Regulation of Extracellular Sodium & Water

Question 1

Na⁺ concentration detection @ ECF

Answer 1

• Na = major osmotic substance @ ECF ==> osmotic equilibrium w/cells
• losses/gains in Na ==> changes in ECF volume (vs. Na concentration)
• Due to the two-fold greater volume of the cellular over the ECF compartment, gains (or losses) of sodium from the ECF result in two-fold greater changes in ECF volume than they do in ECF sodium concentration.
• Major sensors monitor volume ==> mean arterial pressure (MAP) of major arteries

Question 2

Feedback loop for sodium regulation

Answer 2

• e.g. sodium loss ==> decrease ECF volume/MAP decrease
• ==> increased baroreceptor reflex ==> increased renin ==> AgII ==> increased aldosterone (@ adrenal cortex)
• aldoseterone ==> increased Na reabsorption @ distal tubule and collecting duct

Question 3

Feedback loop for water reabsorption

Answer 3

• regulation of water = monitoring of ECF volume and osmolarity
• severe sweating ==> decreased ECF volume ==> decreased LA filling pressure ==> increased baroceptor reflex to hypothalamus
• ==> activated ADH-synthesizing neurons ==> secrete ADH from post. pituitary
• ==> increased water reaborption @ late distal tubule + collecting duct
• ALSO: increased ECF osmolarity ==> activation of hypothalmic osmoreceptors ==> activated ADH-synth neurons

Question 4

Severe diarrhea implications for water regulation

Answer 4

• severe diarrhea ==> isotonic loss of 3 L of ECF ==> activation of low volume pathway ==> increased ADH secretion
• consume 2L of hypotonic water during recovery ==> decrease osmolarity ==> inhibition of ADH pathway
• which one wins?

Question 5

Main mechanism of ECF water regulation

Answer 5

• primarily an osmoregulatory system w/emergency low-volume override
• water is regulated to acheive osmotic constancy; however at low volumes ADH rapidly dominates osmotic effects
• low volume dominance of ADH secretion ==> defense of circulation in severe hypovolemia

Question 6

Regulation of volume overload

Answer 6

• no suppression of ADH if volume increases above normal
• atrial natriuretic peptide (ANP) located at heart ==> potent diuretic hormone + increases sodium excretion
  
  • increase filtration via dilation of afferent and efferent arterioles
  • decreases ADH secretion
  • inhibits renin secretion/aldosterone
  • prevents effects of remaining levels of ADH and aldosterone