Body Fluids and Fluid Compartments

Overview of Body Fluid Balance and Homeostasis

Importance of Fluid Balance: The maintenance of a relatively constant volume and stable chemical composition of body fluids is essential for homeostasis and optimal cellular function.
Role of the Renal System: The kidneys play a primary role in maintaining water balance and regulating the composition of the extracellular fluid (ECF).
Principle of Balance: Fluid intake must precisely match fluid output to prevent body fluid volumes from increasing (hyperhydration) or decreasing (dehydration).
Sources of Daily Water Intake:
- Fluid Ingestion and Food: Water added to the body via direct fluid ingestion or water contained in food averages approximately $2100\,mL/day$ .
- Metabolic Water: Water synthesized within the body as a byproduct of carbohydrate oxidation accounts for approximately $200\,mL/day$ .
- Total Normal Intake: Combines to approximately $2300\,mL/day$ .
- Variability: Daily water intake varies widely between individuals and across different days depending on climate, personal habits, and levels of physical activity.
Routes of Daily Water Output:
- Insensible Loss: Evaporation through the respiratory tract and skin accounts for an average insensible water loss of approximately $700\,mL/day$ .
- Sweat: Normal baseline sweat loss is approximately $100\,mL/day$ . During strenuous exercise in hot weather, sweat production can increase to $1\text{--}2\,L/hour$ . Uncompensated sweat loss can rapidly deplete body fluid reserves if thirst centers are not stimulated to prompt fluid intake.
- Feces: Normal gastrointestinal water loss in feces is approximately $100\,mL/day$ . In pathological states such as severe diarrhea, fecal fluid loss can escalate to several liters per day, posing a life-threatening threat.
- Urine: Excretion by the kidneys represents the most important and dynamically regulated route for maintaining overall fluid and electrolyte balance. Urinary output ranges from $0.5\,L/day$ in a severely dehydrated individual to $20\,L/day$ in a hyperhydrated individual.
Electrolyte Intake Variability:
- Essential electrolytes required by the body include sodium ( $\text{Na}^+$ ), chloride ( $\text{Cl}^-$ ), and potassium ( $\text{K}^+$ ).
- Daily dietary intake of sodium varies significantly between individuals, ranging from as low as $20\,mEq/day$ to as high as $300\text{--}500\,mEq/day$ .
- The kidneys continuously adjust the excretion rate of water and electrolytes to match intake precisely and to compensate for abnormal fluid and electrolyte losses occurring in disease states.

Body Fluid Compartments

Total Body Water (TBW) Statistics:
- In an average $70\,kg$ adult male, total body water constitutes approximately $60\%$ of total body weight, equal to about $42\,L$ of fluid.
- Gender Differences: Females generally possess a higher percentage of body fat than males; because adipose tissue contains little water, total body water in adult females averages approximately $50\%$ of body weight.
- Age Differences: Premature and newborn infants have a significantly higher proportion of water, accounting for $70\text{--}75\%$ of their total body weight.
Major Division of Body Fluid Compartments:
- Total body water is divided into two major compartments: the Intracellular Fluid (ICF) and the Extracellular Fluid (ECF).
Intracellular Fluid (ICF) Compartment:
- Accounts for approximately $40\%$ of total body weight, encompassing about $28\,L$ of fluid in an average $70\,kg$ individual.
- Confined inside the approximately $100\text{ trillion}$ cells of the body.
- Separated from the extracellular fluid compartment by the cell membrane.
- Ionic Composition: Contains very small quantities of sodium ( $\text{Na}^+$ ) and chloride ( $\text{Cl}^-$ ) ions, and almost no calcium ( $\text{Ca}^{2+}$ ) ions. It contains high concentrations of potassium ( $\text{K}^+$ ) and phosphate ( $\text{HPO}_4^{2-}, \text{H}_2\text{PO}_4^-$ ) ions, alongside moderate quantities of magnesium ( $\text{Mg}^{2+}$ ) and sulfate ( $\text{SO}_4^{2-}$ ) ions.
- Protein Content: Cell interiors contain large amounts of protein—nearly four times the protein concentration found in blood plasma.
Extracellular Fluid (ECF) Compartment:
- Includes all body fluids located outside of the cells.
- Accounts for approximately $20\%$ of total body weight, corresponding to about $14\,L$ in an average $70\,kg$ individual.
- Subcompartments:
- Interstitial Fluid: Represents $3/4$ ( $75\%$ ) of the ECF, totaling approximately $11\,L$ .
- Blood Plasma: Represents $1/4$ ( $25\%$ ) of the ECF, totaling approximately $3\,L$ .
- Transcellular Fluid: Considered specialized portions of the ECF, totaling approximately $1\text{--}2\,L$ . Includes synovial fluid, peritoneal fluid, pericardial fluid, intraocular fluid, and cerebrospinal fluid (CSF).
- Capillary Membrane Dynamics: Continuous fluid and solute exchange occurs between blood plasma and interstitial fluid through porous capillary membranes. Because capillary walls are highly permeable to small solutes but impermeable to large plasma proteins, plasma and interstitial fluid have nearly identical electrolyte compositions, with the major difference being a significantly higher protein concentration in blood plasma.
- Homeostatic Control: The precise composition of the ECF is tightly regulated by the kidneys to ensure cells remain bathed in an optimal fluid environment for survival and functioning.

