The partial pressure of a gas is the pressure that the gas would exert if it alone occupied the whole volume of a mixture. It is a measure of how much of that gas is present, and is measured in kilopascals (kPa). The partial pressure of oxygen is written pO2.In a gas mixture, the partial pressure of each gas is proportional to the fraction of the mixture that it makes up:pO2 = fraction of oxygen × total pressure Worked example: air is about 21% oxygen and atmospheric pressure at sea level is about 101 kPa. pO2 = 0.21 × 101 = about 21 kPa.Oxygen always diffuses from a region of higher pO2 to a region of lower pO2, so pO2 works like a concentration gradient. Typical values in the body are:• Alveolar air: about 13 kPa. This is lower than in atmospheric air because inhaled air mixes with air left in the lungs, becomes saturated with water vapour and contains more carbon dioxide.
• Blood leaving the lungs: about 13 kPa, because it has equilibrated with alveolar air.
• Resting tissues: about 5 kPa.
• Actively respiring tissues: can fall to 2 kPa or less, because cells use oxygen rapidly.These differences create the gradients that let oxygen diffuse from alveoli into blood, and from blood into respiring cells. pO2 also determines how saturated haemoglobin is: high pO2 gives high saturation, low pO2 gives low saturation.At high altitude, atmospheric pressure is lower, so pO2 is lower even though air is still 21% oxygen.
• Blood leaving the lungs: about 13 kPa, because it has equilibrated with alveolar air.
• Resting tissues: about 5 kPa.
• Actively respiring tissues: can fall to 2 kPa or less, because cells use oxygen rapidly.These differences create the gradients that let oxygen diffuse from alveoli into blood, and from blood into respiring cells. pO2 also determines how saturated haemoglobin is: high pO2 gives high saturation, low pO2 gives low saturation.At high altitude, atmospheric pressure is lower, so pO2 is lower even though air is still 21% oxygen.