken in by the lungs. Diffusion in the alveoli allows for the exchange of O₂ into the pulmonary capillaries and the removal of CO₂ and other gases from the pulmonary capillaries to be exhaled. In order for the lungs to expel air the diaphragm relaxes, which pushes up on the lungs. The air then flows through the trachea then through the larynx and pharynx to the nasal cavity and oral cavity where it is expelled out of the body.^[2] Exhalation takes longer than inhalation and it is believed to facilitate better exchange of gases. Parts of the nervous system help to regulate respiration in humans. The exhaled air is not just carbon dioxide; it contains a mixture of other gases. Human breath contains volatile organic compounds (VOCs). These compounds consist of methanol, isoprene, acetone, ethanol and other alcohols. The exhaled mixture also contains ketones, water and other hydrocarbons.^[3][4]

It is during exhalation that the olfaction contribution to flavor occurs in contrast to that of ordinary smell which occurs during the inhalation phase.^[5]