Blood and the respiratory system
Blood and the respiratory system

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course

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.

FLIGHT ATTENDANT
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.
Interactive feature not available in single page view (see it in standard view).

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?

a. 

it did not change


b. 

by ¼


c. 

by ½


d. 

by ¾


The correct answer is c.

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)?

To use this interactive functionality a free OU account is required. Sign in or register.
Interactive feature not available in single page view (see it in standard view).

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?

a. 

0.167 mmHg


b. 

6 mmHg


c. 

16 mmHg


The correct answer is a.

a. 


Part 3

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

a. 

0.333 mmHg


b. 

3 mmHg


c. 

13 mmHg


The correct answer is a.

a. 


Part 4

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

a. 

Exhalation


b. 

Inhalation


c. 

Neither, it is constant


The correct answer is a.

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.

SK299_1

Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to University-level study, we offer two introductory routes to our qualifications. You could either choose to start with an Access module, or a module which allows you to count your previous learning towards an Open University qualification. Read our guide on Where to take your learning next for more information.

Not ready for formal University study? Then browse over 1000 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371