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,ji and fat is anhydrous (lacks water). So, in an obese person this fraction will be slightly less (we can say 55% instead of 60%).
• The total body water, which is 60% of the total body weight, is distributed into two components, namely intracellular fluid (ICF) and extra-cellular fluid (ECF).
• ICF makes up 2/3rd of the total body water, and ECF is 1/3rd of the total body water. ICF makes up 40% of the total body weight, and ECF makes up 20% of the total body weight.
• ECF is further divided into two parts: interstitial fluid (ISF), which makes up 3/4th of the total ECF, and Plasma, which makes up 1/4th. The interstitial fluid makes up 15% of the person's total body weight, and plasma makes up 5%.
• It is to be noted that plasma makes up 5% of the total body weight. However, it makes up 1/4th of the ECF and thus 25% of the ECF.
• The total cell volume is 3% of the body weight, RBC, WBC, platelet, all of it.

How To Calculate Interstiatial Fluid Volume?

• 3/4^th of the ECF is Interstitial fluid. (11L) 
• 1/4^th of the ECF is Plasma volume. (3L)
• Children are more prone to dehydration due to their different surface area to volume ratio.

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.

Transcellular Fluid

• Apart from all these types of body water, there is something called transcellular fluid. It is the fluid that is present in certain body cavities.
• For example:
  • Cerebrospinal fluid (CSF), which has a volume of 150 ml approximately.
  • In between two pleurae, visceral and parietal pleura, the intrapleural fluid is present with a total volume of 10-20 ml.
  • The pericardial fluid has an approximate volume of 50 ml.
  • Peritoneal fluid has an approximate volume of 0 ml in case of males and 20 ml in case of females particularly in postovulatory phases. During ovulation, there is rupture of follicle and certain amount of fluid oozes out in the peritoneal cavity thus female peritoneal volume becomes 20 ml.
  • Apart from these, there is also the presence of synovial fluid, which is 1 ml/large joint, approximately. For example, the knee joint, the hip joint, etc.

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.
  • For example, if you are measuring the ECF volume and the dye enters the cells, this will tamper with 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

The 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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