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Body Fluid Compartments – Part 1 Body Composition

Body Fluid Compartments – Part 1 Body Composition. E.S.Prakash, Faculty of Medicine, AIMST University E-mail: dresprakash@gmail.com.

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Body Fluid Compartments – Part 1 Body Composition

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  1. Body Fluid Compartments – Part 1Body Composition E.S.Prakash, Faculty of Medicine, AIMST University E-mail: dresprakash@gmail.com Note to Readers – This is one of the lectures in the Molecular & Cellular Basis of Medicine course, Element 2 [General Physiology, Nerve – Muscle Physiology, Basics of Hematology] in Term 1 of Year 1 of the MBBS programme.

  2. Plasma [about 1/4th of ECF volume] ECF Interstitial Fluid [ISF]; About 3/4th of ECF volume Intracellular Fluid [ICF]; 2/3 of total body water is inside cells

  3. Body fluid volumes in a healthy 70-kg adult male

  4. Measuring body fluid volumes: • Principle: Indicator-dilution principle • Method: • Administer a known amount (A) of a substance that will get distributed in a particular body fluid compartment; • Determine the final steady state concentration (C) of this indicator in blood; • Volume of distribution Vd of the indicator = Amount of indicator injected (A) Concentration in blood after mixing (C)

  5. Indicators used for measuring plasma volume, ECF volume and total body water

  6. Take this problem: • 100 mg of sucrose is injected into a 70 kg man. The plasma sucrose level after mixing is 0.01 mg/ml. If 5 mg has been metabolized during this period, then, what is the ECF volume? • 9.5 L • 14 L • 17.5 L • 10 L

  7. Solution: • Amount of sucrose injected = 100 mg • Amount metabolized = 5 mg • Amount remaining in ECF (A) = 95 mg • Concentration in plasma after mixing (C) = 0.01 mg/ml • Thus, volume of distribution (Vd) of sucrose = A / C = 95 mg / 0.01 mg/ml = 9500 ml = 9.5 L (= ECF volume)

  8. How would you measure interstitial fluid (ISF) volume? • Cannot be measured; it is calculated (estimated).. • ISF volume = ECF volume – plasma volume • How would you measure ICF volume? • Cannot be measured; it is calculated (estimated).. • ICF volume = Total body water – ECF volume

  9. Transcellular fluids • Cerebrospinal fluid • Synovial fluid • Aqueous humor • Pleural fluid • Peritoneal fluid • In health, the total volume of transcellular fluids is < 1 L.

  10. What is the relationship between blood volume and plasma volume? • Blood = plasma + cells in blood • Volume of cells: packed cell volume (PCV) • PCV is also called hematocrit (Hct) • Blood volume = plasma volume × 100 / (100 – Hct) Image from http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/B/Blood.html

  11. Blood volume = plasma volume × (100/100-Hct) • In a healthy 70 kg adult male, plasma volume (measured using the Evans Blue dye) was found to be 3000 ml. Hematocrit was 40%. Calculate his blood volume. • 5000 ml • 5200 ml • 5400 ml • 5600 ml

  12. Measuring red cell volumeNote: measuring is different from calculating something • Technique & principle: • Use Chromium 51 labeled RBC as the indicator; • Inject a known amount of Cr 51 labeled RBC intravenously; • Allow them to mix with RBC in blood; • Measure the fraction of RBC tagged with Cr 51; • Principle: same – indicator dilution principle.

  13. Lean body mass (LBM) • Definition: LBM is fat free mass • Total body mass = fat mass + fat free mass • Note: fat is relatively anhydrous • Note: the water content of LBM is constant • Water content of LBM is constant - 70 ml /100 g tissue

  14. Take this problem: • In a healthy adult male weighing 70 kg, total body water (TBW) was measured to be 42 L. What is his lean body mass (LBM)? What is his fat mass? • Given TBW = 42 L • Assume all this water is in LBM & that fat is water free • We know that water content of LBM is 70 ml/100 g • Thus, if TBW is 42 L, LBM = 60 kg • Since he weights 70 kg, his fat mass is 70-60 = 10 kg

  15. Body compositionLean body mass = fat free mass Total body weight = fat-free mass + fat mass

  16. Determining body fat: • Technique: bioelectric impedance technique • Principle: • Body fluids conduct electricity well; • But fat is anhydrous and therefore is a poor conductor of electricity; • The resistance to flow of a small current between points on the body is proportional to fat mass.

  17. Now you should be able to answer these questions: • Classify body fluid compartments. Mention the volumes of body fluid compartments in a healthy adult male weighing 70 kg. • Explain the principle that is used in the measurement of body fluid volumes and RBC volume. • Mention the indicator that is used to measure the volume of plasma, ECF, and total body water and explain why these indicators are chosen for measuring them. • How you would measure interstitial fluid volume? • How you would measure ICF volume?

  18. Mention the relationship between hematocrit and blood volume. • Describe the meaning of the term ‘lean body mass’. • Mention the relationship between lean body mass and total body mass. • Briefly explain the principle that is used in the estimation of body fat. • Briefly explain gender differences in body composition.

  19. Required Reading: • Ganong WF. Chapter 1. The General and Cellular Basis of Medical Physiology. In: Review of Medical Physiology. Int’l ed, Mc Graw Hill, 2005; pages 1-6.

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