Fluid, Electrolyte, and Acid-Base Balance

Body Water Content

• Infants:
  • ~73% of body weight is water
  • Higher water content due to less body fat and less bone mass
• Adult Males:
  • ~60% of body weight is water
  • More water content due to more skeletal muscle and less adipose tissue
• Adult Females:
  • ~50% of body weight is water
  • Less water content due to more adipose tissue
• Elderly Males and Females:
  • ~45% of body weight is water
• Key Factors: Age, Gender

Fluid Compartments

• Intracellular Fluid (ICF): Inside cells
  • 2/3 of the body’s total fluid volume
• Extracellular Fluid (ECF): Outside cells
  • 1/3 of the body’s total fluid volume
  • Includes plasma (in blood) and interstitial fluid (between cells. Includes lymph, CSF, eye humors)

Water: The Universal Solvent

• Reason: Dissolves many substances (sugar, salt, chemicals)
• Types of Solutes:
  • Non-electrolytes: Do not dissociate in water (glucose, urea, amino acids)
  • Electrolytes: Dissociate into ions in water (sodium chloride, potassium chloride, calcium chloride, magnesium chloride)
    • Conduct electric current
    • Crucial in maintaining osmotic pressure

Electrolyte Concentration

• Expression: Milliequivalent per liter (mEq/L)
• Key Electrolytes: Sodium (Na⁺), Chloride (Cl⁻), Potassium (K⁺)
• Cations and Anions:
  • ECF: Major cation is sodium (Na⁺)
  • ICF: Major cation is potassium (K⁺)

Fluid Movement and Pressure

• Regulation: Osmotic and hydrostatic pressure
• Water Movement:
  • Moves from low to high osmolality
  • Osmolality increase in ECF → water leaves cells → ECF volume increases
  • Osmolality decrease in ECF → water enters cells

Water Intake and Output

• Intake: ~2.5 liters/day
  • Sources: Ingested food, fluids, and metabolic water
• Output:
  • Through kidneys (urine: 60%), skin (evaporation), lungs (exhalation), perspiration, feces
• Balance: Maintained to keep osmolality around 280-300 milliosmoles

Regulation of Water Intake

• Thirst Mechanism: Driven by osmoreceptors in the hypothalamus
  • Triggers:
    • Increased plasma osmolality (≥1-2%)
    • Dry mouth (thick saliva when ECF osmolality increases)
    • Decreased blood volume or pressure (detected by baroreceptors, stimulates angiotensin II release)
• Inhibition: Activated by stretch receptors in stomach and small intestine upon water intake

Influence of Antidiuretic Hormone (ADH)

• Function: Regulates reabsorption of water in kidneys
• Release: Stimulated by increased osmolality or decreased blood volume