Henry's Law states that the solubility of a gas in a liquid is proportional to its partial pressure in the gaseous phase in contact with the liquid. In other words, the number of molecules of gas dissolved in solution is proportional to the partial pressure of the gas.

Henry's law helps to predict the amount of each gas which will go into solution. However, different gases have different solubilities which a constant of proportionality in the law takes into account.

The MCAT loves straightforward questions on Henry's Law because it is one of those physical science principles directly applicable to medical education. Henry's Law is crucial to the understanding of gas exchange in the lungs. Blood entering the lungs contains more carbon dioxide than would correspond to the partial pressure of carbon dioxide in the alveolar air, so carbon dioxide gas leaves the blood and enters the lungs. The opposite situation applies to oxygen gas, so oxygen leaves the alveolar air to enter the blood.