|MadSci Network: Biochemistry|
Think of osmotic pressure as a difference in water activity (or concentration) across a membrane or a semipermeable structure such as the rolled up cylinder of endothelial cells that creates a capillary. That is, think in terms of the water relative to the solute molecules. The osmotic pressure is, then, simply the difference in the chemical potential of water across the membrane or structure. The osmolarity is the concentration of solute in a compartment on one side of that barrier that affects the water activity.
This means that water will flow from the side of high water activity (or concentration) to low water activity. The side of high water activity has a lower osmolarity of dissolved solute molecules relative to the side with low water activity -- lower water activity means more solute molecules per volume of solvent and less water, and, hence higher osmolarity.
So now let's visit the example you present: sodium ions flow into a cell. The cytoplasm in the cells undergoes an increase in sodium concentration (in the absence of water inflow). Since sodium is osmotically active -- it changes the concentration of water -- the water concentration decreases. Across the membrane of the cell, this flow of sodium ions has changed the difference in activity of water across the membrane. The activity is low inside the cell with respect to outside, and so water flows in. This is typically regarded as a "decrease is osmotic pressure of the inside with respect to outside" to account for the water flow direction, which is to the inside.
Clinically, a lot of terms get thrown around. Colloid osmotic pressure, oncotic pressure and the like refer to the osmolarity difference across a membrane, and are generally used in the clinical lab to refer to the total amount of osmotically active solute in a fluid, such as blood, blood plasma, or extracellular fluid (serum). That is, they all refer to the same thing -- the concentration of osmotically active species, salts, sugars, proteins (the "colloids" originally termed when people thought they exerted more osmotic pressure than the salts -- they don't -- the salt is about 200 milliosmoles per liter compared to the 35 to 70 microosmoles per liter of protein). Anyway, you will see "colloid osmotic pressure" of a patients plasma and it means the osmolarity of the solutes in the plasma.
Osmotic pressure and its attendant terms refer to tendency for water to move based on the solute concentration differences. Remember that the other force that can make water move is the hydrostatic pressure. Osmotic and hydrostatic pressures. The hydrostatic pressure drop across a capillary will cause water to move out across the endothelium, unless the tissue (extracellular) fluid or serum has an osmotic pressure that can balance that fluid efflux, which means that it is large enough to create an osmotic pressure-driven flow from tissue to blood that equals the hydrostatic pressure driven flow. Hydrostatic pressure-driven flow in the circulation means blood pressure (or mean arterial pressure)-driven flow.
So all liquid exchange between blood and extracellular fluid is determined by the net sum of the differences in hydrostatic (BP) driven flow and osmotic flows determined chemical solute concentration differences. It is useful to keep the two sources of flow in mind.
Here is one reference for osmotic pressure (most on the web are 'not so good' meaning confusing) http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/ospcal.html
A pithy description of colloid osmotic pressure: http://www.anaesthesiamcq.com/FluidBook/fl2_4.php
Starling's law in a sentence http://cancerweb.ncl.ac.uk/cgi-bin/omd?Starling's+hypothesis
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