L5 Urinary Conc And Dilution

Question 1

When body fluid are hyposomtic

Answer 1

Water excess

Kidneys excrete a dilute urine

As low as 50mOsm/L - water diuresis

Question 2

When body fluids are hyperosmotic

Answer 2

Water deficit

Kidneys excrete a concentrated urine

Up to 1200 mOsm/L - antidiuresis

Kidneys can regulate water excretion independently of solute excretion

Question 3

Tubular reabsorption in LOH and distal/CD

Answer 3

LOH - Na:25%, active water:15%,passive, solute linked

Distal and CD- Na: 8%, active, subject to control by aldosterone
Water: 20%, passive, regulated by ADH

Question 4

Counter current multiplier

Answer 4

Creates a large gradient in the ISF from the corticomedullary border to the tip of the papilla by multiplying a small local gradient created by the epithelium in LOH

Question 5

LOH characteristics

Answer 5

Osmotic gradient from cortex to medulla in interstitial space- used to remove water from urine in the collecting

Descending limb : no active transport of NaCl, very permeable to water

Ascending limb: structurally thin to thick, actively reabsorbs NaCl, Impermeable to water

Question 6

LOH characteristics

Answer 6

Fluid is progressively concentrated as it flows down the descending limb

Fluid is progressively diluted as it flows up the ascending limb

200mOsm/L is the largest osmotic gradient that can be maintained across the wall of the ascending limb (back diffusion= active ion transport from lumen)

Question 7

Countercurrent multiplier

Answer 7

Multiplier because 200 mOsm/L gradient created by active transport has been multiplied (to 1200 mOsm/L)

Countercurrent because flow in opposing directions through the two limbs

Active transport is the essential component

Question 8

Vasa recta as countercurrent exchangers

Answer 8

Vasa recta characterized by hairpin loops

As blood flows down the capillary, NaCl diffuses in and H2O diffuses out

As blood flows up the capillary, the reverse occurs

This serves to protect the ISF gradient

Process is entirely passive

Question 9

Early distal convoluted tubule

Answer 9

NKCC2 not present

there’s a NaCl transporter NCC in the luminal membrane which is blocked by thiazide diuretics

Early portion of distal tubule is not permeable to water (continues to dilute the tubular fluid) this referred to as a diluting segment

Modest amount of electrolyte reabsorption

Osmolarity ~ 50mOs/L

Question 10

Late distal tubule and collecting duct composed of two cell types

Answer 10

Principal cells: reabsorb Na and secret K, site of aldosterone action, site of ADH action

Alpha-intercalated cells: secretes H +, reabsorbs K+

Na reabsorption and K+ secretion depend on the Na, K, ATPase in the basolateral membrane and apical channels for both ions

Question 11

Principal cells

Answer 11

Reabsorbs Na in exchange for K

Aldosterone increases number of apical K+ and Na+ channels (ENaC) and increases Na, K, ATPase activity

ENaC - epithelial Na channel - blocked by amiloride

Question 12

Alpha-intercalated cells

Answer 12

Have H+-ATPase and H+,K+-ATPase

Important for K+ reabsorption and acid/base balance

Proton secretion

Proton out, K in

Question 13

Collecting duct CD

Answer 13

Water and urea permeability of CD are controlled by vasopressin (antidiuretic hormone,ADH)

Water can be high or low:
Dependent on aquaporins in the apical membranes of CD cells
ADH causes the insertion of aquaporins into the apical membrane of CD cells

CD is impermeable to water and urea in absence of ADH