L26 Generation of Medullary Hypertonicity

Question 1

True or false: the ascending limb of the loop of Henle is impermeable

Answer 1

True

The ascending limb has thicker walls than the descending limb. This prevent water leaving

Question 2

True or false: the descending limb of the loop of Henle is permeable to water

Answer 2

True

The descending limb is made of squamous epithelium which allows water to move in from the medulla

Question 3

How does the thick ascending limb of the loop of Henle manipulate the osmolality of the tubular fluid with respect to the medulla?

Answer 3

The wall of the TAL is impermeable to water, but basolateral Na-K pumps (NaK ATPase) can throw sodium out of the tubule and into the medullary interstitium.

This generates an osmotic difference of 200mOsmol between the descending limb and the ascending limb.

Fluid in ascending limb becomes HYPOTONIC wrt fluid in descending limb.

Question 4

What are the two sections of the collecting duct?

Answer 4

Cortical collecting duct (located in the cortex)

Medullary collecting duct (guess where it’s located)

Question 5

True or false: under normal conditions, the distal convoluted tubule, cortical collecting duct and medullary collecting duct are all fairly permeable to water, urea and sodium chloride

Answer 5

False

DCT, CCD and MCD relatively impermeable to H2O, urea and NaCl

Antidiuretic hormone (ADH) increases H2O permeability of these segments according to need (facultative reabsorption)

Question 6

True or false: Majority of facultative water reabsorption occurs in the medullary collecting duct

Answer 6

False

This would negate the medullary hypertonicity, hence the majority of H2O reabsorption takes place in the cortical collecting duct.

Question 7

True or false: ADH increases water reabsorption in the cortical collecting duct

Answer 7

True

Question 8

Fluid leaving the thick ascending limb and reaching the cortical collecting duct has an osmolality of __?__

Answer 8

90mOsmol

Question 9

True or false: Under the influence of ADH, fluid osmolality in the cortical collecting duct rises to 190mOsmol

Answer 9

False

It rises to 290mOsmol

Question 10

What are the 2 main solutes responsible for the osmolality of the renal medulla?

Answer 10

Sodium Chloride and Urea

Question 11

Which part of the collecting duct is permeable to urea?

Answer 11

The medullary collecting duct can be made permeable to urea in the presence of ADH (it adds urea channels as well as aquaporins).

The cortical collecting duct is never permeable to urea.

Question 12

Majority of nephrons (85%) are cortical (short) and don’t extend into the medulla. Does this mean only 15% of nephrons are capable of concentrating urine?

Answer 12

No

All nephrons drain into collecting ducts which pass through the medulla, therefore all functioning nephrons can concentrate urine.