Water and Salt Metabolism

Osmolality

• Def: concentration of all the solutes in a given weight of water
• Estimation: Posm= 2[Na] + Glucose/18 + BUN/2.8
• Effective solutes: impermeable to cell membranes (i.e. Na+, mannitol) and therefore restricted to ECF compartment
  • create osmotic pressure gradients across cell membranes
  • osmotic mov’t of H2O from ICF to ECF compartments
• see Osmolar Gap
   

Water Metabolism

• Represents a balance between the intake and excretion of water.
• “Regulated” and “Unregulated” components of regulation

Thirst

• response to perceived deficits of body fluids
• increased osmolality of ECF (threshold: 1-4% increases in plasma osmolality above basal levels)
• osmoreceptors in anterior hypothalamus
• intravascular hypovolemia (losses of ECF)
• low/high pressure baroreceptors
• circulating angiotensin II (during more severe hypovolemia)

Arginine Vasopressin (A.K.A. Anti-Diuretic Hormone)

• primary regulation of free water excretion by regulation of urinary flow
• Synthesized in hypothalamus
  • Supraoptic nucleus
  • Paraventricular nucleus
• Transported to/stored in posterior pituitary
  • normal pituitary stores allow for > 1week maximal antidiuresis under sustained dehydration
• AVP acts on the collecting tubules to promote water reabsorption

Regulation of AVP Secretion

• Osmolality
  • osmotic threshold: changes of </= 1% plasma osmolality cause increase in AVP
    minute-to-minute regulation b/c of short t ½of AVP
  • the AVP threshold for regulation is much lower than the thirst threshold
• Hypovolemia
  • less effect than osmolality-osmolality represents a more sensitive regulatory system for water balance than does blood or ECF volume
• Nausea (with/without vomiting)
  • most potent stimulus to AVP secretion known to man
  • not significantly involved in determining physiologic regulation of AVP secretion

Sodium Metabolism

• Ensures maintenance of ECF volume
• directly supports blood volume and pressure
• Simple balance between intake and excretion of Na+

Renal Sodium Excretion

• Glomerular filtration rate
  • decreased GFR= decreased Na+ excretion
  • tubuloglomerular feedback
  • compensates for changes in filtered load-as filtered load of Na+increases, Na+absorption by proximal tubule also increases
• Aldosterone
  • increases Na+re-absorption in the distal nephron(with exchange for K+)
  • Stimulation for secretion:
    • renin-angiotensinsystem (angioII)-in response to renalhypoperfusion
    • elevated levels of K+
  • Inhibition of secretion:
    • atrial natriuretic peptide (ANP)
    • hyperosmolality
• Other factors
  • angiotensin II
  • AVP
  • ANP
  • dopamine
  • renal sympathetic nerve activity
  • renal prostoglandins

Hyperosmolality and Hypernatremia

• deficiency of water relative to solute in the ECF
• indicates deficiency of TBW relative to total body solute
• most cases due to losses of body water in excess of body solutes

Pathogenesis of hyperosmolar disorders

• Assessment for Renal Cause
• Urine osmolality
  • appropriately concentrated urine usually eliminates possibility of primary renal cause
  • urine osm >800 mOsm/kg H20= nml AVP secretion and renal response
  • urine osm <800mOsm/kg H2O= presence of renal concentrating defect
  • always check urine for glucose
  • in absence of osmotic diuresis, inadequate urine concentration generally indicates presence of Diabetes insipidus

see SIADH, Diabetes Insipidus