10.10 Hypoxia

10.10.2 Define hypoxia.

Hypoxia is a condition where there is insufficient oxygen reaching the body's tissues.

10.10.4 State the partial pressure of oxygen both inside and outside the lungs at sea level.

As altitude increases, atmospheric pressure decreases.

At approximately 18,000 ft, atmospheric pressure is about half that at sea level.

Although atmospheric pressure decreases with altitude, the percentage of oxygen in the atmosphere remains approximately 21%.

Partial pressure

Partial pressure is the pressure exerted by an individual gas within a mixture of gases.

At sea level:

• atmospheric pressure is approximately 760 mmHg;
• oxygen makes up approximately 21% of the atmosphere; therefore
• the partial pressure of oxygen outside the lungs is approximately 160 mmHg.

Within the lungs, the presence of water vapour and carbon dioxide reduces the partial pressure of oxygen to approximately 105 mmHg.

Key point

Sea level oxygen partial pressure:

Outside the lungs — approximately 160 mmHg

Inside the lungs — approximately 105 mmHg

10.10.6 Explain the mechanical effect of the partial pressure of oxygen on oxygen transfer in the lungs.

The oxygen in the alveoli of the lungs has a higher partial pressure than the oxygen in the blood returning to the lungs after circulating around the body.