4 – Active Renal Reabsorption and Transport Maximum

Question 1

Reabsorption vs. filtration:

Answer 1

-reabsorption is an ACTIVE process, filtration is not
*reabsorption is very SELECTIVE

Question 2

What are the different segments of renal tubules for where reabsorption occurs?

Answer 2

-proximal tubules
-Loop of Henle (descending and ascending limbs)
-distal tubule
-connecting tubule
-collecting tubule
-collecting duct

Question 3

What is the equation for urine excretion?

Answer 3

=glomerular filtration – tubular reabsorption + tubular secretion
*any small change in filtration or reabsorption can lead to HUGE changes in urine volume

Question 4

What are the 2 reabsorption pathways?

Answer 4

1. Through tubular epithelium into the interstitial fluid
2. Through peritubular capillary membranes into the blood (hydrostatic and osmotic forces)

Question 5

Reabsorption through tubular epithelium into interstitial fluid: 2 paths

Answer 5

*can be active (primary and secondary) and passive
1. Transcellular path
2. Paracellular path

Question 6

What are the primary active transporters in the kidneys?

Answer 6

1. Na-K ATPase
2. H ATPase
3. H-K ATPase
4. Ca ATPase

Question 7

Na-K ATPase:

Answer 7

-basolateral membrane
-3Na out, 2K in
*creates 2 main forces for Na diffusion from lumen into epithelial cells
>concentration gradient and negative charge

Question 8

Na reabsorption:

Answer 8

-active Na reabsorption happens in most parts of tubules
>luminal membrane
>basolateral membrane

Question 9

Na reabsorption at luminal membrane:

Answer 9

-Na diffuse into cells because of electrochemical gradient
-Na facilitated diffusion important for secondary active transport of glucose and AA
-assists with secondary active secretion of H+ into proximal tubule

Question 10

Na reabsorption at basolateral membrane:

Answer 10

-active transport against electrochemical gradient

Question 11

How is Na transferred to peritubular capillaries?

Answer 11

-passive ultrafiltration
>hydrostatic and colloid osmotic pressure

Question 12

How are glucose and AA transferred to interstitial fluid?

Answer 12

-basolateral membrane facilitated diffusion through GLUT 2 and GLUT 1
*concentration gradient

Question 13

SGLT2:

Answer 13

-glucose secondary active transport
*very efficient in PROXIMAL TUBULE (90% are SGLT2, 10% are SGLT1)

Question 14

Na assists with secondary active secretion of H+ into proximal tubule at luminal membrane:

Answer 14

-counter transport mechanism through Na-H exchanger (NHE)

Question 15

Pinocytosis:

Answer 15

-active reabsorption of proteins
-used in the proximal tubules (if they made it through, ex. diabetes or high BP)

Question 16

Pinocytosis ‘steps’:

Answer 16

1. Proteins are trapped in invaginations of cell membrane
2. Vesicles form inside cytoplasm
3. Protein is digested int AA (lysosomes)
4. AA are reabsorbed

Question 17

Fanconi’s syndrome:

Answer 17

-when reabsorption process in renal tubules (especially proximal tubules) is compromised due to DAMAGE of the tubules
-primary (genetic) or secondary (toxins, reactions to drugs) condition

Question 18

What happens when someone has Fanconi’s syndrome?

Answer 18

-electrolytes and nutrients (Na, K, glucose, AA, HCO3) are excreted and lost in urine

Question 19

How can you manage Fanconi’s syndrome?

Answer 19

-supply nutrients and electrolytes
-supporting acid-base balance
*response varies depending on degree of damage

Question 20

Transport maximum indicates:

Answer 20

-limit of reabsorption or secretion of a substance
*when ALL nephrons have reached their MAXIMUM reabsorption capacity
Ex. 375mg/min for glucose

Question 21

Transport maximum is related to:

Answer 21

-saturation of transport proteins
-enzymatic activities

Question 22

If more than 375mg of glucose is filtered every minute, what happens?

Answer 22

-all excess glucose appears in urine
*can’t reabsorb anymore than 375mg/min

Question 23

Threshold level of excretion:

Answer 23

-below transport maximum
-when you start seeing traces of glucose in urine
Ex. 250mg/min

Question 24

Threshold level of excretion vs. transport maximum of glucose?

Answer 24

-threshold level of excretion=250mg/min
-transport maximum=375mg/min
*some glucose will appear in urine above 250mg/min
**WHY? Transport maximum for all nephrons is NOT the same