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MCAT Physics Question — Breath Rate

A 70kg man has a tidal volume of 500mL, a residual volume of 1.2L, an inspiratory reserve volume of 3L, and an anatomic dead space of 150mL. At rest, what breath rate is needed to provide the lungs with 5L of fresh air per minute?

A. 2 breaths per minute

B. 10 breaths per minute

C. 15 breaths per minute

D. cannot be determined

Click for Explanation

At rest, fresh air delivered to lungs is defined as the tidal volume minus the anatomic dead space. The tidal volume is volume of air taken in with each breath. The anatomic dead space is comprised of of anatomy along the respiratory path lacking alveoli for gas exchange (think mouth, trachea, bronchi).  Inspiratory reserve volume is not used at rest, and residual volume does not affect fresh air delivery. The normal human respiratory rate at rest is 12-18 breaths per minute.

500mL-150mL=350mL fresh air per breath. 15 breaths per minute are required to deliver 5L of fresh air in a minute.

A) 2 breaths per minute, incorrect, At rest inspiratory reserve volume is not used.

B) 10 breaths per minute, incorrect, Dead space must be subtracted from tidal volume to calculate fresh air delivery.

C) 15 breaths per minute, correct.

D) cannot be determined, incorrect, All necessary measurements are provided in the question stem.

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