Which best describes what happens during exhalation?

• A. The intercostal muscles contract, expanding the rib cage.
• B. Oxygen diffuses from the blood into the alveoli.
• C. The diaphragm contracts and moves downward, increasing the volume of the thoracic cavity and drawing air into the lungs.
• D. The diaphragm relaxes and moves upward, decreasing the volume of the thoracic cavity and forcing air out of the lungs.

Answer: (D)

Verified step by step guidance

1. Understand the process of exhalation: Exhalation is the phase of breathing where air is expelled from the lungs. It is primarily driven by the relaxation of respiratory muscles.
2. Identify the role of the diaphragm: During exhalation, the diaphragm relaxes and moves upward into a dome-shaped position. This reduces the volume of the thoracic cavity.
3. Analyze the effect on pressure: As the thoracic cavity's volume decreases, the pressure inside the lungs increases relative to the atmospheric pressure. This pressure gradient forces air out of the lungs.
4. Consider the role of intercostal muscles: The intercostal muscles also relax during exhalation, allowing the rib cage to return to its resting position, further decreasing thoracic cavity volume.
5. Summarize the correct answer: The diaphragm relaxes and moves upward, decreasing the volume of the thoracic cavity and forcing air out of the lungs. This is the best description of what happens during exhalation.