01-03 Sodium and Water I: Intro

Question 1

Distinguish volume regulation from osmoregulation.

Answer 1

 Sodium and water are independently regulated!
 Hypo / hypernatremia are water problems.
 Hypo / hypervolemia* are sodium problems.
*Meaning ∆s in ECV

Question 2

osmolarity vs. osmolality

Answer 2

 Osmolality- the concentration of solute per kg of solvent (mosm/kg)
 Osmolarity – the concentration of solute per liter of solvent (mosm/L)

Question 3

specific gravity

Answer 3

wt. of substance vs. wt. of an equal volume of distilled water

Question 4

Normal serum osmolarity

—what concentration of Na+

Answer 4

280-290 mOsm/L

—[Na+] = 140mmol/L

Question 5

Equation for estimating osmolarity

Answer 5

2x[Na+] + [glucose]/18 + [BUN]/2.8

*Actual osmolarity slightly ↑er due to Ca++, PO43-, albumin, etc.

 Plasma osmolarity can be directly measured by freezing point depression

 ↑ Osmolar gap (> 10 mOsm/L), a foreign substance is present
– Alcohols, paraproteins

Question 6

Urea vs. albumin

Answer 6

urea can cross membranes freely whilst albumin CANNOT

Question 7

Just remember

Answer 7

Changes in intracellular [K+] will have similar effects on water movement as ∆s [Na+], but will not be directly reflected in serum K+ levels

Question 8

Cerebral edema is caused by ______natremia

Answer 8

hyponatremia

Question 9

Brain makes ________ to make up for ∆s in osmolarity?

Answer 9

idiogenic osmoles

 Production or removal of these takes 2-3 days

Question 10

How does the body perform osmoregulation?

Answer 10

osmoreceptors in hypothal sense ↑ Osm → trigger ADH + thirst response → increased water intake & retention*; periph vasocon
*dependent on medullary conc gradient & distal tubule NaCl reabs mod’d by ADH

Question 11

ADH as osmoregulator and volume regulator

Answer 11

ADH can modulate
—in severe hypovolemia acts more as “vasopressor” and MUCH more is release (10X as much)
—volume trumps osmolarity

Question 12

How does the body regulate volume?
—sensor name/location
—response to hypovolemia

Answer 12

1. JGA (between vascular pole of renal corpuscle and returning DCT of same nephron)
   —senses ∆ing Na+ flow
   —activates RAAS
2. BARORECEPTORS (carotid sinus)
   —sense ‪↓‬s in pressure
   —stimulates adrenergic system, ADH
3. ATRIAL STRETCH RECEPTORS (Venae cavae, pulm aa., atria)
   —sense both directions
   —if ‪↓‬: ADH, renin and aldosterone
   —If ↑‬: ANP

Question 13

What % of ECFV is in ECV?

Answer 13

1/4; interstitial makes up the other 3/4

Question 14

Where is macula densa?

—How does it respond to hypovolemia?

Answer 14

—located in wall of DCT
—macula densa senses ∆s in Na+ Flow
(1) ‪↓‬ resistance in AA via vasodil, which ↑ glom. hydrostatic pressure and helps maintain (GFR)
(2) ↑ renin release from the juxtaglomerular* cells of AA and EA (the major renin storage sites)
*JGA cells also have β1-Rs

Question 15

Actions of A-II

Answer 15

—direct vascon (esp EA)
—↑ Na+ & HCO3- reabs in PT
—stimulates aldo release

Question 16

Aldosterone
—made where?
—actions

Answer 16

 Synthesized in the adrenal cortex
 Acts in the collecting tubule
 Stimulates Na+ channels in principal cells
—– Enhances Na+ absorption, K+ secretion
 Also stimulates H+ secretion in intercalated cells

Question 17

Actions of ANP

Answer 17

 Direct vasodilator
– Lowers blood pressure
– AA dilation leads to ↑ GFR
 Stimulates Na+ excretion