Reabsorption and Secretion

Question 1

How are many substances e.g. glucose and amino acids reabsorbed in the kidneys?

Answer 1

Carrier mediated transport systems

Question 2

What happens when the carriers become saturated?

Answer 2

They reach their maximum transport capacity (Tm)

Question 3

What happens when Tm is exceeded?

Answer 3

Excess substrate enters the urine

Question 4

What is the renal threshold?

Answer 4

Plasma threshold at which saturation occurs

Question 5

What is the renal threshold for glucose?

Answer 5

Plasma conc. > 10mmol/l

Question 6

What happens to the glucose when the renal threshold is exceeded?

Answer 6

It will appear in the urine

Question 7

Do the kidneys regulate glucose concentration?

Answer 7

No

Question 8

Is glycosuria in diabetic patients a sign of kidney failure?

Answer 8

No it is a sign of insulin failure

Question 9

is the amino acids Tm high or low?

Answer 9

It is set so high that urinary excretion does not occur (amino acids are too important to be excreted)

Question 10

How is Tm set for substances such as sulphate and phosphate ions?

Answer 10

The Tm is set so that normal plasma conc. causes saturation allowing the kidneys to regulate the level

Question 11

How are sodium ions reabsorbed?

Answer 11

Active transport: creates a gradient across the tubule wall

Question 12

Where are the majority of sodium ions reabsorbed?

Answer 12

Proximal tubule

Question 13

Where are the active sodium pumps located?

Answer 13

Basolateral surfaces (they have a high density of mitochondria)

Question 14

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

Answer 14

The microvilli and large number of sodium ion channels

Question 15

How are chlorine ions reabsorbed?

Answer 15

They diffuse passively across the membrane down the electrical gradient established and maintained by the active transport of sodium ions

Question 16

What happens to the water in the tubules?

Answer 16

The active transport of sodium out of the tubule creates an osmotic force which draws water out of the tubule

Question 17

What happens to all the other substrates in the tubules after the water and sodium have been reabsorbed?

Answer 17

The tubule fluid is more concentrated and the substances now all have outgoing conc. gradients

Question 18

What does the rate of reabsorption of non-actively reabsorbed solutes depend on?

Answer 18

Amount of water removed and permeability of the membrane to the particular solute

Question 19

How much urea is reabsorbed and why?

Answer 19

Approx. 50% as the tubule membrane is only moderately permeable to urea

Question 20

Name two substances for which the tubular membrane is impermeable?

Answer 20

Inulin and mannitol

Question 21

What effect does the sodium concentration on glucose transport?

Answer 21

High sodium conc. facilitates glucose transport

Low sodium conc. inhibits glucose transport

Question 22

What is the function of the secretory mechanisms?

Answer 22

Transporting substances from the peritubular capillaries into the tubule lumen and providing a 2nd route into the tubule

Question 23

Why are the secretory mechanisms important?

Answer 23

Important for substances that are protein-bound and for rapid elimination of potentially harmful substances

Question 24

How can the carrier mechanism be used for drugs?

Answer 24

The carrier mechanisms are not specific and so can be used for both the organic mechanism and certain drugs

Question 25

Where are the substances secreted?

Answer 25

The proximal tubule

Question 26

Why is the handling of potassium ions so important?

Answer 26

Potassium is the major cation in cells and is essential for life

Question 27

What is the normal ECF [potassium]?

Answer 27

4 mmol/l

Question 28

What happens if the potassium conc. > 5.5mmol/l?

Answer 28

Hyperkalaemia: causes ventricular fibrillation and death eventually

Question 29

What happens if the potassium conc < 3.5mmol/l?

Answer 29

Hypokalaemia: causes cardiac arrhythmias and death eventually

Question 30

Where is the majority of potassium rebsorbed?

Answer 30

Proximal tubule

Question 31

How is the amount of potassium excreted controlled?

Answer 31

Changes in the secretion of potassium in the distal parts of the tubules

Question 32

Which adrenal cortical hormone regulates potassium secretion?

Answer 32

Aldosterone

Question 33

Which part of the kidneys is effected by aldosterone?

Answer 33

Distal tubule

Question 34

Which cells in the kidneys secrete hydrogen ions?

Answer 34

Tubule cells