Gas Physiology

Gases in nature

Gases in nature are rarely seen on their own, more commonly being part of a mixture. For example, air is a mixture of nitrogen (78%), oxygen (21%), and a small amount of other gases such as carbon dioxide and water vapour.

For oxygen to make it to our tissues it needs to cross the thin alveolar membrane then dissolve into the blood. This is a passive process, occurring by diffusion from an area of relatively high partial pressure to an area of relatively low partial pressure.

Pressure vs concentration

When discussing gas exchange in the body we tend to talk about 'partial pressure'. But why?

Some gases dissolve easily in water, and some don’t. Nitrogen, for example, is effectively insoluble in water, which is why we don’t end up with nitrogen in our blood despite it making up almost 80% of the air we breathe. Carbon dioxide is readily soluble in water, which is one of the reasons it is used instead of other gases to make drinks fizzy. Oxygen is somewhat soluble in water – more so than nitrogen but less so than CO₂.

Imagine we have a beaker half full of water and half full of oxygen. Initially the concentration of oxygen is 100% in the top of the beaker and 0% in the water.

Some oxygen will dissolve into the water, but because oxygen isn’t highly soluble it will never reach a state in which 50% is dissolved and 50% remains undissolved. This seems strange, because we might expect it to continue moving down its concentration gradient.

However if we calculate the pressure of oxygen (PO₂) we see that initially the pressure is 10kPa (for example) in the top half and 0kPa in the water. When no more oxygen can dissolve the PO₂ is 10kPa in both halves of the beaker.

Due to its relatively low solubility, the pressure exerted by oxygen dissolved in the water is equal to the pressure exerted by the oxygen above the water.

This means that oxygen (and other gases) dissolve down a pressure gradient, rather than a concentration gradient.

Gases in the blood

In the example above we were dealing with the pressure of one gas. In a mixture of gases we use partial pressure when talking about individual gases, because each gas contributes a part of the overall gas pressure.

In the context of gases dissolving into and out of the blood these will move according to their partial pressure gradients.

For example, if the partial pressure of oxygen in air was 30kPa and the partial pressure of oxygen in blood was 12kPa, we would expect oxygen to move down it’s partial pressure gradient into the blood.

Note that in reality the partial pressure of oxygen in the blood never fully equalises with the partial pressure of oxygen in air.