Body Fluids Summary

Body Fluids

  • Animal bodies are approximately 60% water.
  • Water intake: drinking, moist foods, metabolic by-products.
  • Water loss: insensible loss, sweating, vocalizing, urination, defecation. Sick animals: vomiting, diarrhea, hemorrhage, fever. Dogs die within 2 to 4 days, cattle 6 to 8 days if they do not consume water.

Body Fluid Compartments

  • Intracellular fluid (ICF): 2/32/3 of total body water, inside cells.
  • Extracellular fluid (ECF): 1/31/3 of total body water, outside cells.
    • Interstitial fluid: 3/43/4 of ECF, surrounds cells.
    • Plasma: 1/41/4 of ECF, in blood vessels.

Body Fluid Composition

  • Body fluids contain solutes (ions, electrolytes, non-electrolytes).
  • Ions: charged particles (cations +, anions -), abundant electrolytes.
  • Electrolytes: conduct electrical current in solution (acids, bases).
  • Non-electrolytes: molecules with covalent bonds (glucose, lipids).
  • Proteins: large molecules in ECF (plasma > interstitial fluid > ICF).

Osmolality

  • Osmolality: solute concentration in fluid.
  • Normal serum osmolality: 278-300 mOsm/kg.
  • Regulation: hormonal feedback loop (ADH).
  • Increased osmolality: thirst, ADH release, water reabsorption.
  • Decreased osmolality: reduced thirst, inhibited ADH, increased urine excretion.

Types of Solutions

  • Isotonic: equal solute concentration to reference cell (e.g., 0.9% NaCl).
  • Hypertonic: higher solute concentration than cell (e.g., 3% or 5% NaCl).
  • Hypotonic: lower solute concentration than cell (pure water).

Osmosis

  • Water moves to equalize osmolality.
  • Osmosis: solvent diffusion from dilute to concentrated solution.
  • Increased ECF solute: water moves out of cells.
  • Decreased ECF solute: water moves into cells.

Fluid Exchange

  • Regulated by osmotic and hydrostatic pressures.
  • IF and ICF exchange: across plasma membranes (diffusion, transport mechanisms, aquaporins).
  • Plasma and IF exchange: across capillary walls.

Capillary-Interstitial Fluid Exchange

  • Capillary filtration pressure: pushes water out.
  • Capillary colloidal osmotic pressure: pulls water in.
  • Interstitial hydrostatic pressure: opposes water out.
  • Interstitial colloidal osmotic pressure: pulls water out.

Disturbances of Water Balance

  • Dehydration: fluid loss (water or water & solutes).
    • Causes: hemorrhage, burns, vomiting, diarrhea, sweating, deprivation.
    • Signs: sticky mucosa, thirst, dry skin, oliguria.
  • Hypotonic hydration: ECF diluted, hyponatremia, water moves into cells.
    • Effects: nausea, vomiting, cramping, cerebral edema.
  • Edema: fluid accumulation in interstitial space.
    • Causes: increased capillary hydrostatic pressure/permeability.