Reabsorption and secretion

Question 1

Mechanisms of reabsorption

Answer 1

By carrier mediated transport system eg glucose, amino acids, organic acids, sulphate and phosphate ions - Tm mechanism

Reabsorption of Na+ ions (active transport - sodium pump)

Question 2

Why does substrate enter the urine?

Answer 2

Carriers have a maximum transport capacity Tm which is due to saturation of the carriers.

If Tm is exceeded, then the excess substrate enters the urine.

Question 3

What is the Renal threshold?

Answer 3

The plasma concentration at which saturation occurs

Question 4

At what plasma concentration of glucose will all of it be reabsorbed into the blood?

Answer 4

10 mmoles/l = Renal plasma threshold for glucose.

Question 5

What happens when the level of plasma glucose exceeds 10 mmoles/L

Answer 5

it appears in the urine

If plasma [glucose] = 15 mmoles/l, 15 will be filtered, 10 reabsorbed and 5 excreted.

Question 6

True or false: Glucose is freely filtered

Answer 6

True

Whatever the plasma concentration of glucose = what will be filtered = linear relationship.

Freely filtered means glucose is neither protein-bound nor complexed with macromolecules.

Question 7

What regulates blood glucose?

Answer 7

Insulin and the counter-regulatory hormones responsible for its regulation

Question 8

What is the appearance of glucose in the urine of diabetic patients caused by?

Answer 8

Failure of insulin, NOT the kidney

Question 9

What substances do the kidneys regulate by means of the Tm mechanism?

Answer 9

Sulphate and phosphate ions

Any increase above the normal level will be excreted, therefore achieving its plasma regulation.

Question 10

How is Na+ absorbed?

Answer 10

By active transport (sodium pumps), which establishes a gradient for Na+ across the tubule wall

Majority reabsorbed from the proximal tubule

Question 11

But how come Na2+ can cross the brush bored of the proximal tubule?

Answer 11

The brush border of the proximal tubule cells has a higher permeability to Na+ ions than most other membranes in the body.

Partly because of the enormous surface area offered by the microvilli and the large number of Na+ ion channels, which facilitate this passive diffusion of Na+.

This reabsorption of Na+ ions is key to the reabsorption of the other components of the filtrate.

Question 12

What creates osmotic force?

Answer 12

The active transport of Na+ out of the tubule followed by Cl- (moves down gradient) creates an osmotic force, drawing H2O out of the tubules.

This concentrates all the substances left in the tubule creating outgoing concentration gradients.

Question 13

What does the rate of absorption of non-actively reabsorbed solutes depend on (2)

Answer 13

Amount of H2O removed, which will determine the extent of the concentration gradient.

The permeability of the membrane to any particular solute.

Question 14

What happens with urea?

Answer 14

Tubule membrane is only moderately permeable to urea, so only about 50% is reabsorbed, the remainder stays in the tubule.

Question 15

Which substances is the tubular membrane impermeable to?

Answer 15

Insulin
Mannitol - diuretic

Despite a concentration gradient being established favouring their reabsorption, they cannot gain access through the tubule membrane so that all that is filtered stays in the tubule and passes out in the urine.

Question 16

Look

Answer 16

It is the active transport of Na+ that establishes the gradients down which other ions, H2O and solutes pass passively.

Anything which decreases active transport eg decreased BF => results in
disruption of renal function.

Question 17

Glucose shares the same carrier molecule as which substance?

Answer 17

Na+. It’s the Sodium-dependent glucose transporter

High [Na+] in the tubule facilitates and low [Na+] inhibits glucose transport.

Question 18

What provides a second route into the tubule?

Answer 18

Secretory mechanisms transport substances from the peritubular capillaries into the tubule lumen

Question 19

Why are secretory mechanisms important?

Answer 19

Important for substances that are protein-bound, since filtration at glomerulus is very restricted.

Also for potentially harmful substances, means they can be eliminated more rapidly from the body.

Question 20

Why is K+ maintenance so important

Answer 20

Normal ECF[K+] = 4mmoles/l.

If it increases to 5.5mmoles/l = hyperkalaemia => decreases resting membrane potential of excitable cells and eventually causes ventricular fibrillation and death.

If [K+] < 3.5 mmoles/l = hypokalaemia arises => increases resting membrane potential ie hyperpolarizes muscle, cardiac cells => cardiac arrhythmias and eventually death.

Question 21

Where is K+ filtered?

Answer 21

Primarily at the proximal tubule

Question 22

Which adrenal cortical hormone also regulates K+

Answer 22

Aldosterone

Question 23

How does aldosterone regulate K+

Answer 23

An increase in [K+] in ECF bathing the aldosterone secreting cells stimulates aldosterone release which circulates to the kidneys to stimulate increase in renal tubule cell K+ secretion (secretion into the tubule to get excreted)

i.e Increase in [K+] causes increases in Aldosterone release which stimulates increase in secretion and therefore excretion of K+

Question 24

What else does aldosterone stimulate via a different pathway?

Answer 24

Na+ reabsorption at the distal tubule