FILL IN EVALUATION FORM

PARTIAL PRESSURES OF GASES

• The partial pressure of a gas is a measure of the concentration of a gas in a mixture of gases

• The partial pressure (measured in mmHg or kPa) is the pressure that a particular gas exerts in a gas mixture. The pressure is caused by the impact of moving molecules against a surface.

• Gases which are dissolved in body fluids or tissues also have partial pressures. The dissolved gases move around and impact against surfaces exerting pressure in the same way a gas exerts its own partial pressure in a gas phase.

• Atmospheric pressure at sea level is approximately 760 mmHg

• Oxygen constitues 21 % of the atmospere, so its partial pressure is 21 % of 760 = 159 mmHg. Nitrogen constitutes 78 %, so its partial pressure is 78 % of 760 = 593 mmHg. Carbon dioxide constitutes only 0.04 %, so its partial pressure is 0.04 % of 760 = 0.3 mmHg.

Gas	Partial pressure in inspired air	Partial pressure in alveolar air
H2O	Variable	47 mmHg
CO2	0.3 mmHg	40 mmHg
O2	159 mmHg	105 mmHg
N2	593 mmHg	568 mmHg
Total	760 mmHg	760 mmHg

• The partial pressure of each gas in the respiratory mixture forces gas molecules to dissolve into the alveolar membrane and then into the blood of the pulmonary capillaries

• Conversely, the partial pressure of each gas already in solution in the blood means that some gas molecules escape back into the alveoli

• There is diffusion in both directions but it is the difference between the two partial pressures that determines the net direction of diffusion

• In the case of oxygen, PO₂ in inspired air is 159 mmHg and is 40 mmHg in venous blood. The net direction of diffusion therefore is from the alveoli to the blood resulting in a PO₂ in arterial blood of 95 mmHg.

• In the case of carbon dioxide, PCO₂ in inspired air is 0.3 mmHg and is 46 mmHg in venous blood. The net direction of diffusion therefore is from the blood to the alveoli resulting in a PCO₂ in arterial blood of 40 mmHg.

Back to gaseous exchange in the alveoli

Back to oxygen transport