Physiology > Gases > Dissolved Gases & Concentration
Dissolved gases & concentration
Units of concentration
In chemistry class, you express concentration in terms of moles per liter (molar) or similar units to express the number of molecules in a given volume. Those units might work well for gases dissolved in water, but they don't work well in the gas phase.
Gases diffused down their partial pressure gradients. Therefore, in physiology, gas concentration is usually expressed in terms of partial pressure units. In physiology classes (and in some figures in Campbell), partial pressure is expressed in mm of mercury, or Torr. In this class, we'll often use the intuitively familiar units, atmospheres (atm), where 1 atm is the standardized pressure of air at sea level.
Suppose you have an open beaker of water. When a gas diffuses from the air into the water, the rate of diffusion depends on the concentration gradient between the air and the water. Individual molecules may move in any direction, but the net result will be that the concentration gradient gets weaker over time. When ΔC/x = 0, there is no more concentration gradient and no more net diffusion. The air and the water are at equilibrium. At that point, what will the molar concentration of the gas be? That depends on the partial pressure of the gas and its solubility. Oxygen isn't very soluble in water, but it is very soluble in blood, thanks to oxygen-carrying pigments such as hemoglobin. The function of hemoglobin will be covered in depth in lecture.
This page updated September 20, 2011