Costanzo Renal Physiology: Concentration and Dilution of Urine

Question 1

What is the function of concentration or dilution of urine?

Answer 1

Urine is concentrated in response to water deprivation and urine is diluted in response to water intake.

Question 2

How does the body achieve concentration or dilution of the urine?

Answer 2

By altering the relative amounts of water and solutes excreted.

Question 3

What is hyperosmotic urine?

Answer 3

Urine with an osmolarity greater than blood osmolarity.

Question 4

what hormone stimlulates the production of hyperosmotic urine?

Answer 4

High ADH levels stimulate concentration of urine (hyperosmotic urine)

Question 5

What is the corticopapillary osmotic gradient?

Answer 5

the difference between high (1200mOs) osmolarity in the papilla and low osmolarity in the cortex (300mOs)

Question 6

What is the countercurrent multiplication in the loop of Henle?

Answer 6

Depends on NaCl reabsorption in the thick ascending loop of Henle and counter current flow in the ascending and descending loops.

Question 7

What effect does ADH have on the thick ascending loop?

Answer 7

ADH stimulates the reabsorption of NaCl in the thick ascending loop via the Na+/K+/Cl- cotransporter

Question 8

What is the effect of ADH on urea recycling?

Answer 8

Urea moves from the inner medullary collecting ducts to medullary interstitial fluid.

Question 9

What are the vasa recta and how do they function?

Answer 9

The vasa recta are the capillaries that feed the ascending and descending loops of Henle. They maintain the corticopappillary gradient by acting as osmotic exchangers with the interstitial fluid.

Question 10

How is water absorbed in the proximal tubule?

Answer 10

Water is absorbed isosmotically in the proximal tubule. (TF/P = 1)

Question 11

What happens to water in the thick ascending loop of Henle?

Answer 11

The thick ascending loop is impermeable to water therefore the osmolarity of water in the thick ascending loop is decreased as Na+, K+, and 2Cl- are reabsorbed. This dilutes the filtrate (diluting segment) (TF/P<1) (100mOs)

Question 12

What happens to water early distal tubules?

Answer 12

Water is further diluted in the early distal tubule because it also is impermeable to water.

Question 13

What happens to water in the late distal tubule and collecting ducts?

Answer 13

Normally the late distal tubule and collecting ducts is also impermeable to water. However, in the presence of ADH the principal cells become permeable to water and water is reabsorbed until the osmolarity of the filtrate is equal to the surrounding fluid (TF/P=1)

Question 14

What is hyposmotic urine?

Answer 14

urine that is dilute (osmolarity less than blood osmolarity)

Question 15

Under what hormonal conditions is dilute urine made?

Answer 15

Dilute urine is made when circulating ADH is low or non-functional

Question 16

What happens to the corticopappillary gradient when there is little or no ADH?

Answer 16

It is decreased because ADH stimulates both countercurren multiplication and urea cycling.

Question 17

What happens to water in the proximal tubule under low ADH conditions?

Answer 17

The same as any other time water is absorbed isosmotically.

Question 18

What happens to water in the thick ascending loop of henle under low ADH conditions?

Answer 18

NaCl is reabsorbed just as before making the water dilute (TF/P < 1)

Question 19

What happens to water in the Early distal tubules under low ADH conditions?

Answer 19

NaCl is reabsorbed water is not. Water is further diluted.

Question 20

What happens to water in the late distal tubule under low ADH conditions?

Answer 20

The distal tubule and collecting ducts will remain impermeable to water and the osmolarity of the urine will not equilibrate. ( as low as 50mOs)

Question 21

What is Free-water clearance?

Answer 21

The ability of the kidney to dilute or concentrate the urine. In the presence of ADH there should be solutes in the water. In the absence of ADH there should be solute free water.

Question 22

What is the equation for Free-water clearance?

Answer 22

C(h2O) = V - C(osm)

C(H2O) = free water clearance 
V = urine flow rate
C(osm) = Osmolar clearance.

Question 23

What is the free water clearance when the urine is isosmotic with the plasma?

Answer 23

the free water clearance is zero (loop diuretics)

Question 24

What is the free water clearance with urine that is hyposomotic to plasma (low ADH)?

Answer 24

Free water clearance is positive. (high water intake, diabetes)

Question 25

What is the free water clearance with urine that is hyperosmotic to plasma (high ADH)?

Answer 25

The free water clearance is negative (water deprivation)