Reabsorption & Secretion

Question 1

describe starling forces in glomerular and peritubular capillaries?

Answer 1

Question 2

what is a mechanism of reabsorption?

Answer 2

Many substances are reabsorbed by carrier mediated transport systems eg glucose, amino acids, organic acids, sulphate and phosphate ions.

Question 3

what do carries have in relation to capacity?

Answer 3

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

Question 4

what happens if Tm is exceeded?

Answer 4

the excess substrate enters the urine.

Question 5

what do carrier proteins enable?

Answer 5

larger molecules such as glucose to cross the membrane.

Capacity is limited by number of carriers.

Question 6

wat is renal threshold?

Answer 6

plasma threshold at which saturation occurs.

Question 7

what is the titration curve for glucose?

Answer 7

Glucose is freely filtered, so whatever its [plasma] that will be filtered.

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

Question 8

what is the Rena plasma threshold for glucose?

Answer 8

In man for plasma glucose up to 10 mmoles/l, all will be reabsorbed.

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

Question 9

do the kidneys regulate glucose?

Answer 9

Kidney does NOT regulate [glucose], (insulin and the counter-regulatory hormones responsible for its regulation).

Question 10

what is the normal plasma glucose?

Answer 10

Normal [glucose] of 5 mmoles/l, so Tm is set way above any possible level of (non-diabetic) [glucose]. Ensures that all this valuable nutrient is normally reabsorbed.

Question 11

in what type of patients will glucose appear in urine?

Answer 11

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 12

how is Tm set for amino acids?

Answer 12

set so high that urinary excretion does not occur, regulated by insulin and counter-regulatory hormones.

Question 13

what substances aren’t regulated by the means of Tm mechanism?

Answer 13

sulphate and phosphate ions

Question 14

what ions are the most abundant in ECF?

Answer 14

Na+ ions very large amount filtered daily

180 l/day x 142 mmoles/l = 25560 mmoles/day, 99.5% is reabsorbed.

Question 15

where does the majority of Na+ ion reabsorption occur?

Answer 15

65-75% occurs in the proximal tubule

Question 16

what is the mechanism of reabsorption for Na+ ions?

Answer 16

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

Question 17

how does the Na+ pump drive the whole process?

Answer 17

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 18

is Na+ permeable to cell membranes?

Answer 18

no

Question 19

where is there a higher permeability to Na+ ions?

Answer 19

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 20

what is the reabsorption of Na+ ions key to?

Answer 20

the reabsorption of the other components of the filtrate.

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+.

Question 21

what does the active transport of Na+ out of the tubule create?

Answer 21

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 22

what does the rate of reabsorption of non-actively reabsorbed solutes depends on?

Answer 22

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 23

is tubule membrane permeable to urea?

Answer 23

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

Question 24

what substances is the tubular membrane impermeable to?

Answer 24

inulin and mannitol, the tubular membrane is impermeable.

Question 25

It is the active transport of Na+ that establishes the gradients down which other ions, H2O and solutes pass passively –> what effect would decreasing active transport have on renal function?

Answer 25

Anything which ↓ active transport eg ↓BF → disruption of renal function.

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.

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

Question 26

what is tubular secretion?

Answer 26

Third renal process. Secretory mechanisms transport substances from the peritubular capillaries into the tubule lumen and ∴provide a second route into the tubule.

Question 27

what is tubular secretion particularly important for?

Answer 27

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 28

give examples of substances secreted at the proximal tubule?

Answer 28

Carrier mechanisms are not very specific so that eg organic acid mechanism, which secretes lactic and uric acid can also be used for substances such as penicillin, aspirin and PAH (para-amino-hippuric acid).

Similarly, organic base mechanism for choline, creatinine etc, can be used for morphine and atropine.

Question 29

what is a normal level of K+?

Answer 29

K+ is the major cation in the cells of the body and the maintenance of K+ balance is essential for life.

Normal ECF[K+] ≈ 4mmoles/l.

Question 30

when is the level of K+ indicative of hyperkalaemia?

Answer 30

If it ↑ to 5.5mmoles/l = hyperkalaemia → ↓resting membrane potential of excitable cells and eventually ventricular fibrillation and death. Remember the Nernst equation!

Question 31

when is the level of K+ indicative of hypokalaemia?

Answer 31

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

Question 32

how is K+ handles renally?

Answer 32

Changes in K+ excretion are due to changes in its secretion in the distal parts of the tubule. Any ↑in renal tubule cell [K+] due to increased ingestion will → K+ secretion, while any ↓ in intracellular [K+] → reduced secretion.

Question 33

what effect does aldosterone have on K+ secretion?

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

Question 34

H+ secretion?

Answer 34

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

Question 35

describe K+ regulation by aldosterone?

Answer 35