expiration

physiology

Also known as: exhalation

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June 26, 2026

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Expiration, or exhaling, is the process of air moving out of the lungs. This occurs when the air pressure within the lungs becomes higher than the atmospheric pressure. This pressure difference is created by the relaxation of the diaphragm and other respiratory muscles, which allows the chest cavity and lungs to return to their resting volume.12

Expiration is a crucial part of the breathing cycle, controlled by neural networks in the brainstem. These networks regulate the muscles involved in breathing, ensuring rhythmic air exchange. The respiratory system can adjust its patterns to meet metabolic demands, such as during exercise, and respond to changes in blood gases like carbon dioxide. While normal expiration is largely passive, forceful expiration involves the contraction of abdominal and intercostal muscles.3415

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Respiratory mechanisms
…inhalation (inspiration) and exhalation (expiration). Inspiration and expiration are equally long, equally deep, and transport the same amount of air during the same period of time, approximately half a litre (one pint) of air per breath at rest in most adults. Recordings (made with a device called a pneumograph)…
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…inhalation (inspiration) and exhalation (expiration). Inspiration and expiration are equally long, equally deep, and transport the same amount of air during the same period of time, approximately half a litre (one pint) of air per breath at rest in most adults. Recordings (made with a device called a pneumograph)…
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human respiratory system
The mechanics of breathing
…blown from the lungs (expiration). The flow of air is rapid or slow in proportion to the magnitude of the pressure difference. Because atmospheric pressure remains relatively constant, flow is determined by how much above or below atmospheric pressure the pressure within the lungs rises or falls.
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…blown from the lungs (expiration). The flow of air is rapid or slow in proportion to the magnitude of the pressure difference. Because atmospheric pressure remains relatively constant, flow is determined by how much above or below atmospheric pressure the pressure within the lungs rises or falls.
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diaphragm
…and the thoracic cage produce expiration. The diaphragm is also important in expulsive actions—e.g., coughing, sneezing, vomiting, crying, and expelling feces, urine, and, in parturition, the fetus. The diaphragm is pierced by many structures, notably the esophagus, aorta, and inferior vena cava, and is occasionally subject to herniation (rupture). Small…
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…and the thoracic cage produce expiration. The diaphragm is also important in expulsive actions—e.g., coughing, sneezing, vomiting, crying, and expelling feces, urine, and, in parturition, the fetus. The diaphragm is pierced by many structures, notably the esophagus, aorta, and inferior vena cava, and is occasionally subject to herniation (rupture). Small…
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human respiratory system
Central organization of respiratory neurons
…discharge in both inspiration and expiration. It is thought that the respiratory cycle of inspiration and expiration is generated by synaptic interactions within these groups of neurons.
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…discharge in both inspiration and expiration. It is thought that the respiratory cycle of inspiration and expiration is generated by synaptic interactions within these groups of neurons.
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