Body Fluids and Fluid Compartments | Physiology for MBBS
Body fluids provide the internal environment required for normal cellular function. Water forms the major component of body fluids and acts as the medium for transport, metabolism, temperature regulation and maintenance of homeostasis.
Understanding the distribution of water between intracellular and extracellular compartments is essential for understanding dehydration, overhydration, electrolyte disturbances and fluid therapy.
1. Total Body Water
Total body water (TBW) is the total amount of water present in the body. In a healthy young adult, it generally accounts for approximately 50–65% of body weight, depending mainly on age, sex and body composition.
Women generally have a lower percentage of body water because they usually have a greater proportion of adipose tissue. Adipose tissue contains less water than lean tissue.
Therefore, lean individuals generally have a higher percentage of body water than individuals with obesity. Total body water also decreases with aging because the proportion of adipose tissue tends to increase while lean body mass decreases.
2. Significance of Body Fluids
Homeostasis
Body cells survive in an internal environment called the milieu intérieur. This environment supplies cells with oxygen, glucose, amino acids, electrolytes and other substances required for normal function.
Transport
Water acts as a transport medium for nutrients, hormones, enzymes, electrolytes and metabolic products between different parts of the body.
Metabolic Reactions
Intracellular water provides the medium in which many biochemical and metabolic reactions occur.
Maintenance of Tissue Structure
Appropriate cellular water content contributes to the normal shape, consistency and mechanical properties of tissues.
Temperature Regulation
Water has a high heat capacity and participates in sweating and evaporation, helping maintain normal body temperature.
3. Body Fluid Compartments
In a typical 70-kg adult, total body water is approximately 40 L. It is divided into two major compartments:
| Compartment | Approximate Volume | % of TBW | Important Point |
|---|---|---|---|
| Intracellular fluid (ICF) | ~22 L | ~55% | Fluid inside cells |
| Extracellular fluid (ECF) | ~18 L | ~45% | Fluid outside cells |
The ECF is further divided into interstitial fluid, plasma and smaller specialized fluid compartments.
| ECF Subcompartment | Approximate Share of TBW | Approximate Volume |
|---|---|---|
| Interstitial fluid and lymph | ~20% | ~12 L |
| Plasma | ~7.5% | ~2.75 L |
| Fluid in bone | ~7.5% | Part of specialized ECF |
| Fluid in dense connective tissue | ~7.5% | Part of specialized ECF |
| Transcellular fluid | ~2.5% | Small specialized compartment |
Transcellular Fluid
Transcellular fluid is a specialized portion of extracellular fluid found in epithelial-lined spaces.
- Cerebrospinal fluid (CSF)
- Intraocular fluid
- Digestive secretions
- Pleural fluid
- Pericardial fluid
- Peritoneal fluid
- Synovial fluid
- Fluid within parts of the urinary tract
4. Composition of Body Fluids
Body fluids contain water and dissolved organic and inorganic substances.
Organic Substances
- Glucose
- Amino acids
- Proteins
- Fatty acids and other lipids
- Hormones
- Enzymes
Inorganic Substances
- Sodium
- Potassium
- Calcium
- Magnesium
- Chloride
- Bicarbonate
- Phosphate
- Sulfate
| Feature | ECF | ICF |
|---|---|---|
| Major cation | Sodium (Na⁺) | Potassium (K⁺) |
| Other important ions | Chloride and bicarbonate | Magnesium and phosphate |
| Important organic components | Glucose and circulating nutrients | Proteins and intracellular metabolites |
| Typical pH | ~7.4 | ~7.0 |
ECF = Na⁺ rich
ICF = K⁺ rich
5. Measurement of Body Fluid Volumes
Body fluid compartments are estimated using the indicator dilution principle.
Indicator Dilution Principle
A known amount of an indicator is introduced into a fluid compartment. After the indicator has mixed adequately throughout the accessible compartment, its concentration is measured in a sample.
If some indicator has been excreted before the measurement, the amount excreted must be subtracted from the administered amount.
Properties of an Ideal Indicator
- It should distribute throughout the compartment being measured.
- It should mix thoroughly within a reasonable time.
- It should not be rapidly removed during the measurement.
- It should not significantly alter the volume or composition of the compartment.
- Its concentration should be accurately measurable.
