Reabsorption

Question 1

How is glucose reabsorbed across the membrane into the ICF?

Answer 1

Via carrier mediated transport systems

Question 2

What substances are reabsorbed via carrier mediated transport systems?

Answer 2

Glucose
Amino acids
Organic acids
Sulphate and phosphate ions

Question 3

What happens when Tm is suceeded?

Answer 3

The excess substrate enters the urine and is excreted

Tm is the maximum transport capacity - one saturated the remaining substances are excreted

Question 4

What determines the transport capacity (Tm)?

Answer 4

Number of carriers

Question 5

What is the renal plasma threshold for glucose?

Answer 5

10 mmol/L

If plasma glucose goes above 10mmol/l then it will be excreted in the urine (this is what occurs in diabetes)

Question 6

What substances does the kidney not regulate their Tm?

And why is this of clinical significance?

Answer 6

Glucose and amino acids
They are regulated by insulin and other glucose counter regulatory hormones therefore if there is glucose in the urine (in diabetics) it is failure of insulin not the kidneys

Question 7

What substances does the kidney regulate the Tm of?

Answer 7

Sulphate and phosphate ions

Question 8

What other hormone regulates phosphate excretion ?

Answer 8

Parathyroid hormone

Question 9

By what mechanism is Na ions reabsorbed?

Answer 9

Via active transport

Question 10

What percentage of Na is reabsorbed per day?

Answer 10

99.5%

Question 11

Where does majority of Na reabsorption occur?

Answer 11

Proximal tubule

Question 12

Describe the reabsorption process of Na.

Answer 12

Na enters the cells through membrane proteins down its electrochemical graident
Na is pumped out the basolateral side of cell by the Na K ATPase
This active transport creates an electrochemical graident which allows Na to enter the cell from the lumen passively down a conc gradient

Question 13

Where are the active Na pumps located?

Answer 13

Basolateral surfaces where there is high amounts of mitochondria

Question 14

How does the reabsorption of Na draw water out of the tubules?

Answer 14

Reabsorption of Na establishes an electrochemical gradient which allows negative ions such as Cl to diffuse passively across the proximal tubular membrane down its conc gradient
This creates an osmotic force which drives H20 out of the tubules
Removal of water from proximal tubule = concentrated substances = ongoing concentration gradients = ongoing diffusion of other ions

Question 15

Is the tubular membrane permeable to urea?

Answer 15

Only moderately permeable therefore only 50% is reabsorbed

Question 16

What is responsible for the passive diffusion of impermeable ions across the membrane for reabsorption?

Answer 16

Na

Active transport establishes electrochemical gradient

Question 17

How would decreased blood flow to the kidney result in dehydration?

Answer 17

Decreased blood flow = decreased 02
02 is required for the ATPase responsible fort the active transport for Na
If this gradient isnt established then Na and other ions such as Cl wont be reabsorbed and either will H20
= increased excretion of water = dehydration

Question 18

Does a high or low Na fascilitate glucose transport?

Answer 18

High Na

Glucose crosses the membrane via sodium dependant glucose transporter so requires it for transport

Question 19

Describe how glucose enters and leaves the cell for reabsorption.

Answer 19

Glucose enters the proximal tubule cell via Na dependant glucose transporter
It then leaves the cell into the ISF via GLUT transporter protein

Question 20

what is the renal threshold?

Answer 20

plasma threshold at which saturation occurs

Question 21

what is the Tm for amino acids?

Answer 21

it is unlimited in order for amino acids to be reabsorbed so we don’t waste it

Question 22

what effect does PTH have on phosphate and calcium in the kidney and how?

Answer 22

PTH increases phosphate excretion in distal tubule and increases Ca reabsorption in ascending L.O.H, distal tubule and collecting duct
reduces the number of channel proteins

Question 23

what enzyme is involved in bicarbonate reabsorption?

Answer 23

carbonic anhydrase

Question 24

how much bicarbonate is reabsorbed by the kidney?

Answer 24

99.9%

Question 25

how is bicarbonate reabsorbed?

Answer 25

carbonic anhydrase IV splits H2C03 into C02 and H20
C02 and H20 is then split into H and HC03
H is then secreted into the tubule and HC02 is reabsorbed into the peritubular capillaries in exchange for Cl

Question 26

where is majority of bicarbonate reabsorbed?

Answer 26

proximal tubule