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.

Also read: Physiology Important Questions For NEET PG/FMGE

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 

Also read: Chemical Regulation of Respiration: Role of Chemoreceptors and Reflexes

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.

Also read: Cell Physiology: Overview of Membrane, Cytoskeleton

Composition of Body Fluids 

• Plasma has more positively charged ions than ISF 
• Because of more protein, which is negatively charged. 
• Total osmolarity - sum of all osmotically active substances.
• Corrected osmolarity is slightly lower than total osmolarity 
• Due to interactions between positively and negatively charged osmotically active Molecules.
• Osmotic pressure of 1 mOsmol/L impermeable substances = 19.3mmHg
  • Total osmotic pressure of the plasma = 19.3 x 281 = 5441mmHg (approx. 5500mmHg

Also read: Cell Physiology: Overview of Membrane, Cytoskeleton

Osmolar Gap

• Total osmolarity can be measured with Osmometer with the help of a principle known as Freezing point depression
• Calculation of osmolarity
  • Osmolarity  = 2x [ Na⁺ mmol/L] + Glucose (mg%)  18 +  BUN (mg%)2.8
  • BUN= urea2.14
  • If all units are given in mmol/L, then 2x [ Na⁺] + Glucose + BUN
• Measured osmolarity > calculated osmolarity
• Osmolar gap= Measured osmolarity – Calculated osmolarity
  • Normally, the Osmolar Gap is 10
  • Increased osmolar gap is seen in 
    1. Alcohol poisoning
    2. Sorbitol
    3. Huge amount of protein
    4. Hyperlipidaemia

Osmolarity and Tonicity

• Osmolarity is a measure of the concentration of osmotically active particles (osmoles) in one liter of solution.
• Osmoles per kilogram of water is osmolality.
• Difference between Osmolarity and Osmolality is 1%
• Tonicity of a solution predicts the effect of the solution on cell volume at equilibrium (depends on impermeant solutes).
  • Cell osmolarity is 300 mOsm/L, solution is 500 mOsm/L
    • The solution is hyperosmotic compared to the cell.
    • However, without knowing the concentration of impermeable solutes, it is difficult to determine whether the solution is hypertonic, hypotonic, or isotonic.
  • Solution - 200 mOsmol/L NaCl (impermeable) and 300 mOsmol/L urea (permeable)
    • Solution is hyperosmotic but hypotonic in comparison to cell
    • Urea can freely diffuse across the cell membrane while NaCl cannot.
  • Solution - 300 mOsmol/L NaCl (impermeable) and 200 mOsmol/L urea (permeable)
    • Solution is Isotonic in comparison to the cell, and cell volume will remain the same.

Also read: Biostatistics Multiple-Choice Questions for Health Sciences

Isotonic Solutions

• 5% mannitol 
• 5% Dextrose - isotonic in vitro, with a tonicity of 290-300 mOsmol/L.
  • However, once infused into the plasma, glucose is rapidly taken up by cells, leaving behind water, making it hypotonic.
• 0.9% NaCl (normal saline) - isotonic.
• Ringer's lactate solution (lactated Ringer's or Hartmann's solution) - isotonic and closely resembles plasma composition, except it lacks magnesium.
• Ringer's solution has the same constituents as plasma but does not contain sodium lactate.

Darrow-Yannet Diagram    

• At equilibrium, the osmolality of ICF and ECF are equal
  • 280mOsmol/L

Gains / Loss of fluids	D-Y Diagram	Explanation
Gain of Isotonic Fluid		- When an isotonic solution is administered, the ECF volume increases without any change in osmolality. - Example: 0.9% NaCl infusion
Loss of Isotonic Fluid		- In cases of hemorrhage, urinary loss, acute vomiting, or diarrhea→isotonic fluid  lost from the body→  decrease in extracellular fluid (ECF) volume without altering osmolality.
Gain of Hypotonic Fluid		- Examples: 0.45% NaCl, Polydipsia, SIADH. - SIADH → excess water reabsorption by the kidneys → hypotonic fluid gain. - Psychogenic polydipsia →hypotonic fluid gain (excessive water intake). - Extracellular fluid (ECF) volume expansion → ↓ ECF osmolality.  - Since intracellular fluid (ICF) osmolality remains higher, the condition is not in steady state. - Hypotonic ECF moves into the ICF by osmosis → ICF volume expansion and ↓ osmolarity. - Hypotonic fluid therapy is not recommended, as it can cause intracellular edema due to cell volume expansion. - If 1 liter of hypotonic fluid is infused - 1/3rd will enter ECF – 333ml  - 2/3rd will enter ICF – 667ml
Loss of Hypotonic Fluid		- In conditions like diabetes insipidus or inadequate water intake → excess water is excreted (insufficient ADH). - This leads to a decrease in extracellular fluid (ECF) volume, but since only water is lost and solutes remain, ECF osmolality and tonicity increase, making it hypertonic compared to intracellular fluid (ICF). - Water moves from ICF to ECF through osmosis, ↓ ICF volume and ↑ its osmolarity. - Volume of ICF and ECF  ↓  and Osmolarity ↑  in both compartments in equilibrium
Gain of Hypertonic Fluid		- Infusing 3% NaCl (hypertonic solution) expands ECF volume and increases its tonicity. - The ECF becomes hypertonic relative to the ICF, causing water to move out of cells, decreasing ICF volume and increasing ICF tonicity.
Loss of Hypertonic Fluid		- In adrenal insufficiency, solute is lost in excess of water. - Hypertonic fluid loss decreases ECF volume, making it hypotonic relative to ICF.  - Water moves from ECF to ICF, increasing ICF volume slightly.  - This results in reduced ECF volume, increased ICF volume, and decreased tonicity in both compartments, reaching a new steady state.

Also read: Neural Regulation of Respiration: Control & Mechanisms

MCQ’s

Q. A research fellow was studying the volume and electrolytes in different body water compartments. During his experiment, he took a sample and measured the electrolytes as Na+ of 10mEq/L and K+ of 140 mEq/L. The analysis indicates which of the following compartment:

1. ECF
2. ICF
3. ISF
4. Plasma

Q. Calculate the blood volume with the followings: Weight of the patient 60kg, Hematocrit 45%

1. 4.8 L
2. 5.0 L
3. 5.45 L
4. 6 L

Q. 100mg of sucrose is injected into a 70Kg man. The plasma level of sucrose after mixing is 0.01mg/ml. If 5 mg has been metabolized during this period, then what is the ECF volume?

1. 9.5L 
2. 14 L
3. 17.5 L
4. 10 L

Important Points to Remember 

• Blood makes up to 8% of the body weight
• Preterm infants have a higher body water content, with 90% of their birth weight being water, compared to 75% in term infants.
• Indicator dilution principle or volume of distribution method is used to measure the exact amount of body water
• ICF volume = (TBW - ECF)
• ISF = (ECF – Plasma volume)
• Total osmolarity can be measured with Osmometer with the help of a principle known as Freezing point depression
• Osmolar gap= Measured osmolarity – Calculated osmolarity

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