Sodium and Water Balance — Flashcards

Flashcard 1: The _____ and thick ascending limbs (loop of Henle) are impermeable to water

Answer: thin

Flashcard 2: When CH2O is _____, urine is iso-osmotic relative to plasma

Answer: zero

Flashcard 3: In the presence of _____, aquaporin 2 channels are inserted in the luminal membrane of principal cells of the late distal tubule and collecting duct

Answer: ADH

Flashcard 4: When CH2O is _____, urine is hyper-osmotic relative to plasma

Answer: negative

Flashcard 5: A _____ shift in renal pressure natriuresis curve is caused by angiotensin II by increasing Na and water retention

Answer: rightward

Flashcard 6: The _____ cells of the late distal tubule and collecting duct reabsorb Na+ and H2O

Answer: principal

Flashcard 7: The major action of ADH (vasopressin) is to increase _____ of the principal cells in the late distal convoluted tubule and collecting duct

Answer: water permeability (and thus reabsorption)

Flashcard 8: Atrial natriuretic peptide dilates afferent renal arterioles and constricts efferent renal arterioles, which _____ excretion of Na+ and water, along with downregulating ENaC on the collecting tubules

Answer: increases

Flashcard 9: What is the formula for renal clearance ($C_x$)?

Answer: $C_x = \frac{U_x V}{P_x}$

Extra: $U_x$ = Urine concentration of X $P_x$ = Plasma concentration of X $V$ = Urine flow rate

Flashcard 10: What equation is used to calculate **free-water clearance** ($C_{H_2O}$)?

Answer: $$C_{H_2O} = V - C_{osm}$$ (where $V$ = urine flow rate; $C_{osm}$ = osmolar clearance)

Extra: Free-water clearance ($C_{H_2O}$) represents the volume of solute-free water that must be added to or removed from the urine to make it isotonic with plasma. - If **$C_{H_2O} = 0$**: Urine is isosthenuric (isosmotic to plasma). - If **$C_{H_2O} > 0$**: Free water is excreted (dilute urine; $U_{osm} < P_{osm}$). Seen in low ADH states (e.g., diabetes insipidus, water intoxication). - If **$C_{H_2O} < 0$**: Free water is reabsorbed (concentrated urine; $U_{osm} > P_{osm}$). Seen in high ADH states (e.g., SIADH, dehydration).