6. Marker Substances for Fluid Compartments
| Compartment | Common Marker Substances |
|---|---|
| Total body water | Deuterium oxide (D₂O), tritium oxide (T₂O), antipyrine |
| Extracellular fluid | Sodium, chloride, bromide, sulfate, thiosulfate, inulin, mannitol, raffinose and sucrose |
| Plasma volume | Evans blue and labeled plasma proteins |
7. Measurement of Total Body Water
A marker used to measure total body water must distribute throughout both intracellular and extracellular compartments.
Deuterium oxide (D₂O) and tritium oxide (T₂O) distribute through total body water. Antipyrine can also be used, although its distribution may be slower.
8. Measurement of Extracellular Fluid Volume
An ECF marker should cross the capillary membrane but should not significantly enter cells.
Examples include inulin, sucrose and mannitol.
Sodium and chloride can also be used as indicators, producing measurements referred to as sodium space and chloride space.
Example of Indicator Dilution
Suppose 150 mg of sucrose is administered and 10 mg is subsequently excreted. If the plasma concentration is 0.01 mg/mL:
ECF volume = 140 mg ÷ 0.01 mg/mL
ECF volume = 14,000 mL = 14 L
9. Measurement of Plasma Volume
Plasma volume is measured using an indicator that remains predominantly within the vascular compartment by binding strongly to plasma proteins.
Evans blue and labeled plasma proteins are examples of plasma-volume indicators.
10. Calculation of Interstitial and Intracellular Fluid
Some fluid compartments cannot be measured directly and are calculated from other measured compartments.
If TBW = 40 L and ECF = 18 L:
ICF = 40 − 18 = 22 L
11. Osmolality and Osmolarity
Osmolality
Osmolality describes the concentration of osmotically active particles per kilogram of water.
Osmolarity
Osmolarity describes the concentration of osmotically active particles per litre of solution.
In clinical practice, the two terms are sometimes used interchangeably because their numerical difference is generally small. Osmolality is particularly useful when discussing physiological body-fluid concentration.
Mole and Osmole
A mole represents an amount of a substance based on its molecular weight. An osmole represents the number of osmotically active particles generated in solution.
A substance that dissociates into multiple particles contributes more osmotic particles than a non-dissociating substance at the same molar concentration.
12. Movement of Water Between ICF and ECF
Water moves across cell membranes in response to differences in effective osmotic concentration.
Water moves from ICF → ECF, causing cellular water loss and cell shrinkage.
Water moves from ECF → ICF, causing cellular water gain and possible cell swelling.
13. Tonicity
Tonicity describes the effective osmotic effect of a solution on cell volume. It depends mainly on solutes that do not rapidly cross the cell membrane.
Small molecules such as urea can cross cell membranes relatively easily and therefore have less sustained effect on cell volume. Effective osmoles produce sustained water movement across the cell membrane.
| Solution | Effect on Cell | Example |
|---|---|---|
| Isotonic | No significant net movement of water; cell volume remains relatively stable. | 0.9% NaCl |
| Hypertonic | Water leaves the cell → cell shrinkage. | 2% NaCl |
| Hypotonic | Water enters the cell → cell swelling and possible lysis. | 0.3% NaCl |
Hypertonic ECF → water leaves cells → cells shrink.
Hypotonic ECF → water enters cells → cells swell.
14. Maintenance of Water Balance
Water balance is maintained by coordinated control of water intake and water loss. The major regulatory systems involve the hypothalamus, thirst mechanism, antidiuretic hormone (ADH), and kidneys.
Hypothalamic osmoreceptors detect changes in plasma osmolality and participate in the regulation of thirst and ADH secretion. The kidneys adjust urinary water excretion according to the body's hydration status.
15. Dehydration
Dehydration is a state in which water loss exceeds water replacement, resulting in a reduction in total body water.
