Fluid, Electrolyte, and Acid-Base Homeostasis: Study Notes

Fluid, Electrolyte, and Acid-Base Homeostasis

Overview of Fluid, Electrolyte, and Acid-Base Homeostasis

Fluid, electrolyte, and acid-base homeostasis are essential for maintaining the internal environment of the body. These processes ensure that the volume, concentration, and pH of body fluids remain within narrow limits, supporting normal physiological functions.

• Body fluids include blood plasma, interstitial fluid, cytosol, cerebrospinal fluid, lymph, exocrine secretions, and other specialized fluids. Water is the main component.

• Fluid balance refers to maintaining the appropriate volume and concentration of intracellular and extracellular fluids, primarily through water balance.

• Functions of water:

  • Acts as a polar solvent, transporting solutes.

  • Distributes body heat.

  • Cushions and lubricates organs and tissues.

• Principle of mass balance: What is gained by the body must equal what is lost.

• Factors impacting fluid balance: Water intake, physical activity, kidney function, medications, digestive activities.

• Electrolytes are substances that dissociate into ions in water, conducting electricity. Nonelectrolytes do not dissociate into ions.

• Electrolyte balance is also governed by mass balance and is influenced by fluid balance.

• Acids dissociate in water to release H+ ions (e.g., HCl, H2CO3).

• Bases accept H+ ions (e.g., HCO3-).

• pH scale measures hydrogen ion concentration:

  • pH < 7: Acidic

  • pH > 7: Basic

  • pH = 7: Neutral

Fluid Homeostasis

Fluid homeostasis involves the regulation of water distribution and movement between body compartments, as well as the mechanisms of water gain and loss.

• Total body water in a standard 70 kg adult is about 60% of body weight (~42 kg). Varies with gender, age, body mass, and adipose tissue.

• Fluid compartments:

  • Intracellular fluid (ICF): ~60% of body fluids (~26 L).

  • Extracellular fluid (ECF): Includes plasma and interstitial fluid.

• Movement of water: Water moves freely between compartments, influenced by:

  • Hydrostatic pressure: Pushes water from high to low pressure.

  • Osmotic pressure: Pulls water toward higher solute concentration (osmosis).

• Tonicity: Measurement of osmotic pressure gradient between two fluids.

  • Isotonic: No net movement of water.

  • Hypotonic ECF: Water enters cells, cells swell.

  • Hypertonic ECF: Water leaves cells, cells shrink.

• Water losses:

  • Obligatory water loss: ~500 ml urine daily.

  • Sensible water loss: ~100 ml in feces.

  • Insensible water loss: ~600 ml from skin, ~300 ml from expired air.

  • Total daily loss: ~2.5 L.

• Water gains:

  • Metabolic water: ~250 ml.

  • Ingested liquids: ~1500 ml (thirst mechanism via hypothalamic osmoreceptors).

  • Food: ~750 ml.

• Renin-angiotensin-aldosterone system (RAAS): Activated by decreased plasma volume/blood pressure, stimulates thirst and water reabsorption.

• Hormonal regulation:

  • ADH: Increases water reabsorption in kidneys, decreases urine volume.

  • Decreased ADH: More water eliminated, decreased ECF volume.

• Imbalances:

  • Dehydration: Decreased ECF volume, increased osmolarity; cells lose water and crenate.

  • Overhydration: Increased ECF volume, decreased osmolarity; cells swell, risk of hyponatremia and cerebral edema.

  • Isosmotic imbalances: Hypovolemia (blood loss), hypervolemia (fluid excess), edema (fluid accumulation in interstitial space).

Electrolyte Homeostasis

Electrolyte homeostasis is crucial for maintaining electrical activity, fluid balance, and physiological functions. The main electrolytes include sodium, potassium, calcium, phosphate, chloride, and magnesium.

• Sodium (Na+): Most abundant extracellular cation.

  • Maintained by Na+/K+ ATPase pumps and low membrane permeability.

  • Critical for depolarization in excitable cells (neurons, muscle).

  • Regulated by angiotensin-II and aldosterone (increase retention), ANP (decreases retention).

  • Imbalances: Hypernatremia (high Na+, dehydration), hyponatremia (low Na+, overhydration).

• Potassium (K+): Most abundant intracellular cation.

  • Maintained by Na+/K+ ATPase pumps.

  • Key for resting membrane potential.

  • Regulated by insulin, aldosterone, epinephrine.

