What physical change occurs in the body during expiration?

• A. The diaphragm contracts and lowers
• B. The diaphragm relaxes, reducing thoracic cavity volume
• C. The rib cage expands
• D. Air is drawn into the lungs

Answer: (B)

During expiration, the process involves the relaxation of the diaphragm, which results in a reduction of the thoracic cavity volume. When the diaphragm relaxes, it moves upward into a more dome-shaped position. This change decreases the volume of the thoracic cavity, subsequently increasing the pressure in the lungs relative to the outside atmosphere. This pressure difference causes air to flow out of the lungs.

Understanding this mechanism is crucial, as it highlights the role of the diaphragm as a key muscle in the respiratory system. While contractions of the diaphragm are necessary for inhalation (increasing thoracic volume and drawing air in), its relaxation is essential for expiration. This is how the respiratory cycle operates effectively, alternating between inhalation and exhalation.