Fluids

Body Fluids

  • Total body water: 60% of body weight

  • Decreases with age and body fat percentage

  • Functions:

    • Lubricant

    • Solvent for metabolism

    • Transport medium

    • Regulates temperature

Fluid Distribution

  • 40% of body weight is fluid

  • Extracellular fluid: 33%

    • Interstitial fluid: 25%

    • Blood plasma: 8%

  • Intracellular fluid: 66%

Osmolality and Movement

  • Osmolality: solute concentration in blood

  • Osmosis: water movement across a semipermeable membrane (low to high concentration)

  • Diffusion: particle movement down concentration gradient

  • Active transport: particles moved against gradient, requires energy

Pressures

  • Hydrostatic pressure: 'pushing' pressure, fluid movement out of vessels

  • Osmotic pressure: 'pulling' pressure, influences fluid movement into vessels

  • Colloidal osmotic pressure (COP): influenced by albumin, affects water movement

Diuresis

  • Increased urine production by kidneys, influenced by:

    • Medications (e.g., diuretics)

    • Caffeine and alcohol

  • Types of diuretics:

    • Loop diuretics (Furosemide)

    • Thiazides

    • Potassium-sparing

    • Osmotic

Tonicity

  • Tonicity relates to osmotic pressure and effect on water movement:

    • Isotonic: equal to plasma (e.g., 0.9% saline)

    • Hypotonic: lower concentration than plasma (e.g., 0.45% saline)

    • Hypertonic: higher concentration than plasma (e.g., 3% saline)

Fluid Imbalance

  • Categories:

    • Volume imbalance: too much or too little fluid

    • Concentration imbalance: too many or too few solutes

  • Volume deficit (hypovolemia): causes include vomiting, diarrhea, burns

  • Symptoms: increased HR, weight loss, dizziness

  • Volume excess (hypervolemia): causes include excessive sodium and water retention

Edema

  • Excess interstitial fluid, due to:

    • Increased capillary hydrostatic pressure

    • Decreased colloidal osmotic pressure

    • Increased interstitial osmotic pressure

    • Blocked lymph drainage

Third Spacing

  • Shift of fluid to non-functional spaces (e.g., pleural, pericardial)

  • Often involves protein-rich fluid accumulation.

Renal Physiology and Diuretics

  • Nephron sections: proximal convoluted tubule, loop of Henle, distal convoluted tubule

  • Loop diuretics prevent sodium and water reabsorption, leading to increased urine output.