Contraction of the external intercostal muscles will lead to a(n) ___ in alveolar pressure and a subsequent ____ .

Question

BlackTom AI · Accepted Answer

Contraction of the external intercostal muscles elevates the ribs and expands thoracic volume, so alveolar pressure falls below atmospheric pressure, producing inhalation.

1) Relevant knowledge points and concepts
- Boyle’s law: for a fixed amount of gas at constant temperature, pressure and volume are inversely related (P ∝ 1/V; P1V1 = P2V2).
- Mechanics of breathing: contraction of external intercostals (and the diaphragm) increases thoracic cavity and lung volume. Air flows from region of higher pressure to lower pressure.
- Resting airway physics: at end-expiration alveolar pressure ≈ atmospheric pressure; changing lung volume creates pressure gradients that drive airflow.

2) Step-by-step reasoning
- Step 1: External intercostal contraction raises the ribs and enlarges the thoracic cavity.
- Step 2: Lung (alveolar) volume increases as the lungs expand to fill the cavity.
- Step 3: By Boyle’s law, when volume increases, alveolar pressure decreases relative to the initial state.
- Step 4: Alveolar pressure becomes lower than atmospheric pressure.
- Step 5: Air flows into the lungs down the pressure gradient until alveolar pressure again equals atmospheric pressure — this is inhalation.

3) Intermediate steps / simple relation
- Qualitative application of P1V1 = P2V2: if V2 > V1 then P2 < P1. Here V2 (expanded alveoli volume) > V1 (pre-inhalation), so P_alveoli drops below atmospheric pressure.

4) Why the correct answer is correct
- The physiological action (external intercostal contraction) increases volume, lowers alveolar pressure, and this negative pressure relative to atmosphere draws air in — i.e., inhalation. Thus “drop, inhalation” is the correct pairing.

5) Why the other options are incorrect
- "rise, exhalation": external intercostal contraction does not raise alveolar pressure; it increases volume, so this is reversed.
- "rise, inhalation": a rise in alveolar pressure would drive air out, not in, so it cannot cause inhalation.
- "drop, exhalation": a pressure drop produces inflow (inhalation), not outflow (exhalation).

6) Conclusion
The final answer is “drop, inhalation,” which matches the provided correct answer.

Contraction of the external intercostal muscles will lead to a(n) _ in alveolar pressure and a subsequent __ .单项选择题

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Contraction of the external intercostal muscles will lead to a(n) ___ in alveolar pressure and a subsequent ____ .单项选择题