Guyton & Hall: Regulation of Body Fluid Compartments, Extracellular and Intracellular Fluids, and Edema

Overview of Body Fluid Homeostasis and Balance

• Stability and Composition: The maintenance of a relatively constant volume and stable composition of body fluids is essential for homeostasis.
• Clinical Significance: Abnormalities in the control systems that maintain body fluid constancy are some of the most common and important problems in clinical medicine.
• Fundamental Balance Principles:   - Survival requires the maintenance of fluid and electrolyte balance.   - Fluid Balance: $ext{Fluid Loss} = ext{Fluid Intake}$.   - Electrolyte Balance: $ext{Electrolyte Loss} = ext{Electrolyte Intake}$.
• Regulatory Mechanisms:   - Fluid Intake: Regulated by thirst and dietary habits.   - Electrolyte Intake: Governed primarily by dietary habits.   - Fluid Output: Regulated mainly by the kidneys.   - Electrolyte Output: Regulated mainly by the kidneys.

Daily Intake and Loss of Water

• Sources of Water Intake:   - Ingested: Liquids or water contribute approximately $2100\,mL/day$.   - Metabolic Synthesis: Water synthesized in the body by the oxidation of carbohydrates contributes approximately $200\,mL/day$.   - Total Intake: Standard total daily intake is approximately $2300\,mL/day$, though this is variable based on climate, habits, and physical activity.
• Routes of Water Loss:   - Insensible Water Loss: Occurs without conscious awareness.     - Evaporation through Respiratory Tract: Approximately $300\text{--}400\,mL/day$. Humidified air is exhaled; water loss increases in cold air.     - Diffusion through Skin: Approximately $300\text{--}400\,mL/day$. This is independent of sweating and is minimized by the cholesterol-filled, cornified layer of the skin. If this layer is damaged (e.g., by burns), the evaporate rate can increase significantly.   - Sweat: Highly variable depending on physical activity and environment. Standard loss is $100\,mL/day$, but can reach $1\text{--}2\,L/hour$. The thirst mechanism is activated to balance these losses.   - Feces: Standard loss is $100\,mL/day$. In cases of severe diarrhea, this can increase to several liters per day, becoming life-threatening.   - Kidneys: Urine excretion is the most important means of regulating water and electrolyte balance. Range varies from $0.5\,L/day$ in dehydrated states to $20\,L/day$ in overhydrated states.

Comparative Intake and Output: Normal vs. Heavy Exercise

• Fluid Intake Comparison:   - Fluid Ingested: Normal = $2100\,mL/day$; Prolonged Heavy Exercise = Variable (Thirst-driven).   - Metabolism: Normal = $200\,mL/day$; Prolonged Heavy Exercise = $200\,mL/day$.   - Total Intake: Normal = $2300\,mL/day$.
• Fluid Output Comparison:   - Insensible (Skin): Normal = $350\,mL/day$; Prolonged Heavy Exercise = $350\,mL/day$.   - Insensible (Lungs): Normal = $350\,mL/day$; Prolonged Heavy Exercise = $650\,mL/day$.   - Sweat: Normal = $100\,mL/day$; Prolonged Heavy Exercise = $5000\,mL/day$.   - Feces: Normal = $100\,mL/day$; Prolonged Heavy Exercise = $100\,mL/day$.   - Urine: Normal = $1400\,mL/day$; Prolonged Heavy Exercise = $500\,mL/day$.   - Total Output: Normal = $2300\,mL/day$; Prolonged Heavy Exercise = $6600\,mL/day$.

Body Fluid Compartments and Distribution

• Extracellular Fluid (ECF):   - Represents $1/3$ of total body water ($TBW$) or approximately $20\%$ of total body weight.   - Interstitial Fluid: Comprises $3/4$ of the ECF ($11\,L$).   - Blood Plasma: Comprises $1/4$ of the ECF ($3\,L$).
• Intracellular Fluid (ICF):   - Represents $2/3$ of total body water ($TBW$) or approximately $40\%$ of total body weight.   - Volume is approximately $28\,L$ (contained inside cells).
• Total Body Water Variations:   - Babies: $70\text{--}75\%$ of body weight.   - Adult Men: $60\%$ of body weight.   - Adult Women: $50\%$ of body weight (typically lower due to higher body fat percentage).
• Effects of Body Build, Gender, and Age (TBW %):   - Normal Build: Male ($60\%$), Female ($50\%$), Infant ($70\%$).   - Lean Build: Male ($70\%$), Female ($60\%$), Infant ($80\%$).   - Obese Build: Male (<50\%), Female (<42\%), Infant (<60\%).

