Teaching Plasma Osmolarity vs. Colloid‐Osmotic Pressure in Medical Physiology

Body fluid distribution is a clinically highly relevant concept in human physiology. It is generally covered early in medical physiology courses by discussing osmosis, defining fluid compartments and walking through scenarios of hyper‐, iso‐, and hyposmotic volume contraction and expansion with discussion on volume and osmolarity changes of the intra‐ and extracellular fluid compartments. Yet, many students still struggle with clinical application of these concepts. Even after traditional calculation of Starling forces in a typical systemic capillary, they often have trouble with the concept of plasma colloid‐osmotic (oncotic) pressure and its relevance to edema formation. In the renal section, where hypoalbuminemia and electrolyte disturbances are both discussed confusion between plasma osmolarity and oncotic pressure often arises. Over the past four years, teaching medical physiology to physician assistant students we developed an alternate approach. During the initial fluid compartment lecture we emphasize the concept of “osmotically active substance” and compare permeability of the cell vs. capillary membrane. We also introduce plasma osmolarity and oncotic pressure and discuss their primary determinants. These concepts are then revisited during the cardiovascular and renal section of the course. We found this approach to be highly successful in preventing the confusion described above, and feel this to be a major step in reforming classic medical physiology education.