Renal Transport Mechanisms

Question 1

What gets reabsorbed in the Proximal Convoluted Tubule and how much (%)?

Answer 1

Glucose - 100%
Amino Acids - 100%
Urea - 50%
Sodium - 65-70%
Potassium - 70%
Phosphate - 70%
Calcium - 70%
Magnesium - 30%
H2O - 65-70%

Question 2

What is reabsorbed in the Proximal Straight Tubule and how much (%)?

Answer 2

Phosphate - 15%

Question 3

What is reabsorbed in the Thick Ascending Limb of the Loop of Henle and how much (%)?

Answer 3

Sodium - 25%
Potassium - 20%
Calcium - 25%
Magnesium - 60%

Question 4

What is reabsorbed in the Distal Convoluted Tubule and how much (%)?

Answer 4

Sodium - 5%
Calcium - 8%
Magnesium - 5%
H2O - variable
Urea - Variable

Question 5

What is reabsorbed in the collecting duct and how much (%)?

Answer 5

Sodium - 3%
H2O - Variable
Urea - Variable

Question 6

What is excreted in the urine and how much (%)?

Answer 6

Sodium - <1%
Calcium - <1%
Potassium - 1-100%
Magnesium - 5%
Phosphate - 15%
H2O - Variable
Urea - Variable

Question 7

What channel is the key player in reabsorption of most of the solutes in the Proximal tubule?

Answer 7

Na/K-ATPase on the basolateral side (not facing the tubule lumen)

It moves 3 Na+ into the interstitial fluid between the the tubule and peritubular capillaries while also moving 2 K+ into the tubule cell.

Question 8

How does most Na+ leave the tubule lumen and enter the cell?

Answer 8

Through a Na+/H+ exchanger (NHE)

Found along the proximal tubule

Question 9

What is the consequence of the NHE and how is it regulated?

Answer 9

H+ continously enter the tubule lumen causes acid to form.

HCO3- buffer system in play - Carbonic Anhydrase Enzyme found on apical membrane and within lumen.

H+ binds with HCO3- to form Carbonic Acid, which then dissociates into H2O and CO2 (both of which diffuse into the tubule lining cells where intracellular carbonic anhydrase reforms carbonic acid and gets dissociated into HCO3- and H+

H+ is sent back into the lumen in exchange for a Na+ ion.

Question 10

How does chloride anion leave the tubule lumen?

Answer 10

Passive, paracellular movement due to concentration gradient

Question 11

How does water leave the tubule lumen?

Answer 11

Water can leave paracellularly (such as in the descending limb of the loop of henle), but this cannot occur in the ascending limb due to proteins blocking the path between cells.

Water can leave transcellularly through Aquaporin Channels.

AQP1 - found in the proximal tubule
AQP2 - found in the collecting duct and is under the control of Antidiuretic hormone

Question 12

How does glucose leave the tubule lumen?

Answer 12

Two types of Sodium-Glucose Transporters:

SGLT1 (GLUT1) - High affinity, low capacity; found in the late proximal tubule when glucose concentration is low. Transport 10% of the glucose

SGLT2 (GLUT2) - Low affinity, high capacity; found in the early proximal tubule. Transport 90% of the glucose.

They couple the downstream movement of Na+ into the tubule cells with the upstream movement of glucose into the tubule cells.

Question 13

How does diabetes affect glucose reabsorption?

Answer 13

SGLT2 inhibition occurs in type 2 diabetes, resulting in glucose excretion in the urine in the body’s desperate .

Transporter reaches its Transport maximum and cannot meet the reabsoprtion rate to remove all of the glucose entering the glomerulus

Question 14

Where is the Na/K/Cl Cotransporter found and how many ions of each does it move?

Answer 14

Ascending limb of the Loop of Henle
1 Na+
1 K+
2 Cl-

Question 15

What does the Loop Diuretic Act on?

Answer 15

Blocks the Cl- binding site on the Na/K/Cl co transporter in the ascending limb of the loop of henle, preventing reabsorption of NaCl

Question 16

What do Thiazide Diuretics act on?

Answer 16

Inhibit the Na/Cl symporter in the Distal Convoluted Tubule, preventing NaCl reabsorption.

Question 17

How is urea moved in the nephron?

Answer 17

Reabsorption occurs in the proximal tubule passively (50%)

Secretion of Urea occurs in the Thin Descending Limb and the Thin Ascending Limb of the Loop of henle, bringing urea conentration back to 100%.

Reabsorption occurs again in the inner medullary Collecting duct through Urea transport Proteins (UT-A) that are stimulated by ADH

Urea reabsorbed in the collecting duct is the source of urea that is secreted back into the loop of henle

Question 18

Where does Aldosterone come from and what is its action?

Answer 18

Aldosterone is secreted by the cortex of the adrenal gland in response to Angiotensin II or in response to increased Serum K+

It is known as the “salt-retaining hormone” because it acts on the thick ascending limb, the distal convoluted tubule, and the cortical portion of the collecting duct to promote NaCl and water reabsorption

Causes urine volume and urine Na+ concentration to decrease

Also causes K+ secretion and increased urine K+