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*Urinary System > Reabsorption + Secretion > Flashcards

Reabsorption + Secretion Flashcards

1
Q

What substances are reabsorbed by carrier mediated transport systems?

A

e.g. glucose, amino acids, organic acids, sulfate and phosphate ions

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2
Q

What is Tm? How does it relate to carriers?

A

Maximum transport capacity

Carrers have a Tm which is due to saturation of carriers; if it is exceeded, then excess substrate enters the urine

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3
Q

Capacity is limited by number of carriers; what is ‘renal threshold’?

A

Plasma threshold (mg/mL) at which saturation occurs

Tm (mg/min), is transport rate at saturation

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4
Q

How much of glucose is filtered?

A

All - it is freely filtered

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5
Q

Up to what value will glucose be reabsorbed?

A

10mmoles/l

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6
Q

What happens if concentration of glucose exceeds 10mmoles/l? So what is the distribution of 15mmoles/l plasma {glucose]?

A

Beyond this level of plasma {glucose}, it appears in the urine = renal plasma threshold for glucose

So 15mmoles/l - 15 filtered, 10 reabsorbed and 5 excreted

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7
Q

Does the kidney regulate [glucose]?

A

NO; insulin and the counter-regulatory hormones are responsible for its regulation

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8
Q

What is the normal [glucose]?

A

5mmoles/l

So Tm is set way above any possible level of (non-diabetic) [glucose], ensuring all this valuable nutrient is normally reabsorbed

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9
Q

What does the appearance of glucose in the urine of diabetic patients indicate?

A

Glycosuria - due to failure of insulin NOT the kidney

Any patient w glucose in urine should be followed up

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10
Q

For amino acids, is Tm set high?

A

YES, set so high that urinary excretion does not occur, as with glucose

(AAs regulated by insulin and counter-regulatory hormones same as glucose)

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11
Q

Does the kidney regulate sulfate and phosphate ions?

A

YES, by means of the Tm mechanism

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12
Q

How does the kidney regulate sulfate and phosphate ions by the Tm mechanism?

A

Tm is set at a level whereby the normal [plasma] causes saturation; so any increase above normal level will be excreted, therefore achieving its plasma regulation

(also subject to PTH regulation for phosphate, PTH decreases reabsorption)

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13
Q

Which ions are the most abundant in ECF?

A

Na+

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14
Q

What does this abundance of Na+ ions in ECF mean in terms fo filtration?

A

A large amount is filtered every day

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15
Q

How much Na+ is reabsorbed

A

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

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16
Q

How much of Na+ reabsorption occurs in the proximal tubule?

A

65-75%

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17
Q

How are Na+ ions reabsorbed? (NOT by Tm mechanism)

A

Active transport (establishes a gradient for Na+ across tubule wall)

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18
Q

What drives the whole process of Na+ transport?

A

Na+ pumps

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19
Q

Where are active Na+ pumps located? Why?

A

On the basolateral surface - side which is exposed to the interstitial fluid, not side which is exposed to the tubule lumen, because there is a high density of mitochondria

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20
Q

Active Na+ pumps decrease [Na+] in epithelial cells, meaning waht for the gradient of Na+ ions?

A

Increases the gradient for Na+ ions to move into cells passively across the luminal membrane

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21
Q

Na+ is NOT permeable at cell membranes tho, so what about the brush border of the proximal tubule cells has a higher permeability to Na+ ions than most other membranes of the body?

A

Enormous surface area offered by microvilli and large number of Na+ ion channels, which faciliate this passive diffusion of Na+

22
Q

Reabsorption of Na+ ions is key to what?

A

Reabsorption of other components of the filtrate

23
Q

How do Cl- ions diffuse across the proximal tubular membrane?

A

Cl- diffuse across passively down the electrical gradient established and maintained by the active transport of Na+

24
Q

What force does the active transport of Na+ out of the tubule followed by Cl- create? What happens as a result?

A

Osmotic force

H2O is drawn out of tubules by osmosis meaning the substances left in the tubule are concentrated creating outgoing concentration gradients

25
Q
A
26
Q

What 2 factors does rate of reabsorption of these non-actively reabsorbed solutes depend on?

A
  1. Amount of H2O removed, which will determine the extent of the concentration gradient
  2. The permeability of the membrane to any particular solute
27
Q

How much urea is reabsorbed? Why?

A

50%

Because the tubule membrane is only moderately permeable to urea and so remainder stays in the tubule

28
Q

For some substances the tubular membrane is impermeable, name 2; so what happens to them?

A

Inulin

Mannitol

Despite conc gradient favouring their reabsorption, they cant gain access through tubule membrane so all that is filtered stays in the tubule and passes out in urine

29
Q

It is the active transport of Na+ that establishes the gradients down which other ions, H2O and solutes pass passively, so anything that decreases active transport e.g. decreased BF, leads to what?

A

Disruption of renal function

30
Q

How is transport of sodium related to transport of substances such as glucose, amino acids etc?

A

They 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)

31
Q

What is the function of the SGLT (sodium-dependent glucose transporter)?

A

Allows Na+ to move into the proximal tubule cell down its electrochemical gradient and pulls glucose into the cell against its concentration gradient

32
Q

How are glucose and Na+ diffused out of the proximal tubule cell?

A

Glucose diffuses out the basolateral side of the cell using the GLUT protein

Na+ is pumped out by Na+-K+-ATPase

33
Q

What is tubular secretion?

A

Secretory mechanisms transport substances from the peritubular capillaries into the tubule lumen and provide a second route into the tubule

34
Q

What substances is tubular secretion important for? Why?

A

Substances that are protein-bound, since filtration at glomerulus is very restricted

Also for potentially harmful substances, means they can be eliminated more rapidly

35
Q

Tm -limited carrier-mediated secretory mechanisms are known for a large number of endogenous as well as exogenous substances such as…

A

drugs

36
Q

What is the benefit to carrier-mediated secretory mechanisms not beinf very specific?

A

So that e.g. 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)

37
Q

Similary, organic base mechanism for choline, creatinine etc can be used for what?

A

Morphine and atropine

38
Q

Where are all of these substances secreted?

A

Proximal tubule

39
Q

What is the major cation in the cells of the body? (making its balance essential for life)

A

K+

40
Q

What is normal ECF[K+]

A

~4mmoles/l

41
Q

What happens in hyperkalaemia ([K+] up to 5.5mmoles/l)?

A

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

(Nernst equation)

42
Q

What happens in hypokalaemia ([K+] <3.5mmoles/l)?

A

Increased resting membrane potential i.e. hyperpolarises muscle, cardiac cells -> cardiac arrythmias and eventally death

43
Q

Where is K+ filtered at the glomerulus primarily reabsorbed?

A

Proximal tubule

44
Q

What are changes in K+ excretion due to?

A

Changes in its secretion in the distal parts of the tubule

45
Q

Any increase in renal tubule cell [K+] due to increased ingestion will result in…

A

Increased K+ secretion

46
Q

Any decrease in intracellular [K+] will result in…

A

reduced secretion

47
Q

In addition, K+ secretion is regulatd by what hormone?

A

Aldosterone

48
Q

How is aldosterone production stimulated by potassium?

A

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

49
Q

In addition to regulating potassium, what else does aldosterone act on?

A

Also stimulates Na+ reabsorption at the distal tubule (but by a diff reflex pathway)

50
Q

How are H+ ions secreted?

A

Actively secreted from the tubule cells (not the peritubular capillaries) into the lumen - A/B balance

51
Q

Summarise Biomed Sessions vid on function of nephron

A

*Urinary System (26 decks)

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