Reabsorption & Secretion

Question 1

Which substances are reabsorbed by carrier mediated transport systems?

Answer 1

glucose, amino acids, organic acids, sulphate and phosphate ions.

Question 2

What does Tm stand for?

Answer 2

Transport capacity - 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

Describe the filtration capability of glucose

Answer 4

1. Glucose is freely filtered, so whatever its [plasma] that will be filtered.
2. In man for plasma glucose up to 10 mmoles/l, all will be reabsorbed.

Beyond this level of plasma [glucose], it appears in the urine = Renal plasma threshold for glucose.

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

Question 5

After 10 mmol/l of glucose - literally every other portion of glucose in the blood will be excreted in the urine

Answer 5

Question 6

Does the kidney regulate blood glucose concentration?

Answer 6

No (insulin and the counter-regulatory hormones responsible for its regulation).

Question 7

What does the presence of glucose in the urine suggest?

Answer 7

The appearance of glucose in the urine of diabetic patients = glycosuria, is due to failure of insulin, NOT, the kidney. N.B. Any patient with glucose in their urine should be followed up.

Question 8

Describe the Tm (transport capacity) of amino acids

Answer 8

For amino acids, Tm is also set so high that urinary excretion does not occur, regulated by insulin and counter-regulatory hormones.

Question 9

What substances does the kidney regulate by means of the transport capacity mechanism?

Answer 9

Some examples include:

sulphate and phosphate ions.

Question 10

How does the kidney regulate levels of sulphate and phosphate levels?

Answer 10

This is because Tm is set at a level whereby the normal [plasma] causes saturation.

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

(Also subject to PTH regulation for phosphate, PTH ¯ reabsorption).

Question 11

Where do most of the sodium ions get reabsorbed? How is it reabsorbed?

Answer 11

65-75% occurs in the proximal tubule

Not reabsorbed by a Tm mechanism, but by active transport, which establishes a gradient for Na+ across the tubule wall.

99.5% of sodium is reabsorbed in total

Question 12

Which side of the tubule cell contains the sodium potassium pump?

Answer 12

The side which is exposed to the interstitial fluid - not the side which is exposed to the tubule lumen

Active Na+pumps are located on the basolateral surfaces, where there is a high density of mitochondria.

This decreases [Na+] in the epithelial cells, increasing the gradient for Na+ ions to move into the cells passively across the luminal membrane.

Question 13

What aspect of the proximal tubule cells makes them effective at allowing sodium diffusion?

Answer 13

Na+ is not permeable at cell membranes!

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+.

Question 14

How does the reabsorption of sodium aid the reabsorption of other components of the filtrate?

Answer 14

Negative ions such as Cl- diffuse passively across the proximal tubular membrane down the electrical gradient established and maintained by the active transport of Na+.

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

H2O removed by osmosis from the tubule fluid concentrates all the substances left in the tubule creating outgoing concentration gradients.

Question 15

What determines the rate of reabsorption of non-actively reabsorbed solutes?

Answer 15

a) amount of H2O removed, which will determine the extent of the concentration gradient.
b) the permeability of the membrane to any particular solute.

Question 16

Describe the permeability of the membrane to urea

Answer 16

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

Question 17

What substances os the tubular membrane impermeable to?

Answer 17

For some substances eg inulin and mannitol, the tubular membrane is impermeable.

Question 18

How is the transport of sodium related to the transport of glucose?

Answer 18

Substances such as glucose, amino acids etc, share the same carrier molecule as Na+ (symport).

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

Question 19

What might disrupt renal function if the sodium transport requires ATP?

Answer 19

Decreased blood flow

Question 20

What is the function of the SGLT protein?

Answer 20

SGLT = Sodium-dependent glucose transporter

Allows sodium to travel down the electrochemical gradient and pulls glucose into the cell against the concentration gradient

Question 21

What else is sodium reabsorption linked to?

Answer 21

Na+ reabsorption also linked to HCO3- ion reabsorption (A/B).

Question 22

What is another route into the tubule lumen?

Answer 22

Tubular secretion:

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

Question 23

What substances is tubular secretion important for?

Answer 23

Important for substances that are protein-bound, since filtration at glomerulus is very restricted. Also for potentially harmful substances, means can be eliminated more rapidly

Question 24

What is the benefit of non-specific carrier mechanisms allowing secretion of organic acids?

Answer 24

Means the mechanisms that secrete lactic acid and uric acid can also be used for substances such as penicillin, aspirin, and PAH (para-ammino-hippuric acid)

Question 25

What transport mechanism can be used to secrete morphine and atropine?

Answer 25

organic base mechanism for choline, creatinine

Question 26

Where are these substances excreted?

Answer 26

At the proximal tubule

Question 27

What is the major cation in cells?

Answer 27

Potassium - normal ECF [K+} is around 4mmoles/l

Question 28

What is the consequence of hyperkalaemia (5.5 mmoles/l)

Answer 28

Decreased resting membrane potential of excitable cells and eventually ventricular fibrillation and death

Question 29

What is the result of hypokalaemia?

Potassium less than 3.5 mmoles/l

Answer 29

Increases resting membrane potential (hyperpolarizes muscle, cardiac cells) - cardiac srrhythmias and eventually death

Question 30

Where is potassium normally reabsorbed?

Answer 30

Potassium filtered at the glomerulus and is reabsorbed primarily at the proximal tubule

Question 31

What brings about changes in potassium excretion?

Answer 31

Chenges in secretion in the distal parts of the tubule.

Any ­increase in renal tubule cell [K+] due to increased ingestion will stimulate K+ secretion, while any decrease in intracellular [K+] results in reduced secretion.

Question 32

What hormone is responsible for regulating potassium balance?

Answer 32

Adrenal cortical hormone aldosterone

Question 33

When does aldosterone get produced?

Answer 33

An ­in [K+] in ECF bathing the aldosterone secreting cells stimulates aldosterone release which circulates to the kidneys to stimulate ­in renal tubule cell K+ secretion.

Question 34

What is the effect of aldosterone on sodium?

Answer 34

Aldosterone also stimulates Na+ reabsorption at the distal tubule but by a different reflex pathway.

Question 35

H+secretion: H+ions are actively secreted from the tubule cells (not the peritubular capillaries) into the lumen A/B Balance.