Lecture Notes: Fluid Balance - Chapter 27 Lecture 1

Introduction to Fluid Balance

• Body fluid consists of water and dissolved solutes
• Mechanisms regulate:
  • Total volume
  • Distribution
  • Solute concentration
  • pH
• Importance of maintaining homeostasis:
  • Prevent fatal conditions
  • Example: Excessive blood loss, sweating, vomiting, or diarrhea
  • Intake of plain water can lead to hyponatremia (low sodium)

Fluid Compartments and Exchange

• Intracellular vs. Extracellular fluid
  • 2/3 of fluid is intracellular
  • 1/3 is extracellular (80% interstitial, 20% plasma)
• Exchange points:
  • Cell membranes (intracellular/interstitial)
  • Capillaries (plasma/interstitial)
• Fluid balance involves both water and electrolytes

Fluid and Electrolyte Balance

• Water follows solute ("where salt goes, water follows")
• Water and electrolyte balance are interconnected
  • Electrolytes: Charged molecules (e.g., sodium, chloride)
  • Osmosis: Movement of water across semi-permeable membranes

Water Balance and Sources

• Water makes up 45%-75% of body weight
• Sources of water gain:
  • Ingested liquids and food
  • Metabolic water from reactions (dehydration synthesis)
• Water loss through:
  • GI tract, lungs, skin, kidneys
  • Sensible vs. insensible perspiration

Regulation of Fluid Gain and Loss

• Main regulator: Thirst
  • Dehydration triggers thirst mechanism
  • Decreased saliva, increased blood osmolarity
  • Involves hypothalamic osmoreceptors and hormone release
• Hormones regulating urine and fluid balance:
  • Antidiuretic Hormone (ADH):
    • Released from posterior pituitary
    • Decreases urine formation, increases fluid reabsorption
  • Atrial Natriuretic Peptide (ANP):
    • Released from atria of the heart
    • Does opposite of ADH (increases urine to reduce fluid volume)
  • Aldosterone:
    • Part of the renin-angiotensin-aldosterone system
    • Steroid hormone from adrenal cortex
    • Decreases urine formation, promotes reabsorption

Thirst Mechanism

• Thirst triggered by:
  • Dry mouth and pharynx
  • Increased blood osmolarity and osmoreceptor stimulation
  • Decreased blood volume and pressure
• Hormonal responses:
  • ADH, Aldosterone, and Angiotensin II formation
  • Stimulate thirst center in hypothalamus
• Drinking restores normal body water levels

Summary

• Water and solute balance critical for physiological function
• Hormonal regulation maintains balance through feedback mechanisms
• Understanding these mechanisms is essential for managing fluid-related disorders