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.
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.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?
Answer
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)?
Answer
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?
Part 3
ii) If k = 1, what will be the pressure if the volume is 3 litres?
Part 4
Is the pressure in the lungs higher during exhalation or inhalation?
Answer
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.
OpenLearn - Blood and the respiratory system
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