Explain the individual stages of pulmonary ventilation in detail, making sure to indicate and explain the pressure changes that occur in the thorax during inhalation and exhalation.

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Pulmonary ventilation is primarily enabled through the contraction and expansion of the diaphragm, a layer of muscle that separates the thorax from the abdominal cavity. Ventilation is under nervous system control. Each breath is initiated by neurons in a respiratory control center located in the medulla oblongata. These neurons stimulate...

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Pulmonary ventilation is primarily enabled through the contraction and expansion of the diaphragm, a layer of muscle that separates the thorax from the abdominal cavity. Ventilation is under nervous system control. Each breath is initiated by neurons in a respiratory control center located in the medulla oblongata. These neurons stimulate the diaphragm and the external intercostal muscles to contract. When these muscles contract, the ribs raise, and the rib cage expands, increasing the total volume of the thorax. The expanded volume of the thorax and lungs brings about a negative pressure system in comparison with the atmospheric pressure outside of the body. The difference in pressure (higher atmospheric pressure in comparison with the lower pressure inside the thoracic cavity) forces air into the lungs. This is called inspiration.

Expiration, or the release of gas from the lungs, occurs whenever the neurons of the respiratory control center stop producing impulses. This causes the external intercostal muscles and the diaphragm to relax. The diaphragm subsequently moves upward, and the rib cage contracts, reducing the total volume of the thoracic cavity. This change in volume corresponds to the formation of a positive pressure system in the thorax in comparison with the external, atmospheric pressure. This positive pressure system (higher partial pressure in the thorax in comparison with the atmospheric pressure) forces air against the pressure gradient, outside of the body.

Because the lungs and thorax have a degree of elasticity, exhalation is an unforced maneuver whereas inhalation requires muscle contraction. The relaxation of the external intercostal muscles and diaphragm is unforced because the elastic tension in the thorax is released, allowing the organs to recoil. Forced exhalation is actually the result of the contraction of the abdominal muscles, not of changes that occur in the diaphragm.

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