Lecture 5- Renal Tubular Function

Question 1

high water intake

Answer 1

excretion of dilute urine (50mOsm/L)

Question 2

typical water intake

Answer 2

excretion of urine of normal osmolarity (500 mOsm/L)

Question 3

low water intake

Answer 3

excretion of concentrated urine (120 mOsm/L)

Question 4

reabsorption of water and solutes in proximal tubule is

Answer 4

iso-osmotic

Question 5

In the thin descending limb there is increased water permeability so tubular osmolarity increases/decreases ?

Answer 5

increases- (due to high osmolarity of interstitial fluid in renal medulla drawing water out)

Question 6

In the thick ascending limb, only solutes are absorbed so fluid is

Answer 6

hyposmotic

Question 7

In the distal tubule and collecting duct, there is continued absorption of solutes, but not is water if ADH absent so

Answer 7

further ↓ in urine osmolarity

Question 8

Diabetes Insipidus

Answer 8

• Patients either fail to make ADH or principal cells fail to respond to ADH
• Lack of production/response to ADH > excrete large volumes of urine (up to max excretion of 18L/day)

Question 9

What is the maximal urine excretion a day?

Answer 9

18L/day

10% of filtered water

Question 10

key factors in excretion of concentrated urine (2)

Answer 10

1. high osmolarity of medullary interstitium

2. high water permeability of DT and CD (from high ADH)

Question 11

How much solute must you excrete a day?

Answer 11

600 mOsm (50% urea)
since mas osmolarity of urine is 1200 mOsm, the minimum volume excreted is 0.5L

Question 12

High ADH - high water permeability of DT and CD will lead to

Answer 12

high water reabsorption

Question 13

What does extensive water reabsorption in the DT and CD prevent?

Answer 13

excessive dilution of medullary intersitium

Question 14

Urea is how much percent of max medullary osmolarity?

Answer 14

40% (500 mOsm/L) of the 1200 mOsm/L

Question 15

What is urea?

Answer 15

a waste product from AA catabolism

Question 16

How much urea do the kidneys need to clear?

Answer 16

25-35gm uea/day (50% of filtered urea is excreted

Question 17

urea in PT

Answer 17

↑[urea] due to water reabsorption that’s faster than urea

Question 18

urea in thin loop of henle

Answer 18

↑[urea] due to high permeability & concentration in medullary interstitium

Question 19

urea in thick loop of henle, DT, cortical CD

Answer 19

impermeable to urea

↑ ADH = ↑ water reabsorption from DT and CCD = ↑ [urea]

Question 20

medullary CD

Answer 20

permeable to urea > urea diffuses into medullary interstitium

Question 21

urea cycles from loop of henle back to medullary collecting duct maintains high/low concentration?

Answer 21

high

Question 22

vasa recta

Answer 22

delivers blood to medulla without “washout” of solutes

Blood flow is slow (1-2% of total renal BF)

Question 23

Countercurrent exchange

Answer 23

due to high permeability of vasa recta to solutes and water

Question 24

Vasodilators

Answer 24

↑ BF > ↑ “washout” of solutes > ↓ ability to form concentrated urine

Question 25

purpose of free water clearance

Answer 25

to determine whether water is excreted in excess of solutes or solutes are excreted in excess of water (relative to plasma)

Question 26

Osmolar Clearance

Answer 26

volume of plasma cleared of solutes

C(osm) = U (osm) x V/P(osm)

Question 27

Free water clearance

Answer 27

difference between urine flow rate and osmolar clearance

can be positive or negative

Question 28

positive C(H2O)

Answer 28

water excretion > solute excretion

Question 29

negative C(H2O)

Answer 29

water excretion < solute excretion