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 represents 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 volumes change in clinically relevant situations. Figure r-2-2 is one way of presenting this information. 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
Body Compartments Intracellular volume This varies with the effective osmolarity of the extracellular compartment, that is, the conce tration of particles that do not penetrate the cell membrane. An increase in osmolarity decreas es intracellular volume, and the opposite occurs with a decrease in body osmolarity Review Questions (Answers below Using the graph presented in Figure 1-2-2, determine the volume and concentration changes associated with the following states of hydration. If you need help, see Table 1-2-1 Examples: hemorrhage(neglect loss of intracelular fluid as red blood cell [RBC] volume), the formation of isotonic urine, and the immediate consequences of diarrhea or vomiting 2. Loss of hypotonic fluid Examples: sweating(dehydration), hypotonic urine formation such as occurs in diabetes insipidus and alcoholism 3. Ingestion of salt tablets 4. Person who drinks I liter of tap(or distilled)water. (This is equivalent to water intoxication. 5. Infusion of hypotonic saline(1/2 normal saline 6. Infusion of isotonic saline Infusion of hypertonic saline(or hypertonic mannitol Primary adrenal insufficiency(volume and salt depletion, volume replacement exceeds salt replacement)
BodyCompartments IntracellularVolume This varies with the effectiveosmolarity of the extracellular compartment, that is, the concentration of particles that do not penetrate the cell membrane. An increase in osmolarity decreases intracellular volume, and the opposite occurs with a decrease in body osmolarity. ReviewQuestions (Answersbelow) Using the graph presented in Figure 1-2-2, determine the volume and concentration changes associated with the following states of hydration. Jfyou need help, see Table 1-2-1. 1. Loss of isotonic fluid Examples:hemorrhage (neglect loss of intracellular fluid as red blood cell [RBC]volume), the formation of isotonic urine, and the immediate consequences of diarrhea or vomiting 2. Loss of hypotonic fluid Examples: sweating (dehydration), hypotonic urine formation such as occurs in diabetes insipidus and alcoholism 3. Ingestion of salt tablets 4. Person who drinks 1 liter of tap (or distilled) water. (This is equivalent to water intoxication.) 5. Infusion of hypotonic saline (112 normal saline) 6. Infusion of isotonic saline 7. Infusion of hypertonic saline (or hypertonic mannitol) 8. Primary adrenal insufficiency (volume and salt depletion, volume replacement exceeds salt replacement) iiie&ical 13
USMLE Step 1: Physiology Changes in Volume and Concentration(dashed lines) 1. Loss of isotonic fluid that might be due to hemorrhage(neglect loss of intracellular fluid as RBC volume), isotonic urine, or the immediate consequences of diarrhea or vomiting Figure 1-2-3 There will be a loss of volume but no change in extracellular effective osmolarity. The fact that extracellular osmolarity is unchanged means no change in intracellular volume 2. Loss of hypotonic fluid that might be due to sweating(dehydration), hypotonic urine, or -2-4 Losing hypotonic fluid from the extracellular space would increase extracellular effective osmolarity(sodium concentration would increase). Fluid would move from the intracel- lular to the extracellular compartment until osmolarity was again equal in the two com- partments. The fluid entering the extracellular space would partially but not completely compensate for the originate insult. edical
/' USMLEStep1: Physiology 14 meCtical ChangesinVolumeandConcentration(dashedlines) 1. Lossof isotonicfluidthat mightbe due to hemorrhage(neglectlossof intracellularfluid as RBCvolume),isotonicurine,or the immediateconsequencesof diarrheaor vomiting: Figure 1-2-3 There willbe a loss of volume but no change in extracellular effectiveosmolarity. The fact that extracellular osmolarity is unchanged means no change in intracellular volume. 2. Loss of hypotonic fluid that might be due to sweating (dehydration), hypotonic urine, or diabetes insipidus: : - - - - - - - - - - - - -1- - - - - --: "I I I I I I I I I Figure 1-2-4 Losing hypotonic fluid from the extracellular space would increase extracellular effective osmolarity (sodium concentration would increase). Fluid would move from the intracellular to the extracellular compartment until osmolarity was again equal in the two compartments. The fluid entering the extracellular space would partially but not completely compensate for the originate insult
3. Ingestion of salt tablets Figure 1-2-5 The salt tablets would increase the effective osmolarity of the extracellular fluid. The result would be a fluid shift from the intracellular to the extracellular compartment. 4. Person who drinks I liter of tap(or distilled )water Figure 1-2-6 The tap water entering the extracellular space would increase its volume and decrease its osmolarity. Because of the decrease in osmolarity, some of the ingested water would dif- fuse into the intracellular space. 5. Infusion of hypotonic saline(half-normal saline) The answer is the same as answer 4 medical 15
BodyCompartments 3. Ingestion of salt tablets: ------------- - - - - - - - - - - -I 1 1 1 1 1 1 1 1 1 1 Figure 1-2-5 The salt tablets would increase the effective osmolarity of the extracellular fluid. The result would be a fluid shift from the intracellular to the extracellular compartment. 4. Person who drinks 1 liter of tap (or distilled) water: -------------------------- Figure 1-2-6 The tap water entering the extracellular space would increase its volume and decrease its osmolarity. Because of the decrease in osmolarity, some of the ingested water would diffuse into the intracellular space. 5. Infusion of hypotonic saline (half-normal saline): The answer is the same as answer 4. meClical 15
USMLE Step 1: Physiology 6. Infusion of isotonic saline FIgure 1-2-7 The infusion of isotonic saline would increase the volume but not the effective osmolar. ty of the extracellular space. Because there was no change in osmolarity, the intracellular volume is unchanged. An additional point is that most of the saline would enter the inter stitial space. a much smaller volume would remain in the intravascular compartment. If plasma, which does contain protein, was infused, however, almost all of the fluid would remain in the vascular space because the proteins do not easily cross capillary membranes 7, Infusion of hypertonic saline(or hypertonic mannitol; mannitol does not cross cell mem branes easily) The hypertonic saline would increase both the volume and effective osmolarity of the extracellular compartment. The increased osmolarity would cause a fluid shift from the intracellular to the extracellular space, reducing intracellular volume and further increas- Ing extracelular volume dical
....- USMLEStep1: Physiology 16 KAPLAN .. . . me leaI 6. Infusion of isotonic saline: Figure 1-2-7 The infusion of isotonic saline would increasethe volume but not the effective osmolarity of the extracellular space.Becausethere was no changein osmolarity, the intracellular volume is unchanged. An additional point is that most of the salinewould enter the interstitial space.A much smaller volume would remain in the intravascular compartment. If plasma, which does contain protein, was infused, however, almost all of the fluid would remain in the vascularspace because the proteins do not easilycrosscapillary membranes. 7. Infusion of hypertonic saline (or hypertonic mannitol; mannitol does not cross cellmembranes easily): ~-------------- Figure 1-2-8 The hypertonic saline would increase both the volume and effective osmolarity of the extracellular compartment. The increased osmolarity would cause a fluid shift from the intracellular to the extracellular space, reducing intracellular volume and further increasing extracellular volume