Our efforts to apply the vapor pressure theory of osmosis to medical physiology have been difficult. The difficulty appears to be due to language differences. Therefore, we’re going to look at a very simple example from a popular medical text.
A cell is placed into pure water. The osmolality of pure water is much less than the solution within the cell. The pure water is hypotonic compared to the solution within the cell. Water will flow into the cell and cause it to swell.
If the cell is placed in a high concentration sugar solution, a hypertonic solution, the cell will shrink because water flows from the cell. The osmolality of the sugar solution is greater than the osmolality inside the cell.
The text mentions that there is a tremendous osmotic pressure difference across the cell membrane when one side of the membrane is exposed to pure water, and the other side contains the cellular solution. Furthermore, it states that this osmotic pressure causes flow across the cell membrane.
The vapor pressure model for osmosis states that vapor pressure is the motive force for osmotic flow across a membrane. With Raoult’s law, we know that the effective vapor pressure of water is reduced as the concentration of solutes is increased. Thus, a cell placed in pure water will expand because the effective vapor pressure of water in the cell is lower than the vapor pressure of pure water. Water flows into the cell. As we add sugar to the pure water, we reduce the effective vapor pressure of the pure water until it equals and then becomes less than the effective vapor pressure inside the cell. Note that we have assumed that the total pressure is the same inside and outside of the cell. The cell membrane cannot support much pressure difference.
Total pressure is like atmospheric pressure and applied osmotic pressure. Vapor pressure is the pressure that forms above a pure substance when placed in a vacuum container.
We are getting closer to applying the vapor pressure theory to the performance of a kidney. This gets us back to the origin of the Osmosis experiment. Jean-Antoine Nollet a French Priest and physicist discovered the phenomenon of osmosis in 1748. Perhaps he just wanted to know why his urine was yellow when wine was a popular beverage. He was completely surprised when water flowed up hill through a membrane into the wine….