  • Imbalances: Hyperkalemia (dangerous, cells depolarize abnormally), hypokalemia (cells hyperpolarize, less responsive).

• Calcium (Ca2+) and Phosphate (PO43-):

  • Bound in hydroxyapatite in bone.

  • Required for muscle contraction, cardiac action potential, signaling, blood clotting, synaptic transmission.

  • Regulated by bone, kidneys, intestines; hormones: PTH and vitamin D3 (calcitriol).

  • Imbalances: Hypercalcemia (neurons less excitable), hypocalcemia (neurons hyperexcitable).

• Other ions:

  • Chloride (Cl-): Abundant in ECF, forms HCl in stomach, participates in bicarbonate formation.

  • Magnesium (Mg2+): Activates enzymes, component of bone.

Acid-Base Homeostasis

Acid-base homeostasis maintains the pH of body fluids within a narrow range (7.35–7.45), essential for enzyme function and cellular processes. Buffer systems, respiratory, and renal mechanisms contribute to this regulation.

• Buffer systems:

  • Chemical buffers: Weak acid and conjugate base pairs resist pH changes.

  • Carbonic acid-bicarbonate system: Most important in blood.

    • Equation: $\mathrm{CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow HCO_3^- + H^+}$

  • Phosphate buffer system: Important in cytosol and kidney tubules.

  • Protein buffer system: Carboxylic acid groups of amino acids act as buffers.

• Physiological buffer systems:

  • Respiratory system: Controls CO2 (volatile acids) in ECF.

  • Renal system: Controls fixed acids and bicarbonate ions in ECF.

  • Kidneys can manufacture new bicarbonate or eliminate excess H+.

• Acid-base imbalances:

  • Acidosis: pH < 7.35; neurons less excitable; caused by excess H+ or loss of HCO3-.

  • Respiratory acidosis: Decreased pH due to excess CO2 (hypoventilation).

  • Metabolic acidosis: Addition of metabolic acids or loss of HCO3-.

  • Alkalosis: pH > 7.45; neurons hyperexcitable; caused by excess base or loss of H+.

  • Respiratory alkalosis: Loss of CO2 (hyperventilation).

  • Metabolic alkalosis: Loss of H+ or excess HCO3-.

• Compensatory mechanisms: Respiratory and renal systems adjust to restore pH.

• Arterial Blood Gases (ABGs): Clinical tool to assess pH, PCO2, and HCO3- levels.

Table: Acid-Base Disorders and ABG Findings

Disorder	pH	PCO2	HCO3-	Compensation
Respiratory Acidosis	Decreased	Elevated	Elevated (if compensated)	Renal: Increased HCO3-
Metabolic Acidosis	Decreased	Decreased (respiratory compensation)	Decreased	Respiratory: Hyperventilation
Respiratory Alkalosis	Elevated	Decreased	Decreased (if compensated)	Renal: Decreased HCO3-
Metabolic Alkalosis	Elevated	Elevated (respiratory compensation)	Elevated	Respiratory: Hypoventilation

Example: Correction of Dehydration

Dehydration triggers a series of homeostatic responses to restore fluid, electrolyte, and acid-base balance.

• Decreased total body water leads to:

  • Decreased blood volume and pressure.

  • Increased extracellular electrolyte concentration (especially Na+).

  • Increased ECF osmolarity draws water out of cells.

  • Increased ECF metabolic acid concentration may cause metabolic acidosis.

• Juxtaglomerular cells release renin, activating RAAS.

• Angiotensin-II effects:

  • Vasoconstriction (increases blood pressure).

  • Increases Na+ and water reabsorption.

  • Stimulates thirst, ADH, and aldosterone secretion.

  • Aldosterone stimulates K+ and H+ secretion, helping restore pH.

• Fluid homeostasis is restored as these mechanisms correct imbalances.

Key Equations

• Carbonic acid-bicarbonate buffer system: $\mathrm{CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow HCO_3^- + H^+}$

Summary Table: Fluid Compartments

Compartment	Volume (L)	% of Total Body Water	Main Components
Intracellular Fluid (ICF)	~26	~60%	Cytosol, inside cells
Extracellular Fluid (ECF)	~16	~40%	Plasma, interstitial fluid

Additional info: These notes expand on brief points with academic context, definitions, and examples to provide a comprehensive, exam-ready summary of fluid, electrolyte, and acid-base homeostasis.