Fluid, Electrolyte, and Acid-Base Balance: Structured Study Notes

Fluid, Electrolyte, and Acid-Base Balance

Body Fluids and Compartments

The human body is composed primarily of water, which is distributed across various compartments. Understanding the distribution and composition of body fluids is essential for maintaining homeostasis.

• Body Water Content:

  • Males: 60% water, 40% solid materials

  • Females: 50% water, 50% solid materials

  • Fat tissue is hydrophobic and contains less water; skeletal muscle is highly hydrated.

  • Water content by age:

    • Early embryo: 97% water

    • Newborn infant: 77% water

    • Adult male: 60% water

    • Adult female: 50% water

    • Elderly adult: 45% water

• Fluid Compartments:

  • Intracellular Fluid (ICF): Fluid within cells (cytosol, nucleoplasm, mitochondrial matrix). 33% in males, 27% in females.

  • Extracellular Fluid (ECF): Fluid outside cells, including:

    • Interstitial fluid: Between cells (21.5% in males, 18% in females)

    • Intravascular: Blood plasma (4.5% in both sexes)

    • Other: Lymph, cerebrospinal fluid, eye humors, synovial fluid, serous fluids, GI secretions (<1%)

• Composition of Body Fluids:

  • Electrolytes: Dissociate in solution to form ions, conducting electricity. Examples: salts, acids, bases.

    • Cations: Na+, H+, K+, Ca2+, Mg2+

    • Anions: Cl-, HCO3-, HPO42-, SO42-

  • Non-electrolytes: Do not dissociate; include proteins, carbohydrates, lipids, creatinine, urea.

  • Electrolyte concentrations are measured in milliequivalents per liter (mEq/L).

  • Comparison of ECF and ICF:

    • ECF (blood plasma, interstitial fluid): High Na+, high Cl-, higher protein in plasma.

    • ICF: High K+, high HPO42-

• Fluid Movement Between Compartments:

  • Blood plasma links external and internal environments, ICF and ECF.

  • Capillary exchange:

    • Arteriole end: Hydrostatic pressure forces fluid out.

    • Venous end: Colloid osmotic pressure draws fluid in.

  • Movement between interstitial and intracellular spaces is regulated by membrane permeability.

    • Water moves freely by osmosis.

    • Nutrients and oxygen move into cells; wastes and CO2 move out.

    • Transport mechanisms: Facilitated diffusion, primary/secondary active transport, endocytosis, exocytosis.

Water Balance

Fluid balance is achieved when water intake equals water output. Hormonal regulation is crucial for maintaining hydration and sodium balance.

• Obligatory Water Output (2500 mL/day):

  • Urine: 1200 mL

  • Evaporation via skin: 750 mL

  • Evaporation via lungs: 400 mL

  • Feces: 150 mL

• Water Intake (2500 mL/day):

  • Solid foods: 1000 mL

  • Liquids/beverages: 1200 mL

  • Metabolic water: 300 mL

• Hormonal Regulation:

  • ADH (Antidiuretic Hormone): Produced by hypothalamus, secreted by neurohypophysis. Increases water reabsorption in kidneys when blood volume is low.

  • ANP (Atrial Natriuretic Peptide): Secreted by heart atria; blocks ADH, increases urine output when blood volume is high.

  • Aldosterone: Secreted by adrenal cortex (zona glomerulosa) when Na+ is low; increases Na+ and water reabsorption in kidneys.

• Disorders of Water Balance:

  • Dehydration: Water loss exceeds intake; causes include hemorrhage, burns, vomiting, diarrhea, sweating, deprivation, diuretics. Cells crenate in hypertonic ECF.

  • Diabetes insipidus: Chronic dehydration due to low ADH.

  • Hypotonic hydration: Water gain exceeds loss; can cause cell lysis if kidneys fail to excrete excess water.

Electrolyte Balance

Electrolyte balance involves regulating the levels of minerals in body fluids. Hormones play a key role in maintaining proper concentrations.

• Sodium (Na+):

  • Regulated by aldosterone (increases reabsorption) and ANP (reduces reabsorption).

• Potassium (K+):

  • Moves opposite to Na+; aldosterone increases K+ secretion, ANP decreases it.

  • Imbalances are rare but dangerous.

• Chloride (Cl-):

  • Follows Na+ movement; reabsorbed with Na+.

• Calcium (Ca2+):

  • Regulated by PTH (raises Ca2+) and calcitonin (lowers Ca2+).

• Phosphate (HPO42-):

  • Regulated by PTH (lowers phosphate) and calcitonin (raises phosphate), opposite to calcium.

• Electrolyte Disorders:

  • Hypernatremia/hyponatremia: Na+ > 145 mEq/L or < 135 mEq/L

  • Hyperkalemia/hypokalemia: K+ > 5.5 mEq/L or < 3.5 mEq/L

  • Hyperchloremia/hypochloremia: Cl- > 105 mEq/L or < 95 mEq/L

  • Hypercalcemia/hypocalcemia: Ca2+ > 5.2 mEq/L or < 4.5 mEq/L

  • Hyperphosphatemia/hypophosphatemia: HPO42- > 2.9 mEq/L or < 1.6 mEq/L

  • Hypermagnesemia/hypomagnesemia: Mg2+ > 2.2 mEq/L or < 1.4 mEq/L

Acid-Base Balance

Acid-base balance is vital for physiological function. The body uses chemical and physiological buffers to maintain pH within a narrow range.

