Beckers Lecture Notes Physiology USMLE Step 1 – This book for the subject of Physiology by Becker is one of the best books a medical student can find out in the market
Only osmotic forces determine the distribution of fluid between the intracellular and extracellular compartments. Osmosis is simply the net diffusion of water across a membrane. Consider the following model:
• The concentration of the dissolved substance determines t he concentration of water (t concentration or dissolved substance .J. concentration or water).
• The water diffuses from the higher concentration in Y to the lower concentration in X.
• If the membrane was permeable to the dissolved substance, it would equalize its concentration between the two compartments, and there would be no water concentration gradient and no net diffu sion of water (osmosis).
• To have an osmotic effect across a membrane, it cannot penetrate the membrane. The concentration or all dissolved substances in a compartment that cannot penetrate the surrounding membrane is the effective osmolarity.
• In the extracellular fluid, the main dissolved substance that cannot penetrate the cell membrane is sodium (Na+). A negative ion must remain with the ECF Na+ to maintain electrical neutrality; for example, ct·, HC03 -, and HPO,-.
• Thus, ECF effective osmolarity is approximately 2x [Na•].
• Glucose penetrates membranes slowly and contributes some osmotic effect, particularly with hyperglycemia. Urea easily penetrates most membranes, but not all (blood·brain barrier, sections or nephron). Some include urea in the ECF effective osmolarity; others ignore it.