Reabsorption and Secretion

Question 1

Where is the main site of reabsorption in the nephron?

Answer 1

The proximal convoluted tubule

Question 2

What is Tm?

Answer 2

This is the renal saturation point where no more substrate can be reabsorbed because all of the transporters are saturated

Question 3

What happens to the excess substrate once the renal saturation pint is reached?

Answer 3

it enters the urine

Question 4

What key substances are reabsorbed by carrier mediated transport systems?

Answer 4

Glucose
Amino Acids
Organic acids
Sulphate 
Phosphate ions

Question 5

What happen to the shape of the binding site of a carrier proteins once the shape changes to allow release?

Answer 5

The binding site fits the molecule less well, allowing it to be released more easily

Question 6

Is carrier mediated transport, like that used for glucose, active transport?

Answer 6

No, although it is carrier mediated, there is no energy input, the process simply takes advantage of the normal chemical gradient

Question 7

Is glucose freely filtered from the plasma?

Answer 7

Yes

Question 8

What is the normal renal saturation for reabsorption of glucose? What happens if the plasma glucose exceeds this limit?

Answer 8

10mmoles/l

The excess glucose will end up in the urine

Question 9

Do the kidneys have a role in the regulation of glucose levels? Why?

Answer 9

No, this is demonstrated by the fact that if the glucose levels increase beyond the renal saturation point (Tm) the excess will be secreted into the urine, but no mechanism exists on the part of the kidneys to reduce the blood glucose, they will simply absorb as much as possible up to their saturation point

Question 10

How do the kidneys play a role in the regulation of certain substances like sulphate and phosphate ions via the Tm mechanism?

Answer 10

Tm is set at a level whereby the normal plasma levels cause saturation, this means that the kidneys will never be able to reabsorb more than the normal plasma levels, thereby excreting any excess which is above the normal level i.e. indirectly regulating the amount of the substances in the body

Question 11

Which is the most abundant of the nutrient in the ECF? In what concentration does it usually exist?

Answer 11

Na+

140mmoles/l

Question 12

Where does 70% of Na+ reabsorption occur?

Answer 12

The proximal convoluted tubule

Question 13

How is Na+ reabsorbed?

Answer 13

Not by a Tm mechanism, but instead via active transport, creating a gradient for Na+ to cross the tubule wall and into the interstitial fluid

Question 14

How is the sodium able to pass through the proximal tubule cell membrane if it is lipid soluble?

Answer 14

Being lipid soluble, the sodium would just pass into the membrane and stay there. However, the brush border of the specialised proximal tubule cells has a much higher permeability to Na+ than most other membranes in the body, thanks to the huge surface area - microvilli, and the large number of Na+ ion channels

Question 15

Describe the sodium gradient created by active transport to allow it to pass out of the tubule

Answer 15

The Na+ pumps are located on the basolateral surface where there is a high density of mitochondria
This decreases the Na in the proximal tubule cell which therefore attract sodium through them via the high to low gradient, allowing the sodium to pass through the tubule out into the interstitial fluid

Question 16

The reabsorption of which substance has a knock-on permissive effect for the reabsorption of many other substances? How does this happen?

Answer 16

Na+
When it is drawn out of the cells, water is drawn with it, creating an osmotic force which favours the pulling of the other substances passively out of the cell along the chemical gradient e.g. anions, H20, K+, Ca2+, urea

Question 17

What two factors dictate the rate of reabsorption of the passively reabsorbed substances from the tubule to the extracellular fluid?

Answer 17

1. The amount of H20 removed which determines the extent of the concentration
2. However, the permeability has an ultimately dominant effect on the reabsorption - if the membrane is totally non-permeable, molecules will not be reabsorbed, regardless of the amount of water removed from the tubule due to the action of Na+ e.g. insulin, mannitol

Question 18

Describe the carrier mediated mechanism of glucose reabsorption

Answer 18

The glucose uses a Sodium-Dependant glucose transporter - this is a symporter system which is nt active transport as there is no direct energy input

Question 19

Why is the symporter transport mechanism for glucose reabsorption necessary?

Answer 19

A diffusion driven mechanism would not be efficient enough to reabsorb all of the glucose, which it needs to be able to maintain up to the saturation point

Question 20

Describe secretion from the peritubular capillaries into the tubule

Answer 20

Secretory mechanisms transport substances from the capillaries into the lumen of the tubule and provide a secondary route into the tubule

Question 21

What is an important factor to allow filtration at the glomerulus?

Answer 21

That substances are protein-bound

Question 22

Describe the carrier mechanisms which allow for secretion from the capillaries into the tubules for excretion

Answer 22

They are broadly non-specific generalised carriers which usually accept more than one substance for excretion - this is important for secretion of drug metabolites and metabolic waste products etc.

Question 23

What is the major ICF ion?

Answer 23

K+

Question 24

What is the normal ECF[K+]?

Answer 24

4mmoles/l

Question 25

What is hyperkalaemia?

Answer 25

An ECF K+ of higher than 5.5mmoles/l
It decreases the resting membrane potential of excitable cells and eventually results in VF and death

POTASSIUM IS KEY

Question 26

What is hypokalaemia?

Answer 26

An ECF K+ of less than 3.5mmoles/l
It increases the resting membrane potential i.e. hyperpolarises muscle, affecting cardiac cells which leads to arrhythmias and death

POTASSIUM IS KEY

Question 27

What portion of K+ filtered at the glomerulus is reabsorbed? Where

Answer 27

All of it, primarily at the proximal c tubule

Question 28

What affects K+ secretion?

Answer 28

Changes in its secretion at the distal tubule. This is triggered by any increase in renal tubule K+ concentration (due to increased dietary ingestion), increasing K+ secretion into the distal tubule

Any decrease in intracellular K+ will result in decreased secretion into the distal tubule

Question 29

What hormone can regulate K+ secretion? How?

Answer 29

Aldosterone
An increase in the K+ concentration in the ECF surrounding aldosterone secreting cells will cause aldosterone release which circulates to the kidney and stimulates renal distal tubule K+ secretion