Fluid and Electrolyte Balance

Fluid, Electrolyte, and Acid-Base Balance: An Overview

• General
  • Most of the body is composed of water, essential for distributing gases, nutrients, wastes.
  • Body water is divided into:
  • Extracellular Fluid (ECF): Interstitial fluid, plasma, etc.
  • Intracellular Fluid (ICF): Cytoplasm of the cells

Processes for Stabilizing Fluid Compartment Volumes and pH

• Fluid Balance:
  • Achieved when water intake equals water loss
  • Primary water gain via the digestive system and loss through the urinary system
• Electrolyte Balance:
  • Balance absorption in the digestive tract with loss at the kidneys
• Acid-Base Balance:
  • pH must remain within normal limits
  • Kidneys secrete hydrogen ions, lungs eliminate carbon dioxide (an acid)

Introduction to Fluid and Electrolyte Balance

• Fluid Compartments
  • ECF and ICF behave as distinct entities
  • Principal Ions:
  • ECF: Sodium (Na+)
  • ICF: Potassium (K+)

Primary Regulatory Hormones of Fluid/Electrolyte Balance

• Antidiuretic Hormone (ADH):
  • Produced in hypothalamus, stored in posterior pituitary
  • Released in response to increased ECF substance concentration
  • Increases water conservation (kidneys) and thirst
• Aldosterone:
  • Released by adrenal cortex, responds to the renin-angiotensin system
  • Promotes sodium retention and water conservation
• Natriuretic Peptides (ANP & BNP):
  • Released by cardiac muscle in response to heart strain
  • Reduce thirst and block ADH and aldosterone, leading to diuresis

Fluid (Water) Balance

• Fluid Movement:
  • Pressures at capillary beds force fluids into interstitial spaces (edema if excess)
  • Interstitial fluid enters lymphatic vessels, moving to venous circulation
  • Water moves through other body cavities (e.g., peritoneal, pericardial)
• Fluid Gains and Losses:
  • Daily loss about 2500 ml from urine, feces, perspiration
  • Gains match losses through food, drink, metabolic processes (approx. 2500 ml/day)
• Fluid Shifts:
  • Rapid movement between ICF and ECF in response to tonicity changes
  • Dehydration (loss exceeds gains) and overhydration (gains exceed losses) can occur

Electrolyte Balance

• Sodium Balance:

  • Dominant cation in the ECF (normal value is 140 meq/L)
  • Sodium concentration balanced by dietary intake and kidney excretion
  • Conditions:
  • Hyponatremia: Low sodium concentration
  • Hypernatremia: High sodium concentration

• Potassium Balance:

  • Dominant cation in ICF with low ECF levels (3.8-5.0 meq/L)
  • Gains through diet; losses via urine
  • Conditions:
  • Hypokalemia: Low potassium
  • Hyperkalemia: High potassium (more serious concerns)

• Other Electrolytes:

  • Calcium: Most abundant in bones, important for muscle contraction, nerve impulse, blood clotting
  • Magnesium: Cofactor in enzymes, efficiently reabsorbed by kidneys
  • Phosphate: Critical for energy (ATP) and structure in nucleic acids
  • Chloride: Main anion in ECF, absorbed with sodium

Acid-Base Balance

• Importance of pH Control:
  • ECF normal pH: 7.35-7.45; deviations dangerous
  • Acidosis: pH < 7.35; Alkalosis: pH > 7.45
• Mechanisms:
  • Balance hydrogen ions' gains and losses
  • Buffer systems neutralize excess H+ for tissue protection
  • Buffer systems:
  • Protein Buffer Systems: Amino acids manage H+
  • Carbonic Acid-Bicarbonate System: CO2 conversion affects pH
  • Phosphate Buffer System: Minor role in pH regulation
• Maintenance Methods:
  • Short-term control via buffers; long-term via respiratory and renal compensation

Disturbances of Acid-Base Balance

• Respiratory Disorders:

  • Respiratory Acidosis: Caused by CO2 retention due to hypoventilation
  • Respiratory Alkalosis: Excessive CO2 elimination due to hyperventilation

• Metabolic Disorders:

  • Metabolic Acidosis: Excess acids; causes include lactic acidosis, ketoacidosis
  • Metabolic Alkalosis: Increased bicarbonate concentrations

• Detection:

  • Usually through arterial blood gases (ABG) measurements
  • Example of compensation: Respiratory acidosis can prompt kidneys to retain bicarbonate to neutralize acidic pH.