Inhalation occurs when the:

• A. diaphragm contracts and moves downward, increasing thoracic volume
• B. intercostal muscles relax, causing the rib cage to move downward
• C. diaphragm relaxes and moves upward, decreasing thoracic volume
• D. pressure inside the lungs increases above atmospheric pressure

Answer: (A)

Verified step by step guidance

1. Understand the mechanics of inhalation: Inhalation is the process of drawing air into the lungs, which requires an increase in thoracic volume and a decrease in pressure inside the lungs relative to atmospheric pressure.
2. Identify the role of the diaphragm: During inhalation, the diaphragm contracts and moves downward, which increases the vertical dimension of the thoracic cavity and contributes to the increase in thoracic volume.
3. Examine the role of intercostal muscles: The external intercostal muscles contract during inhalation, causing the rib cage to move upward and outward, further increasing thoracic volume.
4. Relate thoracic volume to pressure changes: According to Boyle's Law, an increase in thoracic volume leads to a decrease in pressure inside the lungs (intrapulmonary pressure), making it lower than atmospheric pressure and allowing air to flow into the lungs.
5. Eliminate incorrect options: Review the provided choices and eliminate options that describe relaxation of the diaphragm or intercostal muscles, as these actions are associated with exhalation rather than inhalation.