Fluid Compartments

  • Any major category of fluids in the body separated by a selectively permeable membrane with different chemical compositions

    • Lymph, blood, tissue fluid, ICF

Average total body water content is 60% in men and 50% in women

65% ICF

35% ECF

    25% interstitial

    8% blood plasma and lymph

    2% transcellular fluid


Intake

  1. Dehydration = low blood volume and pressure + high blood osmolarity

  2. Hypothalamus’ osmoreceptors respond to angiotensin II and high osmolarity for ECF

  3. Osmoreceptors stimulate ADH secretion - conserve water

  4. Tell cerebral cortex to make us thirsty

  5. Less salivation

  6. Water is drunk = reduced osmolarity = less stimulation from osmoreceptors


Output

  1. Urine volume

  2. Kidneys can only slow down rate of loss when dehydrated, but not control output

  3. Substances reabsorbed by kidneys become ECF

  4. Urine volume linked to sodium reabsorption proportionately

  5. ADH controls water output independent of sodium

  6. Cells, in response to ADH, secrete aquaporins to act as channels for water to diffuse out of

  7. Kidneys reabsorb water and have less urine

    1. Slows down decline of blood volume and osmolarity rise in dehydration

  8. ADH = negative feedback loop


Water Balance Disorders

  1. Hypovolemia (water depletion) - Water and sodium are lost without replacement

    1. Osmolarity fine but total body water decreases

    2. Hemorrhage, diarrhea, chronic vomiting, severe burns

  2. Dehydration - Body eliminates more water than sodium, ECF osmolarity rises

    1. Lack of drinking water

  3. Volume Excess - Sodium and water are retained

    1. Renal failure

  4. Hypotonic Hydration - More water than sodium is retained

    1. Water intoxication

    2. ADH Hypersecretion