Renal System 4

Question 1

QUESTION

Answer 1

NKCC pump inhibited by loop diuretic?? slide 13

How does the osmolarity in the discending limb increase? By removal of water and increase of concentration? -> but osmolarity is about the amount of solute isnt it?

Shouldnt the osmolarity in the interstitium increase, why is it increasing in the discendig limb according to the figure?

Question 2

What are substances that are reabsorbed almost completely, which are excreted to some degree?

Answer 2

reabsorbed: Water, glucose, sodium, calcium, chloride (can depend on the weather, activity,..)

excreted: Urea (nitrogen waste trapped in urea)

Question 3

Where do water and sodium get reabsorbed?

Answer 3

Mostly in the proximal convoluted tubule, in the Loop of Henle, and in the distal convoluted tubule (Fine-tuning)

Question 4

How is sodium reabsorbed in the proximal c. tube?

Answer 4

1) Cotransport with different molecules (glucose, amino acids, vitamins, HCO3-; and Countertransport with H+)

Question 5

How is reabsorption in the proximal tube made possible?

Answer 5

Because Na+ in the tubule is pushed out of the cell into the interstitial fluid through the Na-K pump (need ATP) -> keeps Na+ inside the cell low -> so that Na+ can be easily reabsorbed from the tubular lumen (urine)

Question 6

How is sodium reabsorbed in the ascending limb?

Answer 6

Na-K pump (ATP) on the basolateral membrane push Na out and keeps Na+ low inside the cell -> Cotransport of Na+ with 2xCl- and K+ through NKCC pump on the apical membrane

Question 7

What specific cell is involved in the reabsorption of sodium in the distal convoluted tube?

Answer 7

Principal cells

Question 8

How is sodium reabsorbed in the distal c. tube?

Answer 8

Na+ is pushed out of the cell by the Na-K pump and K+ is pumped inside the principal cell (K+ gets secreted into the tubule lumen (urine) -> Na+ is reabsorbed actively? through the sodium channel

Question 9

What determines how much K+ leaves the principal cell?
(K+ can leave at each side -> urine or interstitial fluid, blood)

Answer 9

Amount of the sodium reabsorbed into distal c. tube determines how much K+ gets secreted

Na+ changes the charge within the principal cell, providing a force for some of K+ to leave, they can leave on each side

Question 10

How is water reabsorbed?

Answer 10

Transcellular route: Water tries to follow Na+ if it can, when Na+ leaves the local osmolarity will drop -> H20 will go from the tubular lumen (urine) into epithelial cells -> into the interstitial fluid, and then into the blood, where there is a higher concentration of solute (osmosis)

Paracellular (vary if there are tight junctions) flows between cells

Question 11

How can H20 pass the cell membrane (lipids, not permeable to water)

Answer 11

Permeability is increased by Aquaporins (AQP water channels) on the surface of the kidney cells

Question 12

Where exactly are aquaporins located?

Answer 12

In the collecting duct cells and distal convolute tubule

Question 13

What determines the number of Aquaporins on the surface of the kidney cells?

Answer 13

Vasopressin (antidiuretic) binds to the vasopressin receptor and starts a cascade, that causes vesicles that carry aquaporins to merge with the membrane

Question 14

Where do most of the water reabsorption occur by aquaporins?

Why is water less reabsorbed in the Loop of Henle?

Answer 14

The distal portion of convoluted tubules and cortical + medullary collecting duct onto the influence of antidiuretic hormones

Because water is not permeable in the Loop of Henle

Question 15

What condition is needed to reabsorb as much water as possible?

Answer 15

-Tubules that are very permeable to water (Aquaporins)
-A very salty interstitium to draw the water out of tubules (H20 follows Na+)

Question 16

Where in the kidney is the level of osmolarity (saltiness) of the interstitium the highest?

Answer 16

It increases towards the medulla (up to 1200 mOsm)

Question 17

What is the Counter-current multiplier

Answer 17

Increase of osmolarity, due to the stepwise shift of fluid, the level of osmolarity is increasing with the length of the tubule,

-> on the ascending limb Na+ is pushed out by NCKK pump -> causing water to go from the descending limp into interstitium because of osmosis (higher concentration of solute),
water can only leave the descending limb because only there it is water permeable

Question 18

What happens with H20 and salt that is reabsorbed into the interstitium?

Answer 18

It goes to the blood -> Vasa recta, it is located adjacent to the Loop of Henle and works similarly in regards to water reabsorption

Question 19

How is the osmolarity maintained, if the water and salt go into the blood (Vasa recta)?

Answer 19

The Vasa recta also release salt and water, which increases the osmolarity in the interstitium

Question 20

How is urea reabsorbed? Urea recycling

Answer 20

Urea enters Bowman capsule -> 50% reabsorbed at proximal tubule -> at the bottom of Loop of Henle urea is secreted so that we have 100% urea again -> at distal tubule 30% is reabsorbed -> at the very end (inner medulla) 55% is reabsorbed -> 15% of urea is excreted

Note: water is almost completely reabsorbed in the meanwhile)

Question 21

How does urea contribute to high osmolarity in the medulla?

Answer 21

Urea contributes to high osmolarity because it is reabsorbed throughout the urea cycle