Final answer:
Exhalation is generally a passive process facilitated by the diaphragm's relaxation and lung tissue recoil. Gravity's influence on this process can shift the position of the diaphragm, affecting the volume changes in the thorax necessary for normal breathing. Forced expiration utilizes additional muscle contractions to actively push air out of the lungs.
Step-by-step explanation:
Understanding Exhalation
Exhalation, or expiring, is predominantly a passive process that involves the relaxation of the diaphragm and the recoil of the lung tissue after inhalation. When in the supine position, gravity exerts an expiratory force on the abdomen, which can influence the relaxation point of the diaphragm and thus affect breathing dynamics. During normal or gentle exhalation, muscle relaxation results in a decrease in the thoracic cavity's volume and an increase in intrapulmonary pressure, causing air to flow out of the lungs. With forced breathing, such as during exercise or while singing, accessory muscles including abdominal muscles contract to aid in forcing air out more vigorously.
Gravity's impact on lung ventilation can be particularly noted when an individual is standing; the lower parts of the lungs are stretched more than the upper parts, which can influence the lung's ventilation. The Valsalva maneuver, typically used during defecation or childbirth, is an example of how the diaphragm and abdominal muscles work synergistically to affect pressures within the thoracic and abdominal cavities.