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# 2.1 Atmospheric pressure

If you have recently taken a flight on a commercial airline, you will be familiar with the instructions that are given in the event of a change in cabin pressure, such as in Video 6 below.

Video 6 An Open University airline safety video.
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These safety measures highlight the importance of pressures for gas exchange in the lungs. To understand this relationship, it is helpful to use Boyle’s law, which states that at a constant temperature (k), an increase in pressure (P) causes a proportional decrease in volume (V). Watch Boyle’s law in action in Video 7 below. (Make sure to open the link in a new window/tab so you can easily navigate back to this page.)

Link to Video 7 – The effect of increasing pressure on volume. [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)]

## Question 2 Increasing pressure

a.

it did not change

b.

by ¼

c.

by ½

d.

by ¾

As the pressure increased by a factor of 2, the volume of the air decreased by ½, from 1 litre to 0.5 litre.

In physiology, the unit of pressure is conventionally measured as millimetres (mm) of mercury (Hg). ‘Millimetres of mercury’ (mmHg) refers to the height of a column of mercury attached to an instrument that detects pressure (e.g. a sphygmomanometer). Other units of pressure, such as that used in Video 7, include bar, pounds per square inch (psi) and pascals (Pa). All units of pressure can be interconverted, so 1 bar = 14.5 psi, 1 psi = 51.7 mmHg and 1 mmHg = 133 Pa.

At sea level, the atmospheric pressure (i.e. the pressure exerted by the gases in the Earth’s atmosphere) is about 760 mmHg. During inhalation, the volume of the lungs increases and the pressure inside the lungs decreases below that of atmospheric pressure. This creates a pressure gradient that draws air into the lungs. During exhalation, the lungs return to their original size, pressure in the lungs rises compared with the atmospheric pressure and air moves out.

## Question 3 Boyle's law

Boyle's law is described by the following formula:

PV = k.

### Part 1

How would you rewrite the formula to calculate pressure (P)?

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P = k/V. To calculate pressure (P), divide the constant (k) by the volume (V).

a.

0.167 mmHg

b.

6 mmHg

c.

16 mmHg

a.

a.

0.333 mmHg

b.

3 mmHg

c.

13 mmHg

a.

a.

Exhalation

b.

Inhalation

c.

Neither, it is constant