• Acids: Electrolytes that release H+ (proton donors); pH < 7. Types:

  • Fixed acids: Remain in body fluids until eliminated by kidneys (e.g., sulfuric, phosphoric acid).

  • Organic acids: By-products of metabolism (e.g., lactic acid, ketone bodies).

  • Volatile acids: Can leave the body via lungs (e.g., carbonic acid).

• Bases: Electrolytes that release OH- (proton acceptors); pH > 7.

• Key Terms:

  • pH: Negative logarithm of H+ concentration. $pH = -\log[H^+]$

  • Neutral: pH = 7; equal H+ and OH-.

  • Salt: Ionic compound with cation other than H+ and anion other than OH-.

  • Buffer: Opposes changes in pH by removing/replacing H+; maintains blood pH (7.35–7.45).

• CO2 and pH: Partial pressure of CO2 (PCO2) inversely affects pH.

  • As PCO2 rises, pH drops (more acidic).

  • As PCO2 falls, pH rises (more basic).

• Buffer Systems:

  • Bicarbonate Buffer System (ECF):

    • NaHCO3 (weak base), H2CO3 (weak acid)

    • Example reactions:

      • $HCl + NaHCO_3 \rightarrow H_2CO_3 + NaCl$

      • $NaOH + H_2CO_3 \rightarrow NaHCO_3 + H_2O$

  • Phosphate Buffer System (ICF and urine):

    • Na2HPO4 (weak base), NaH2PO4 (weak acid)

    • Example reactions:

      • $HCl + Na_2HPO_4 \rightarrow NaH_2PO_4 + NaCl$

      • $NaOH + NaH_2PO_4 \rightarrow Na_2HPO_4 + H_2O$

  • Protein Buffer System: Most abundant; functions in both ICF and ECF.

    • Carboxyl groups (COOH) act as weak acids.

    • Amine groups (NH3) act as weak bases.

• Physiological Buffers: Lungs and kidneys compensate for acid-base imbalances.

Disorders of Acid-Base Balance

Acid-base disorders arise from metabolic or respiratory causes and are compensated by physiological mechanisms.

Condition	pH	PCO2	HCO3-	Causes	Compensations
Metabolic Acidosis	Low	Normal/Low	Low	Alcohol, diabetes, starvation, diarrhea, renal dysfunction	Hyperventilation, kidney reabsorption of HCO3-, secretion of H+
Metabolic Alkalosis	High	Normal/High	High	Vomiting, diuretics, gastric suction, excess NaHCO3, aldosterone	Hypoventilation, kidney reabsorption of H+, secretion of HCO3-
Respiratory Acidosis	Low	High	Normal/High	Hypoventilation, bronchitis, emphysema, cystic fibrosis, narcotics	Kidney reabsorption of HCO3-, secretion of H+, hyperventilation
Respiratory Alkalosis	High	Low	Normal/Low	Hyperventilation, emotions, hypoxia, brain injury	Kidney reabsorption of H+, secretion of HCO3-, hypoventilation

Metabolic/Electrolytic Disorders

Imbalances in fluid and electrolytes can lead to a variety of clinical disorders, each with distinct causes and symptoms.

Disorder	Causes	Results/Symptoms
Diabetes insipidus	Hyposecretion of ADH	Chronic dehydration
Hypotonic hydration	Water gain exceeds loss	Cell lysis, brain swelling
Edema	Medication, pregnancy, liver/heart/kidney disease	Fluid accumulation in tissues
Hypernatremia	Dehydration, IV saline	Thirst, coma, death
Hyponatremia	Solute loss, excess water, aldosterone deficiency, renal disease, excess ADH	Brain swelling, coma, shock
Hyperkalemia	Renal failure, aldosterone deficit, IV KCl, burns	Nausea, arrhythmias, paralysis
Hypokalemia	GI disturbances, Cushing's, diet, diuretics	Arrhythmia, weakness, confusion
Hypercalcemia	Hyperparathyroidism, vitamin D, immobilization, renal disease, malignancy	Arrhythmias, weakness, kidney stones
Hypocalcemia	Burns, hypoparathyroidism, vitamin D deficiency, renal disease	Tetany, convulsions, fractures
Hyperphosphatemia	Kidney failure, hypoparathyroidism, trauma, absorption	Symptoms from Ca2+ changes
Hypophosphatemia	Absorption, urinary output, hyperparathyroidism	Muscle weakness, bone pain
Hyperchloremia	Dehydration, retention, acidosis, hyperparathyroidism	Symptoms with pH imbalance
Hypochloremia	Alkalosis, vomiting, aldosterone deficiency	Symptoms with pH imbalance
Hypermagnesemia	Renal failure, antacids	Lethargy, coma, cardiac arrest
Hypomagnesemia	Alcoholism, malnutrition, diuretics	Tremors, tetany, convulsions
Hyperproteinemia	Protein production, dehydration, liver disease, myeloma, ketoacidosis	Nausea, fatigue, weight loss
Hypoproteinemia	Malnutrition, protein loss, kidney/liver disease, lymphoma, AIDS	Edema, weight gain, diarrhea, atrophy

Example:

Hyperkalemia is a dangerous electrolyte disorder that can result from renal failure or rapid infusion of potassium. It leads to cardiac arrhythmias and muscle weakness, and must be managed promptly to prevent life-threatening complications.

Additional info: The notes expand on brief points by providing definitions, examples, and context for each disorder and buffer system. All equations are formatted in LaTeX as required.