Reabsorption/Secretion in the Proximal Tubule

Question 1

T or F. The proximal tubule (PT) is the site of mass reabsoprtion of glomerular filtrate?

Answer 1

T

Question 2

What is important for regulation of ECF volume?

Answer 2

reabsorbing 2/3 of golmerular ultra filtrate

Question 3

How do you calculate the rate of flow into loop of Henle?

Answer 3

GFR - Reabsorption + secretion

Question 4

What are the major solutes that contribute to isotonic reabsoprtion in the PT?

Answer 4

Na, Cl, and Bicarb

Question 5

T or F. Na reabsorption occurs only at the PT.

Answer 5

F. it occurs all along the nephron

Question 6

What are 3 components found at sites of active Na reabsorption?

Answer 6

1. tight junctions
2. luminal membrane Na channel
3. NaKATPase ion pump (found on basolateral side of cell)

Question 7

What doe the NaKATPase result in?

Answer 7

It’s the driving force for Na Absorption

1. decreased IC [Na]
2. decreased in membrane potential

Question 8

What is used to drive reabsorption of other solutes from the lumen of PT?

Answer 8

the potential energy generated from passive diffusion of Na down its electrochemical gradient

Question 9

What drives Cl transport in PT?

Answer 9

rapid Na absorption resulting in luminal fluid being more negative than ISF

Question 10

What allows paracellular space transport of solutes?

Answer 10

a leaky epithelium

Question 11

What happens in the straight tubule of the PT?

Answer 11

Cl absorption is reduced and bicarb is rapidly absorbed

Question 12

Where are the different types of aquaporins found? AQP 1, 4/5, 2

Answer 12

1 –> luminal membrane
4/5 –> basolateral membrane
2 –> distal tubule

Question 13

What drives water reabsorption in PT?

Answer 13

osmotic gradient facilitated by leaky epithelium w/ high hydraulic conductivity (high Kf value)

Question 14

What does reabsorption mean?

Answer 14

capillary uptake of fluid from ISF

Question 15

T or F. Reabsorption from PT is iso-osmotic but selective.

Answer 15

T

Question 16

How is HCO3 absorbed?

Answer 16

indirectly w/ a H+/Na+ exchanger

1. proton secretion
2. H+ reacts w/ bicarb to form CO2 and H2O
3. CO2 absorbed and converted to Bicarb via CA and HCO3- reabsorbed into blood

Question 17

How is glucose reabsorbed?

Answer 17

via a Na-glucose co-transporter, secondary active transport.

Question 18

What is the Tm of glucose?

Answer 18

370-390, point at which all the glucose transporters are transported

Question 19

What is the threshold of glucose?

Answer 19

point at which glucose is first seen in the blood. It’s when reabsoprtion doesn’t match filtration so glucose is excreted

Question 20

What are some causes of glucosuria?

Answer 20

thirst and nocturia, pregnancy, DM, renal glucosuria (mutation in glucose transporter)

Question 21

What are the transporters for glucose called?

Answer 21

1. SGLT1 – found in int and kidney

2. SGLT2 – found in kidney

Question 22

How are amino acids reabsorbed and how much are?

Answer 22

Na co-transporter, only 0.5-2% excreted

Question 23

When is protein excretion high?

Answer 23

in MS, hemoglobinemia, myoglobinemia

Question 24

How are organic acids reabsorbed?

Answer 24

coupled to Na electrochemical gradient

Question 25

Does phosphate has a high or a low threshold when it comes to reabsorption? What does that mean?

Answer 25

low –> partially excreted continuously in urine

Question 26

What regulates the Tm of phosphate?

Answer 26

hormones –> PTH decreases Tm so promotes phosphate excretion

Question 27

What allows Cl to be passively reabsorbed?

Answer 27

1. concentration gradient created by water reabsorption

2. electrochemical grandient created by Na reabsorption

Question 28

Why is only 60% of Cl reabsorbed in PT?

Answer 28

b/c of active transport of HCO3-

Question 29

If there is an increase in urine flow what effect will that have on urea clearance?

Answer 29

it will increase it

Question 30

how much urea is reabsorbed?

Answer 30

50%

Question 31

What will freely filtered but not reabsorbed substances do?

Answer 31

increase osmolarity and cause diuresis

Question 32

What is the clinical significance of only freely filtered substances?

Answer 32

reduce intracranial and intraocular pressure, promote excretion of toxins, edema

Question 33

What is an example of a freely filtered substance only?

Answer 33

mannitol

Question 34

What is mannitol?

Answer 34

monosaccharide that has no transporters –> reduces water reabsorption and increase excretion