Detailed Compartment Composition

• Intracellular Fluid (ICF) Specifics:   - Separated from ECF by a cell membrane that is highly permeable to water but poorly permeable to electrolytes.   - Major Cations: Potassium ($K^+$), Magnesium ($Mg^{2+}$).   - Major Anions: Proteins (concentration is $4\times$ that of plasma), Organic phosphates, Sulfate ($SO_3^-$).   - Small Quantities: Sodium ($Na^+$), Chloride ($Cl^-$).   - Minimal Presence: Almost zero Calcium ($Ca^{2+}$).
• Extracellular Fluid (ECF) Specifics:   - Major Components: Interstitial Fluid and Plasma.   - Plasma: The non-cellular portion of blood.   - Transcellular Fluid: Specialized ECF contained in spaces like synovial, peritoneal, pericardial, and intraocular spaces, plus the Cerebrospinal Fluid (CSF). Constitutes $1\text{--}2\,L$.   - Composition: High in $Na^+$, $Cl^-$, and Bicarbonate ($HCO_3^-$). Low in $K^+$, $Ca^{2+}$, $Mg^{2+}$, Phosphates ($PO_4^-$), and Sulfate ($SO_3^-$).
• Blood Volume:   - Contains both ECF (plasma) and ICF (within Red Blood Cells or RBCs).   - Average Volume: $7\text{--}9\%$ of total body weight; approximately $5\,L$ in adults.   - Composition: $60\%$ plasma, $40\%$ formed elements.   - Hematocrit: The fraction of blood composed of RBCs.
• The Donnan Effect:   - Result of negatively charged plasma proteins attracting cations and repelling anions.   - Results in a concentration of positively charged ions being slightly higher in plasma than in interstitial fluid.

Movement and Osmotic Equilibrium

• Starling Forces:   - Hydrostatic Pressure: Drives fluid out of the plasma.   - Osmotic/Oncotic Pressure: Regulates the return of fluid to the plasma.
• Osmotic Principles:   - Osmosis: The diffusion of water through a selectively permeable membrane from higher water concentration to lower water concentration until equilibrium is reached.   - Osmolality: Concentration expressed as osmoles per kg of water.   - Osmolarity: Concentration expressed as osmoles per liter of solution.   - Osmotic Pressure: Qualitative measurement of osmosis.   - Solute Contributions: In ECF, $80\%$ of osmolarity is due to $Na^+$ and $Cl^-$. In ICF, approximately $50\%$ is due to $K^+$.
• Tonicity and Cell Volume:   - Isotonic: Same solute concentration as cells (e.g., $0.9\%$ saline). No volume change.   - Hypertonic: Solution has higher solute concentration; causes cells to shrink as water moves out.   - Hypotonic: Solution has lower solute concentration; causes cells to swell as water moves in.
• Osmotic Fluids and Equilibrium:   - Isosmotic Fluids: Same osmolarity as the cell (e.g., $5\%$ glucose solution).   - Equilibrium Time: It typically takes about $30\,minutes$ to achieve osmotic equilibrium after drinking water.
• Fluid Addition Scenarios:   - Add Isotonic NaCl: Increases ECF volume; no change in osmolarity or ICF volume.   - Add Hypertonic NaCl: Increases ECF volume and osmolarity; decreases ICF volume (osmosis out of cells) and increases ICF osmolarity.   - Add Water/Hypotonic Solution: Increases both ECF and ICF volumes; decreases both ECF and ICF osmolarity.

Abnormalities in Sodium Concentration

• Normal Plasma [Na+]: Approximately $142\,mEq/L$.
• Hyponatremia (Decreased Na+):   - Causes via NaCl loss: Diarrhea, vomiting, diuretics, Addison's disease, sodium-wasting renal diseases.   - Causes via Water retention: SIADH (Syndrome of Inappropriate Antidiuretic Hormone), excess water consumption.   - Consequences (Acute): Brain cell edema. Symptoms include headache, nausea, lethargy, and disorientation. Severe cases lead to seizures, coma, permanent brain damage, and death. Because the skull is rigid, a brain volume increase of >10\% can force the brain into the neck (herniation).
• Hypernatremia (Increased Na+):   - Causes via Water loss: Diabetes Insipidus (central or nephrogenic), dehydration, excessive sweating.   - Causes via NaCl excess: Excessive aldosterone secretion (leads to hypernatremia and overhydration).   - Consequences: Initial cell shrinkage; promotes intense thirst and stimulates ADH (Antidiuretic Hormone) secretion.

Edema: Causes and Mechanisms

• Definition: Presence of excess fluid in body tissues.
• Intracellular Edema Causes:   - Hyponatremia.   - Depression of tissue metabolic systems.   - Lack of adequate cellular nutrition.
• Extracellular Edema Causes:   - Increased Capillary Pressure:     - Excessive kidney retention of salt/water (acute/chronic kidney failure, mineralocorticoid excess).     - High venous pressure/constriction (heart failure, venous obstruction, failure of venous pumps).     - Decreased arteriolar resistance (excessive body heat, sympathetic nervous system insufficiency, vasodilator drugs).   - Decreased Plasma Proteins:     - Loss in urine (Nephrotic syndrome).     - Loss from denuded skin (burns, wounds).     - Failure to produce proteins (liver disease, malnutrition like Kwashiorkor).   - Increased Capillary Permeability:     - Immune reactions (histamine release).     - Toxins, bacterial infections, or prolonged ischemia.     - Burns.   - Blockage of Lymphatic Return:     - Cancer, surgery, or infections (e.g., Filaria nematodes).

Safety Factors Preventing Edema

• Total Safety Factor: $17\,mmHg$.   - Low Compliance of Interstitium: When interstitial fluid (IF) pressure is negative, compliance is low. Normal pressure is $-3\,mmHg$. Safety factor = $3\,mmHg$.   - Increased Lymph Flow: Lymph flow can increase $10\text{--}50\text{-fold}$. Safety factor = $7\,mmHg$.   - Washdown of Interstitial Fluid Proteins:     - Increased filtration into the interstitium increases IF hydrostatic pressure.     - This increases lymph flow, which removes more protein from the interstitium.     - Decreased protein concentration in the IF decreases IF oncotic pressure, which subsequently decreases capillary filtration.     - Safety factor = $7\,mmHg$.