In a study to detect extracellular fluid volume, 10 gm mannitol was injected by intravenous route and after waiting for adequate time for equilibration of levels, concentration was measured as 50 mg/100 ml. In this time, 10% mannitol was excreted. What is the calculated volume of ECF?

Ans. b. 18 Litres (Ref. Ganong 24/e p2)Out of 10 gm mannitol, 10% was excreted.Hence, amount of mannitol distributed in ECF = 10-1= 9 gmGiven concentration of mannitol in ECF = 50 mg/100 mlVolume of ECF = Amount/Concentration = 9 g/50 mg x 100 ml = 18 LBody FluidsWater constitutes approximately 60% of total body weightQ.Of this 60%, 40% is present in cells in the form of intracellular fluid (ICF) and 20% is present outside the cells in the form of extracellular fluid (ECF)Q.20% of body weight water in ECF is divided into interstitial fluid (15% of body weight) and plasma (5% of body weight)Q.Body FluidsTotal Body Water (60% of body weight)QIntracellular fluid (ICF)Extracellular fluid (ECF)* 2/3rd of TBWQ (40% of body weight)* 1/3rd of TBW (20% of body weight)QECFPlasmaInterstitial fluid* 25% or 1/4th of ECF* 5% of body weightQ* 75% or 3/4th of ECF* 15% of body weightQMost of the fluids are calculated directly from dilution methods, except for ICF & interstitial fluids. Both of these are calculated indirectly by calculating other body fluids.ICF= Total body water volume - ECF volumeInterstitial fluid = ECF volume - Plasma volumeFluid volumeindicator usedTotal body water volume* Deuterium oxide: D20 (MC used)Q* Tritium oxideQ* AmlnopyrineQ* AntipyrineQExtracellular fluid volume* Inulin (most accurate)Q* Sucrose* Mannitol* Sodium thiosulphatePlasma volume* Evans blueQ* Serum albumin labeled with radioactive iodineQ (1-125)Red cell volume* Cr-51 labeled RBCQ