# Pressure and volume relationship from TBR CBT4

#### Tokspor

This is a PV concept from TBR CBT4 that I am confused about.

Let's say you are at some depth in the ocean where the external pressure of the surrounding water is much greater than the pressure inside your lungs. If you hold your breath, why would the volume inside your lungs then decrease?

The explanation given is that the external pressure of the surrounding water causes the volume in the lungs to decrease. Doesn't holding one's breath mean maintaining a constant volume?

#### BerkReviewTeach

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This is a PV concept from TBR CBT4 that I am confused about.

Let's say you are at some depth in the ocean where the external pressure of the surrounding water is much greater than the pressure inside your lungs. If you hold your breath, why would the volume inside your lungs then decrease?

The explanation given is that the external pressure of the surrounding water causes the volume in the lungs to decrease. Doesn't holding one's breath mean maintaining a constant volume?
I believe the question is as follows:

7. If a scuba diver descended into the water and for a time neither inhaled nor exhaled, then what would happen to the air in his lungs?

A. Both the volume and the mass of the air in his lungs would decrease.
B. Both the volume and the mass of the air in his lungs would increase.
C. The volume of the air in his lungs would increase, while its mass would remain the same.
D. The volume of the air in his lungs would decrease, while its mass would remain the same.

You can treat this as a closed system, given that by not exhaling or inhaling, the amount moles of gas remains constant. Assuming that the body temperature doesn't change during descent, then only the external pressure changes. As external pressure increases (because of descent), the pressure outside will compress the diver's thoracic cavity, causing the lung to undergo a decrease in volume, until the pressure inside oquals the pressure outside. I believe they are looking for Boyle's law applied to a nonideal system.