Reabsorption and Secretion

Question 1

What is Tm?

Answer 1

Maximum transport capacity

Question 2

What happens when Tm is exceeded by carrier transport systems?

Answer 2

The excess substrate enters the urine

Question 3

What is the renal threshold for glucose?

Answer 3

10mmoles/l

Question 4

What occurs when plasma glucose concentration exceeds the renal threshold for glucose?

Answer 4

10mmoles/l will be reabsorbed and the remainder will be excreted although all is filtered

Question 5

When is glucosuria common?

Answer 5

During insulin failure i.e uncontrolled diabetes

Question 6

What percentage of Na+ ions are reabsorbed?

Answer 6

99.5%

Question 7

Where does the majority of Sodium Ion reabsorption occur?

Answer 7

the proximal tubule

Question 8

How are sodium ions reabsorbed?

Answer 8

Via active transport rather than a Tm Mechanism

Question 9

Where are sodium pumps located?

Answer 9

basolateral surfaces

Question 10

Why does the brush border of the proximal tubule have a higher permeability to Na+ ions than other membranes?

Answer 10

Large surface area from microvilli

Large number of Na+ ion channels

Question 11

How do negative ions diffuse across the membrane?

Answer 11

Via passive transport down the electrical gradient maintained by sodium active transport

Question 12

How is an osmotic force created in the tubules of the kidney?

Answer 12

Active transport of Na out of the tubule followed by Cl ion passive transport created an osmotic force which draws H2O out of the tubules into ECF

Question 13

How are substances within the tubule concentrated ?

Answer 13

Since H2O is removed from tubule fluid, the left over substances within the tubule are more concentrated.

This creates an outgoing concentration gradient

Question 14

What is the rate of absorption dependent on?

Answer 14

Amount of water removed which determines extent of concentration gradient

AND

The permeability of the membrane to any particular solute

Question 15

How much urea is absorbed?

Answer 15

~50%

Question 16

What is tubular secretion?

Answer 16

Secretory mechanisms transport substances from peritubular capillaries into tubule lumen and provide a second route into the tubule

Question 17

Why is tubular secretion important for protein bound substances?

Answer 17

Because their filtration at the glomerulus is limited

Question 18

What is the normal concentration of potassium in ECF?

Answer 18

4mmoles/l

Question 19

What occurs if potassium concentration exceeds 5.5 mmol/l?

Answer 19

HYPERKALAEMIA which decreases resting membrane potentials of excitable cells = VF= Death

Question 20

What occurs if potassium drops below 3.5mmol/l ?

Answer 20

HYPOKALAEMIA = increase in resting membrane potential = hyperpolarisation of cells = cardiac arrhythmia’s and cells death

Question 21

Where is potassium filtered?

Answer 21

Within glomerulus, primarily at proximal tubule

Question 22

When are changes in potassium observed?

Answer 22

When changes in secretion at the distal parts of the tubule are witnessed

Question 23

What occurs when there is an increase in renal tubule cell potassium concentration?

Answer 23

Potassium secretion ensues and intracellular potassium concentration decreases which then leads to a decrease in secretion

Question 24

What is potassium secretion regulated by?

Answer 24

Aldosterone

Question 25

When is aldosterone released?

Answer 25

When there is an increase in [K+] in ECF

Question 26

What is the affect of aldosterone on secretion?

Answer 26

It stimulates renal tubule cell [K+] secretion

Question 27

If Drug A’s clearance is greater than inulin clearance, then what would happen to Drug A?

Answer 27

Net secretion of drug A