Classification by Severity
| Severity | Approximate Fluid Loss | Clinical Significance |
|---|---|---|
| Mild | ~5% | Usually manageable with prompt fluid replacement |
| Moderate | ~10% | Requires timely rehydration and clinical assessment |
| Severe | ~15% | Potentially life-threatening and requires urgent treatment |
Types of Dehydration
| Type | Relative Loss | Examples |
|---|---|---|
| Isotonic | Water and sodium lost proportionately | Vomiting, diarrhea |
| Hypertonic | Water loss exceeds sodium loss | Fever, inadequate water intake |
| Hypotonic | Sodium loss exceeds water loss | Some diuretic-related states |
Causes
- Severe diarrhea and vomiting
- Excessive urinary water loss
- Diabetes mellitus with osmotic diuresis
- Diabetes insipidus
- Adrenal disorders
- Inadequate water intake
- Prolonged physical activity in a hot environment
- Excessive sweating
- Excessive use of laxatives or diuretics
Clinical Features
Early or moderate dehydration may produce:
- Thirst
- Dry mouth
- Reduced sweating
- Reduced urine output
- Headache
- Dizziness
- Weakness
- Muscle cramps
Severe dehydration may result in:
- Reduced circulating blood volume
- Reduced cardiac output
- Hypotension
- Syncope
- Hypovolemic shock
- Acute kidney injury
- Altered consciousness
- Seizures or coma in extreme cases
16. Oral Rehydration Therapy
Oral rehydration solution (ORS) is used to replace water and electrolytes lost during dehydration, particularly with gastrointestinal fluid loss.
The effectiveness of ORS depends importantly on intestinal sodium-glucose cotransport. Glucose facilitates sodium absorption, and water follows the absorbed solutes.
Standard ORS formulations contain appropriate amounts of glucose and electrolytes, including sodium and potassium, with a base such as citrate.
ORS is not simply water. It provides water together with glucose and electrolytes to improve intestinal fluid absorption.
17. Water Intoxication and Overhydration
Water intoxication occurs when water intake or retention exceeds the body's ability to excrete free water. Severe excess water can lower plasma sodium concentration and cause cellular swelling, particularly affecting the brain.
Important Causes
- Heart failure with impaired water handling
- Renal impairment
- Excessive ADH activity, including SIADH
- Excessive administration of intravenous fluids
- Conditions with impaired free-water excretion
Clinical Features
- Nausea and vomiting
- Headache
- Drowsiness and altered behavior
- Weakness and muscle cramps
- Confusion
- Seizures in severe cases
- Coma in life-threatening cases
18. High-Yield MBBS Revision
TBW is approximately 50–65% of body weight, depending on age, sex and body composition.
ICF is the largest body-fluid compartment and contains approximately two-thirds of TBW.
ECF contains approximately one-third of TBW and includes plasma and interstitial fluid.
Sodium predominates in ECF, while potassium predominates in ICF.
D₂O and T₂O distribute through total body water.
Inulin and other suitable extracellular markers remain predominantly outside cells.
ICF = TBW − ECF.
Interstitial fluid = ECF − plasma volume.
Water leaves cells and cells shrink.
Water enters cells and cells swell.
19. Quick MBBS Takeaway
Body water → ICF + ECF.
Approximately two-thirds of total body water is intracellular and approximately one-third is extracellular.
ECF is Na⁺-rich, whereas ICF is K⁺-rich.
Changes in ECF osmolality cause water to shift between ECF and ICF until osmotic equilibrium is approached.
Hypertonic ECF → cell shrinkage.
Hypotonic ECF → cell swelling.
Fluid compartments can be estimated using the indicator dilution principle. ICF and interstitial fluid are generally calculated from other measured compartments.
Disturbances of water balance can produce dehydration, hypovolemia, electrolyte abnormalities, cellular swelling and other clinically important conditions.
20. Key Points for Revision
- Total body water varies mainly with age, sex and body composition.
- Lean individuals generally have a higher percentage of body water than individuals with obesity.
- ICF is the largest body-fluid compartment.
- ECF consists mainly of plasma and interstitial fluid.
- Sodium is the principal extracellular cation.
- Potassium is the principal intracellular cation.
- TBW can be measured using indicators that distribute throughout body water.
- ECF can be estimated using indicators that remain outside cells.
- Plasma volume requires an indicator that remains predominantly intravascular.
- ICF = TBW − ECF.
- Interstitial fluid = ECF − plasma volume.
- Osmolality refers to osmoles per kilogram of water.
- Osmolarity refers to osmoles per litre of solution.
- Tonicity describes the effective osmotic influence of a solution on cell volume.
- Dehydration occurs when water loss exceeds replacement.
- Severe water loss can produce hypovolemia and hypovolemic shock.
- Excess free-water retention can produce water intoxication and neurological complications.
Body fluids maintain the internal environment, provide a medium for transport and metabolism, and are distributed mainly between ICF and ECF; changes in their volume or osmolality produce important physiological and clinical disturbances.
Medical Disclaimer: The information provided in this article is strictly for educational, study, and exam-preparation purposes. It does not constitute professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider for clinical decisions.