USMLE Step 1: Physiology effective osmolarity of a particular compartment. Because sodium chloride represents most of the nonpermeant particles of the extracellular Auid, the concentration of sodium chloride rep- resents most of the effective osmolarity of this compartment. Twice the extracellular sodium concentration is usually a good index of body osmolarity If ECF effective osmolarity increases, cells shrink(ICF l) If ECF effective osmolarity decreases, cells swell (ICF T) Interstitial versus Vascular(Plasma) Fluid Movement of fluid between these two compartments occurs across capillary membranes. Capillary membranes are freely permeable to all natural substances dissolved in the plasma, except proteins. Thus, it is the concentration of plasma proteins that determines the effective osmolarity between these two compartments. Capillary exchange is discussed in the peripheral circulation unit. Graphical Representation of Volume versus Solute Concentration in the ICF and ECF It is important to understand how body osmolarity and the intracellular and extracellular vol umes change in clinically relevant situations. Figure 1-2-2 is one way of presenting this informa tion. The y axis is solute concentration or osmolarity. The x axis is the volume of intracellular (2/3)and extracellular(1/3)fluid. If the solid line represents the control state, the dashed lines show a decrease in osmolarity and extracelular volume but an increase in intracellular volume Concentratlon of solute olume CF volume Figure 1-2-2. Darrow- Yannet Diagram Extracellular Volume When there is a net gain of fluid by the body this compartment always enlarges. a net loss of body duid decreases extracellular volume Concentration of Solute Particles This is equivalent to body osmolarity and in most cases is approximated as twice the sodium concentration(mM)of the extracellular Fluid. Remember, at equilibrium the intracellular and extracellular osmolarity will be the same./' USMLEStep 1: Physiology 12 KAPLANii' . me IcaI effective osmolarity of a particular compartment. Because sodium chloride represents most of the nonpermeant particles of the extracellular fluid, the concentration of sodium chloride rep￾resents most of the effective osmolarity of this compartment. Twice the extracellular sodium concentration is usually a good index of body osmolarity. Na = 143 mOsm (mM) If ECF effective osmolarity increases, cells shrink (rCF J-). rf ECF effective osmolarity decreases, cells swell (rCF i). InterstitialversusVascular(Plasma)Fluid Movement of fluid between these two compartments occurs across capillary membranes. Capillary membranes are freely permeable to all natural substances dissolved in the plasma, except proteins. Thus, it is the concentration of plasma proteins that determines the effective osmolarity between these two compartments. (Capillary exchange is discussed in the peripheral circulation unit.) GraphicalRepresentationof VolumeversusSoluteConcentrationinthe ICFand ECF It is important to understand how body osmolarity and the intracellular and extracellular vol￾umes change in clinically relevant situations. Figure r-2-2 is one way of presenting this informa￾tion. The y axis is solute concentration or osmolarity. The x axis is the volume of intracellular (2/3) and extracellular (1/3) fluid. rf the solid line represents the control state, the dashed lines show a decrease in osmolarity and extracellular volume but an increase in intracellular volume. Concentration of Solute ----------------- ------- volume ICF volume ECF ~ ~ 0 Figure 1-2-2.Darrow-Yannet Diagram ExtracellularVolume When there is a net gain of fluid by the body, this compartment always enlarges. A net loss of body fluid decreases extracellular volume. Concentrationof SoluteParticles This is equivalent to body osmolarity and in most cases is approximated as twice the sodium concentration (mM) of the extracellular fluid. Remember, at equilibrium the intracellular and extracellular osmolarity will be the same