Fluid, Electrolyte, and Acid-Base Balance

Fluid, Electrolyte, and Acid-Base Balance

• The study of fluid, electrolyte, and acid-base balance is crucial for maintaining homeostasis in the body. Each component works together to regulate the internal environment.

Water in the Body

• Water: Makes up 99% of the fluid volume outside cells.
  • Extracellular Fluid (ECF): Essential for the cytosol inside cells.
  • Intracellular Fluid (ICF): All cellular operations depend on water as a medium for diffusion of gases, nutrients, and waste.

Key Processes

• The volumes, solute concentrations, and pH of ECF and ICF are stabilized by three interrelated processes:
  • Fluid Balance: Balance between water gained and lost.
  • Electrolyte Balance: Balance of solute concentrations.
  • Acid-Base Balance: Maintenance of pH levels in body fluids.

Fluid Balance

• Fluid balance: Daily balance between water gained and lost.
  • Water Gained:
  • Digestive system = Primary source of water intake.
  • Approx. 2500 mL/day required.
  • Water Lost:
  • Urinary system = Primary route.
  • Includes perspiration and respiration losses.

Electrolyte Balance

• Electrolytes: Ions dissociated from inorganic compounds that conduct electrical currents in solution.
  • Balance involves a fine-tuned process of absorption in the digestive tract and losses in the kidneys.
  • Major electrolytes include sodium, potassium, calcium, magnesium, and phosphate.

Acid-Base Balance

• Acid-Base Balance:
  • Regulates production and loss of hydrogen ions (H+).
  • Normal pH of body fluids: 7.35–7.45.
  • Kidneys and lungs play crucial roles in maintaining pH by secreting H+ and eliminating carbon dioxide (CO2).

Fluid Compartments

• Water Distribution:
  • ICF accounts for ~60% of body weight (male); 50% (female).
  • ECF includes interstitial fluid and plasma, with minor components such as lymph, CSF, and synovial fluid.
  • Ions in ECF: Sodium, chloride, bicarbonate.
  • Ions in ICF: Potassium, magnesium, phosphate.

Regulation of Fluids and Electrolytes

• Homeostatic mechanisms monitor changes in the ECF, which is more readily affected by dietary gains/losses.
• Hormones such as ADH, Aldosterone, and Natriuretic peptides help regulate fluid and electrolyte balance through:
  • ADH: Increases water reabsorption in kidneys.
  • Aldosterone: Promotes sodium retention, leading to water retention.
  • Natriuretic peptides: Lower blood pressure and inhibit water retention.

Fluid Movement and Imbalance

• Fluid Movement:
  • Water circulates freely in ECF, maintaining osmotic equilibrium.
  • Edema results from excess water moving into interstitial fluid.
• Hypo- and Hyperhydration:
  • Dehydration occurs when losses exceed gains.
  • Hyperhydration occurs when excess water moves intracellularly, affecting cell function.

Electrolyte Concentrations

• Sodium (Na+):
  • Main cation in ECF, balances between intake and output.
  • Major role in determining osmotic concentration of ECF.
• Potassium (K+):
  • Main cation in ICF; crucial for cellular function, balances determined by dietary intake and renal losses.
• Calcium (Ca2+) and Magnesium (Mg2+):
  • Important for various biological functions, with their levels regulated by hormonal influences.

Acid-Base Disturbances

• Acidosis: Low blood pH (<7.35); common due to acidic metabolic activity.
• Alkalosis: High blood pH (>7.45), leading to disturbance in normal cellular function.

Buffer Systems

• Buffers: Substances that prevent drastic changes in pH by donating or absorbing H+. Key buffer systems include:
  • Carbonic acid-bicarbonate system.
  • Phosphate buffer system.
  • Protein buffer systems (using amino acids and hemoglobin).

Respiratory and Renal Control

• Mechanisms of pH control include respiratory compensation (altering breathing rates) and renal compensation (altering rates of H+ and bicarbonate excretion), essential for maintaining acid-base homeostasis.