When all muscles of respiration are relaxed and the difference between alveolar pressure and atmospheric pressure is zero, lung volume is equal to:

Question

BlackTom AI · Accepted Answer

At the end-expiratory point when alveolar pressure equals atmospheric pressure (Palv − Patm = 0) and respiratory muscles are relaxed, the lung is at functional residual capacity (FRC).

1) Relevant concepts:
- Alveolar (intrapulmonary) pressure (Palv) and atmospheric pressure (Patm) determine airflow; no flow occurs when Palv = Patm.
- Functional residual capacity (FRC) is the lung volume remaining after a normal, passive exhalation when respiratory muscles are relaxed.
- FRC = expiratory reserve volume (ERV) + residual volume (RV).
- Mechanical equilibrium at FRC: inward elastic recoil of the lungs is exactly balanced by the outward recoil of the chest wall; intrapleural pressure is negative but constant.

2) Step-by-step reasoning:
- When muscles of respiration are relaxed at the end of a normal tidal exhalation, there is no net airflow because Palv = Patm (Palv − Patm = 0).
- That mechanical equilibrium corresponds to the specific lung volume called FRC, not to maximal or minimal lung volumes.
- The balance of forces (lung recoil vs. chest wall recoil) establishes that volume; respiration would begin from this point with the next inspiratory muscle contraction.

3) Calculations / intermediate relations:
- By definition: FRC = ERV + RV.
- Using typical textbook values as an example: ERV ≈ 1200 mL, RV ≈ 1200 mL → FRC ≈ 2400 mL. (Numbers vary with body size; the relation remains.)

4) Why the correct answer is correct:
- FRC is the passive equilibrium volume reached when muscles are relaxed and Palv = Patm; it represents the starting volume at the end of a normal expiration.

5) Why other options are incorrect:
- Vital capacity: the maximal volume change from full inspiration to full expiration (IRV + TV + ERV); not the resting volume with muscles relaxed.
- Tidal volume: the volume moved in a normal breath (TV ≈ 500 mL), not the residual lung volume at muscle relaxation.
- Residual volume: volume remaining after maximal forced expiration; this is part of FRC but is smaller than FRC and does not represent the passive equilibrium after a normal expiration.

6) Conclusion:
The lung volume when all respiratory muscles are relaxed and alveolar and atmospheric pressures are equal is functional residual capacity (FRC), matching the provided answer.

When all muscles of respiration are relaxed and the difference between alveolar pressure and atmospheric pressure is zero, lung volume is equal to:单项选择题

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