Ventilation occurs when:

• A. Muscles cause the volume of the thoracic cavity to change.
• B. The gas molecules pass between air and the blood.
• C. Changes in pressure cause a change in the size of the thoracic cavity.
• D. The lungs contract and relax, altering the size of the thoracic cavity.

Answer: (A)

Verified step by step guidance

1. Understand that ventilation is the process of moving air in and out of the lungs, which is essential for gas exchange between the air and the blood.
2. Recognize that the primary muscles involved in ventilation are the diaphragm and the intercostal muscles. These muscles contract and relax to change the volume of the thoracic cavity.
3. When the diaphragm contracts, it moves downward, increasing the volume of the thoracic cavity. This decrease in pressure inside the thoracic cavity allows air to flow into the lungs.
4. During exhalation, the diaphragm relaxes and moves upward, while the intercostal muscles relax, decreasing the volume of the thoracic cavity and increasing the pressure, which pushes air out of the lungs.
5. Understand that the changes in pressure within the thoracic cavity are crucial for the movement of air, and this is facilitated by the contraction and relaxation of the muscles, altering the size of the thoracic cavity.