Respiratory physiology: Gas Transport

Partial Pressure

In a mixture of gases, each constituent gas exerts a partial pressure. The partial pressure of a gas is that gases contribution to total barometric pressure. Partial pressure may be described as the notional pressure of that constituent gas if it alone occupied the entire volume of the original mixture at the same temperature

Air in the atmosphere is composed of 78% Nitrogen, 21% Oxygen and 1% other gases which include argon and carbon dioxide. 1 atmosphere describes the mean sea level air pressure and is equivalent to 760 mmHg or 101 kPa. The constituent gases contribute to that pressure according to their percentage composition as shown in the table below:





Note expired air is a combination of dead space gas and gas that has taken part in gas transfer at the alveolar interface hence the pO2 in expired air is higher than in alveolar air.

Gases in solution

In medicine we often encounter gases that are dissolved in solution such as the gases above becoming dissolved in blood plasma. The amount of gas that dissolves in solution is directly proportional to the partial pressure of that gas above the liquid. Another way to think is higher pressure increases solubility.

The proportionality constant is called Henry's law constant. Thus Henry's law may be written as:

C=kPgas

Where:

• C: is the solubility of a gas at a fixed temperature in a particular solvent
• k: Henry's law constant
• Pgas: partial pressure of the gas