Composition of Body Fluid Compartments

The quantitative solute and osmolar concentrations across Plasma, Interstitial Fluid, and Intracellular Fluid are detailed below (measured in $\text{mOsm/L } H_2O$ ):
- Sodium ( $\text{Na}^+$ ): Plasma = $142\,mOsm/L$ ; Interstitial = $139\,mOsm/L$ ; Intracellular = $14\,mOsm/L$
- Potassium ( $\text{K}^+$ ): Plasma = $4.2\,mOsm/L$ ; Interstitial = $4.0\,mOsm/L$ ; Intracellular = $140\,mOsm/L$
- Calcium ( $\text{Ca}^{2+}$ ): Plasma = $1.3\,mOsm/L$ ; Interstitial = $1.2\,mOsm/L$ ; Intracellular = $0\,mOsm/L$
- Magnesium ( $\text{Mg}^{2+}$ ): Plasma = $0.8\,mOsm/L$ ; Interstitial = $0.7\,mOsm/L$ ; Intracellular = $20\,mOsm/L$
- Chloride ( $\text{Cl}^-$ ): Plasma = $108\,mOsm/L$ ; Interstitial = $108\,mOsm/L$ ; Intracellular = $4\,mOsm/L$
- Bicarbonate ( $\text{HCO}_3^-$ ): Plasma = $24\,mOsm/L$ ; Interstitial = $28.3\,mOsm/L$ ; Intracellular = $10\,mOsm/L$
- Phosphate ( $\text{HPO}_4^{2-}, \text{H}_2\text{PO}_4^-$ ): Plasma = $2\,mOsm/L$ ; Interstitial = $2\,mOsm/L$ ; Intracellular = $11\,mOsm/L$
- Sulfate ( $\text{SO}_4^{2-}$ ): Plasma = $0.5\,mOsm/L$ ; Interstitial = $0.5\,mOsm/L$ ; Intracellular = $1\,mOsm/L$
- Phosphocreatine: Plasma = $0\,mOsm/L$ ; Interstitial = $0\,mOsm/L$ ; Intracellular = $45\,mOsm/L$
- Carnosine: Plasma = $0\,mOsm/L$ ; Interstitial = $0\,mOsm/L$ ; Intracellular = $14\,mOsm/L$
- Amino Acids: Plasma = $2\,mOsm/L$ ; Interstitial = $2\,mOsm/L$ ; Intracellular = $8\,mOsm/L$
- Creatine: Plasma = $0.2\,mOsm/L$ ; Interstitial = $0.2\,mOsm/L$ ; Intracellular = $9\,mOsm/L$
- Lactate: Plasma = $1.2\,mOsm/L$ ; Interstitial = $1.2\,mOsm/L$ ; Intracellular = $1.5\,mOsm/L$
- Adenosine Triphosphate (ATP): Plasma = $0\,mOsm/L$ ; Interstitial = $0\,mOsm/L$ ; Intracellular = $5\,mOsm/L$
- Hexose Monophosphate: Plasma = $0\,mOsm/L$ ; Interstitial = $0\,mOsm/L$ ; Intracellular = $3.7\,mOsm/L$
- Glucose: Plasma = $5.6\,mOsm/L$ ; Interstitial = $5.6\,mOsm/L$ ; Intracellular = $0\,mOsm/L$
- Protein: Plasma = $1.2\,mOsm/L$ ; Interstitial = $0.2\,mOsm/L$ ; Intracellular = $4\,mOsm/L$
- Urea: Plasma = $4\,mOsm/L$ ; Interstitial = $4\,mOsm/L$ ; Intracellular = $4\,mOsm/L$
- Others: Plasma = $4.8\,mOsm/L$ ; Interstitial = $3.9\,mOsm/L$ ; Intracellular = $10\,mOsm/L$
- Total mOsm/L: Plasma = $301.8\,mOsm/L$ ; Interstitial = $300.8\,mOsm/L$ ; Intracellular = $301.2\,mOsm/L$
- Corrected Osmolar Activity: Plasma = $282.0\,mOsm/L$ ; Interstitial = $281.0\,mOsm/L$ ; Intracellular = $281.0\,mOsm/L$
- Total Osmotic Pressure at $37^\circ C$ : Plasma = $5443\,mm\,Hg$ ; Interstitial = $5423\,mm\,Hg$ ; Intracellular = $5423\,mm\,Hg$

