Blood and the respiratory system
Blood and the respiratory system

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Blood and the respiratory system

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.

Download this video clip.Video player: Video
Skip transcript: Video 6 An Open University airline safety video.

Transcript: Video 6 An Open University airline safety video.

Welcome to OpenAir flight 299, en route to Milton Keynes. Please pay attention to the safety features of this Airbus 320, which are shown on the flight card in the seat pocket in front of you.
In the event that the airplane loses cabin pressure, oxygen masks will drop down automatically. Remain seated with your seat belt fastened, pull down on the mask, and place it over your nose, using the elastic band to secure it to your head. Continue to breathe normally as oxygen will follow automatically into the bag. Remember to secure your own mask before assisting someone else.
Thank you for choosing OpenAir. We wish you a safe and pleasant flight.
End transcript: Video 6 An Open University airline safety video.
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

By how much did the volume of air in the cylinder decrease when the surrounding water pressure increased from 1 bar to 2 bar?


it did not change


by ¼


by ½


by ¾

The correct answer is c.


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).

Part 2

i) If k = 1, what will be the pressure of the gases if the volume of the lungs is 6 litres?


0.167 mmHg


6 mmHg


16 mmHg

The correct answer is a.


Part 3

ii) If k = 1, what will be the pressure if the volume is 3 litres?


0.333 mmHg


3 mmHg


13 mmHg

The correct answer is a.


Part 4

Is the pressure in the lungs higher during exhalation or inhalation?






Neither, it is constant

The correct answer is a.


As the volume of the lungs shrinks during exhalation, the pressure in the lungs increases above that of atmospheric pressure and air moves out of the lungs down the pressure gradient.

If you are unfamiliar with rearranging equations you might find our Mathematics for science and technology course helpful for brushing up.

Returning to the example of the aeroplane, the atmospheric pressure at cruising altitude (e.g. 243 mmHg at 30 000 feet or 9100 metres) is much lower than that at sea level (760 mmHg). If you were exposed to that same pressure as a passenger, the pressure in your lungs would be greater than that of the atmosphere, and you would be unable to draw a breath.

In the next section, you will learn how differences in pressures of gases in the atmosphere versus pressures of those gases in the lungs also drive O2 and CO2 exchange.


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