Reabsorption & Secretion Flashcards

1
Q

Which substances are reabsorbed by carrier mediated transport systems?

A

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

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2
Q

What does Tm stand for?

A

Transport capacity - if Tm is exceeded then the excess substrate enters the urine

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3
Q

What is the renal threshold?

A

The plasma concentration at which saturation occurs

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4
Q

Describe the filtration capability of glucose

A
  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.)

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5
Q

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

A
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6
Q

Does the kidney regulate blood glucose concentration?

A

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

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7
Q

What does the presence of glucose in the urine suggest?

A

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.

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8
Q

Describe the Tm (transport capacity) of amino acids

A

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

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9
Q

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

A

Some examples include:

sulphate and phosphate ions.

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10
Q

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

A

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).

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11
Q

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

A

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

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12
Q

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

A

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.

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13
Q

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

A

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+.

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14
Q

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

A

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.

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15
Q

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

A

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

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16
Q

Describe the permeability of the membrane to urea

A

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

17
Q

What substances os the tubular membrane impermeable to?

A

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

18
Q

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

A

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.

19
Q

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

A

Decreased blood flow

20
Q

What is the function of the SGLT protein?

A

SGLT = Sodium-dependent glucose transporter

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

21
Q

What else is sodium reabsorption linked to?

A

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

22
Q

What is another route into the tubule lumen?

A

Tubular secretion:

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

23
Q

What substances is tubular secretion important for?

A

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

24
Q

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

A

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)

25
Q

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

A

organic base mechanism for choline, creatinine

26
Q

Where are these substances excreted?

A

At the proximal tubule

27
Q

What is the major cation in cells?

A

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

28
Q

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

A

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

29
Q

What is the result of hypokalaemia?

Potassium less than 3.5 mmoles/l

A

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

30
Q

Where is potassium normally reabsorbed?

A

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

31
Q

What brings about changes in potassium excretion?

A

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.

32
Q

What hormone is responsible for regulating potassium balance?

A

Adrenal cortical hormone aldosterone

33
Q

When does aldosterone get produced?

A

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.

34
Q

What is the effect of aldosterone on sodium?

A

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

35
Q

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

A