Final answer:
During quiet respiration, the diaphragm and external intercostal muscles contract. Internal intercostals do not contract during this process; they are mainly active during forced expiration. The correct options are A: Diaphragm and C: External intercostals.
Step-by-step explanation:
The muscles that contract during quiet respiration are the diaphragm and the external intercostals. The diaphragm contracts to increase thoracic cavity volume and decrease pressure, allowing air to flow into the lungs. Meanwhile, the contraction of the external intercostal muscles raises the rib cage, which expands it, further aiding in inspiration. The internal intercostals are not involved in quiet breathing; rather, they are engaged mainly during forced expiration.
Therefore, the correct answer to the question 'Which of the following muscles contract during quiet respiration?' is:
- A: Diaphragm
- B: Internal intercostals - Not during quiet respiration
- C: External intercostals
- D: None of the above - Incorrect
- E: All of the above - Incorrect as internal intercostals do not contract during quiet respiration
So, the correct options are A and C, the diaphragm and external intercostals.