Body Fluid Compartments and its Measurement

In Physiology, body fluid compartments is a significant topic for NEET PG exam. The topic forms a fundamental concept of human physiology and is essential for understanding fluid and electrolyte balance in the body. Topics related to body fluid compartments are likely to be covered in exams like NEET PG, and hence must be studied thoroughly.  

Read this blog further to get a quick overview of this important physiology topic. Also, stay tuned to our medical notes blogs as we will be covering all the high-yield topics from NEET PG Exam Preparation.

COMPARTMENTS

• Total body water is sixty percent of the body weight. TBW = (0.6) x Body weight 
• So, in a 70 kg healthy adult = 60% of 70 = 42L. His total body water is above 42L. In extreme obesity, i.e., there is a lot of fat content and fat is anhydrous (lacks water). So, in an obese person this fraction will be slightly less (we can say 55% instead of 60%).
• Out of this total body water, two-third is ICF, and one-third is ECF.
  • ICF= 0.4 X Body weight → 28 L  
  • ECF= 0.2 x Body weight → 14 L
    • 1L volume is called trans cellular fluid. It contains synovial fluid, intraocular fluid etc.
    • ECF
  • 3/4^th of the ECF is Interstitial fluid. (11L) 
  • 1/4^th of the ECF is Plasma volume. (3L)
• In children, due to the different surface area to the volume ratio of their body, they are more prone to dehydration.

Important Information

Blood is approximately 8% of the body weight. 
Plasma will be 5% of the body weight. 
Remaining 3% will be the volume of the cells.

MEASUREMENTS OF THE BODY FLUID COMPARTMENTS

We measure the body fluid compartments by Dye dilution method or Indicator dilution method.

• It is sometimes also called the Stewart-Hamilton dye dilution technique.
• E.g. – If we put the dye in a particular volume, it will get dispersed evenly and then the sample is collected and the concentration of the dye is measured.
• Greater the volume of that compartment, greater will be the dilution of the dye. So, the concentration of the sample will be lesser as it got diluted more.
• V = I/C
  • V = Volume of the compartment 
  • I = Initial volume of the indicator injected
  • C= Concentration of the indicator after it got dispersed evenly 

Prerequisites for this method

• The dye should get dispersed evenly in the compartment.
• It should not leave the compartment.
  • E.g. – You are measuring the ECF volume & the dye enters the cells. This will tamper the calculation.
• If it leaves the compartment after the dye is injected, then the formula becomes:
  • V = (I-A)/C 
  • where, A = Amount of the dye that left the compartment
• Lesser the concentration, more the dilution, which means the volume was higher.

Indicators used to measure the various compartments          

Main criteria while measuring the body water is that it should diffuse easily through the various membranes. Here are the indicators used to measure the various compartments: 

1. Indicators used for the Total Body water:
   • Deuterium Oxide (D₂O)
   • Tritium (3H₂O)
   • Antipyrine 
   • Aminopyrine 
2. Indicators used for the ECF volume:
   • Non-metabolizable saccharide 
   • Inulin (Best Indicator)
   • Sucrose
   • Mannitol 
3. Indicators used to measure the ICF Volume:
   • ICF volume cannot be measured directly.
   • It is measured by (TBW)-(ECF)
4. Indicators used to measure Plasma volume:
   • Radiolabeled albumin (¹³¹I - labeled)
5. Indicators used to measure the volume of the cell:
   • Chromium 51 (51 Cr tagging of red cells)

Fixed number of cells will be tagged with chromium & will be dispersed into the red cells.

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