Measurement of Body Fluid Compartments

Formula for chamber volume using indicator dilution

Indicator Dilution Method (Dye Dilution Method):
- Principle: Total body water and individual body fluid compartment volumes are determined by injecting a known mass of a marker substance or dye into a compartment, allowing it to disperse evenly throughout the compartment, measuring its concentration in a sample, and applying the dilution formula.
- Mathematical Formula:

$\text{Volume}_B = \frac{\text{Volume}_A \times \text{Concentration}_A}{\text{Concentration}_B}$

Required Information for Calculation:
1. The total amount or mass of indicator injected into the chamber ( $\text{Volume}_A \times \text{Concentration}_A$ ), forming the numerator.
2. The concentration of indicator in the fluid sample after complete dispersion ( $\text{Concentration}_B$ ), forming the denominator.
Worked Example Calculation:
- If $5\,mL$ of a solution containing $20\,mg/mL$ of dye is injected into Chamber B, and the final dispersed concentration is $0.07\,mg/mL$ , the volume of Chamber B is calculated as:

$\text{Volume}_B = \frac{5\,mL \times 20\,mg/mL}{0.07\,mg/mL} = \frac{100\,mg}{0.07\,mg/mL} = 1429\,mL$

Essential Characteristics of Ideal Marker Substances:
1. Must be non-toxic to biological tissues.
2. Must mix thoroughly and evenly throughout the target compartment within the shortest possible time.
3. Must not be excreted rapidly during the mixing phase.
4. Should be completely excreted from the body within a reasonable timeframe.
5. Must not alter the natural color of body fluids.
6. Must not alter the natural volume of body fluids or shift water between compartments.
Measurement of Specific Compartments:
- Total Body Water (TBW):
- Indicators: Radioactive water (tritium, $^3\text{H}_2\text{O}$ ) or heavy water (deuterium, $^2\text{H}_2\text{O}$ ).
- Process: These indicators mix completely with all fluid compartments within a few hours following injection. Concentration is measured from a plasma sample and applied to the indicator dilution formula.
- Extracellular Fluid (ECF) Volume:
- Indicators: Substances that disperse evenly throughout plasma and interstitial fluid but cannot readily cross cell membranes. Examples include radioactive sodium, radioactive chloride, radioactive iothalamate, thiosulfate ion, and inulin.
- Process: Following intravenous injection, mixing occurs across all ECF subcompartments within $30\text{--}60\,minutes$ . Plasma samples are analyzed to calculate volume.
- Subspace Names: Because markers like sodium, chloride, inulin, and sucrose diffuse throughout ECF subcompartments, the measured ECF volume is frequently termed the sodium space, chloride space, inulin space, or sucrose space.
- Intracellular Fluid (ICF) Volume:
- Cannot be measured directly because no marker distributes exclusively within cell interiors without entering ECF.
- Calculation Formula:

$\text{Intracellular Volume} = \text{Total Body Water} - \text{Extracellular Volume}$

Interstitial Fluid Volume:
- Cannot be measured directly because markers that penetrate interstitium also distribute throughout plasma.
- Calculation Formula:

$\text{Interstitial Fluid Volume} = \text{ECF Volume} - \text{Plasma Volume}$

Plasma Volume:
- Indicators: Substances that bind tightly to plasma proteins and do not cross capillary walls into the interstitium in significant amounts. Examples include serum albumin labeled with radioactive iodine ( $^{125}\text{I}\text{-albumin}$ ) and protein-binding dyes such as Evans blue dye (also designated as T-1824).
Total Blood Volume:
- Calculated from plasma volume if the hematocrit (the fraction of blood volume occupied by blood cells) is known.
- Calculation Formula:

$\text{Total Blood Volume} = \frac{\text{Plasma Volume}}{1 - \text{Hematocrit}}$

Fluid Exchange Between ECF and ICF

Plasma vs. Interstitial Fluid Dynamics: Fluid distribution between plasma and interstitial space is regulated by the balance of capillary hydrostatic pressure and plasma colloid osmotic pressure acting across capillary endothelial membranes.
ECF vs. ICF Dynamics: Fluid distribution between intracellular and extracellular compartments is governed by the osmotic effect of small solute particles—primarily sodium ( $\text{Na}^+$ ), chloride ( $\text{Cl}^-$ ), and other electrolytes—acting across cell membranes.
Cell Membrane Permeability: Cell membranes are highly permeable to water molecules but relatively impermeable to small ions like sodium and chloride.
Isotonic Equilibrium: Rapid osmotic movement of water across cell membranes maintains intracellular fluid in constant osmotic equilibrium (isotonicity) with extracellular fluid.

Applied Physiology: Dehydration

Definition: Dehydration is defined as an excessive loss of water from the body.
Fluid Requirements: Minimum daily water intake required for basic physiological maintenance is $1\,L$ , though requirements vary with age and activity level. Active individuals require $2\text{--}3\,L/day$ . Dehydration develops whenever total fluid loss exceeds total fluid intake.
Classification Based on Severity:
- Mild Dehydration: Characterized by a loss of approximately $5\%$ of total body fluid. Typically non-serious; managed successfully with rehydration therapy.
- Moderate Dehydration: Characterized by a loss of approximately $10\%$ of total body fluid. Requires rehydration therapy.
- Severe Dehydration: Characterized by a loss of approximately $15\%$ of total body fluid. Highly severe and life-threatening; requires immediate hospitalization and emergency medical treatment.
Classification Based on Solute-to-Solvent Ratio:
- Isotonic Dehydration: Characterized by balanced, proportional losses of solute ( $\text{Na}^+$ ) and water. Typical causes include acute diarrhea and vomiting.
- Hypotonic Dehydration: Characterized by greater loss of solute ( $\text{Na}^+$ ) relative to water loss. Caused by excessive diuretic medication use.
- Hypertonic Dehydration: Characterized by greater loss of water relative to solute loss. Typical cause includes high fever.
Signs and Symptoms of Dehydration:
- Mild to Moderate Dehydration:
1. Dryness of the mouth
2. Excessive thirst
3. Decreased sweating
4. Decreased urine output (oliguria)
5. Headache
6. Dizziness
7. General body weakness
8. Muscle cramps in legs and arms
- Severe Dehydration:
1. Marked decrease in blood volume (hypovolemia)
2. Reduced cardiac output
3. Low blood pressure (hypotension)
4. Hypovolemic cardiac shock
5. Fainting (syncope)
- Very Severe Dehydration:
1. Organic damage to critical organs including the brain, liver, and kidneys
2. Severe mental depression and confusion
3. Acute renal failure
4. Convulsions
5. Coma

Applied Physiology: Water Intoxication

Definition: Water intoxication—also termed hyperhydration, overhydration, or water poisoning—is a physiological condition defined by a significant increase in total body water content.
Etiology: Occurs when fluid intake significantly exceeds the excretory capacity of the body. Water intoxication caused purely by excess voluntary drinking is rare in individuals with intact organ systems, but specific clinical conditions induce hyperhydration:
1. Heart Failure: The impaired myocardium fails to pump blood efficiently, reducing renal perfusion and fluid excretion.
2. Renal Disorders: Impaired kidney function reduces the ability to excrete water in urine.
3. Hypersecretion of Antidiuretic Hormone (ADH): Pathological states such as Syndrome of Inappropriate Antidiuretic Hormone secretion (SIADH).
4. Iatrogenic Fluid Administration: Excessive intravenous administration of fluid or medications beyond excretory capability.
5. Infant Vulnerability: Neonates carry a elevated risk during their first month of life.
Signs and Symptoms of Water Intoxication:
1. Neurological Changes: The brain is highly vulnerable to cell swelling; behavioral and cognitive changes manifest first.
2. Central Nervous System Depression: Drowsiness and inattentiveness.
3. Gastrointestinal Effects: Nausea and vomiting.
4. Physical Findings: Rapid weight gain, followed by marked generalized weakness and blurred vision.
5. Vascular & Hematologic Complications: Anemia, cyanosis, internal hemorrhage, and circulatory shock.
6. Neuromuscular Dysfunctions: Muscle weakness, cramps, twitching, poor motor coordination, and paralysis.
7. Severe Complications:
- Delirium: Extreme mental disorientation characterized by confusion and visual or sensory illusions.
- Seizures: Sudden, uncontrolled, involuntary muscular contractions.
- Coma: Profound state of unconsciousness characterized by non-responsiveness to external stimuli and loss of voluntary actions.
Treatment Protocols:
- Mild Water Intoxication: Treated solely by strict fluid restriction.
- Very Severe Water Intoxication:
1. Diuretic Therapy: Administration of diuretics to promote free water elimination via urine.
2. ADH Receptor Antagonists: Administration of receptor antagonists to block ADH-mediated water reabsorption in the renal collecting tubules.
3. Hypertonic Saline: Intravenous administration of hypertonic saline solution to restore plasma sodium ( $\text{Na}^+$ ) concentration and resolve osmotic gradients.