Reabsorption and Secretion Flashcards

1
Q

How does reabsorption occur in the peritubular capillaries?

A

In peritubular capillaries:

PPC very low because hydrostatic P overcoming frictional resistance in efferent arteriols

osmotic pressure high compared to normal, loss of 20% plasma concentrates plasma protein

osmotic pressure >> PPC only reabsorption

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2
Q

what is the only thing to happen in the glomerular capillaries?

A

PGC >> Pp

Only filtration occurs at glomerular capillaries

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3
Q

Since aorund 20% of the plasma has filtered into Bowman’s capsule in the glomerulus, (filtration fraction), the blood remaining in the efferent arteriole and then the peritubular capillaries has what?

A

a higher concentration of plasma proteins and therefore increase osmotice pressure

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4
Q

As a consequence, the net result of the low PPC and the high oscmotic pressure causes what?

A

the net result of the low PPC and the high osmotic pressure is that the balance of Starling’s forces in the peritubular capillaries is entirely in favour of reabsorption

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5
Q

what substances are reabsorbed and where about are they mainly reabsormbed?

A

99% H2O, 100% glucose, 99.5% Na+, 50% urea filtered at the glomerulus are reabsorbed within the tubule, mainly at the proximal convoluted tubule

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6
Q

many substances are reabsorbed by what?

A

Many substances are reabsorbed by carrier mediated transport systems

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

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7
Q

do carries have a maximum transport capacity?

A

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

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8
Q

what happens if Tm is exceeded?

A

If Tm is exceeded, then the excess substrate enters the urine

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9
Q

what do carrier proteins allow?

A

Carrier protein enables larger molecules such as glucose to cross the membrane

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10
Q

Capacity is limited by number of _______

A

carriers

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11
Q

what is the renal threshold?

A

Renal threshold = plasma threshold at which saturation occurs

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12
Q

One of the most important substances to consider is glucose.

Describe the titration curve for 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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13
Q

does kidney regulate glucose?

A

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

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14
Q

What is the normal level of glucose ocncentration and why is it this?

A

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

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15
Q

is the appearance of glucose in someones urine a sign of failing kidneys?

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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16
Q

What is the Tm of amino acids and why is this again?

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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17
Q

does the kidneys regulate any substances?

A

Kidney does regulate some substances by means of the Tm mechanism, eg sulphate and phosphate ions

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

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

(Also subject to PTH regulation for phosphate, PTH decrease reabsorption)

18
Q

Na+ ions are the most abundant in the ECF, a very large amount is filtered every day

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

where is most of it rebabsorbed and how is it reabsorbed?

A

65-75% of Na+ion reabsorption 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

19
Q

what is the process of Na+ activley being pumped out across the tubule wall?

A

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

It is the Na+ pumps which drive the whole process

20
Q

WAIT A MINUTE!!! Na+ is not permeable at cell membranes!

how does it cross the brush border of proximal tubule cells?

A

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+

This reabsorption of Na+ ions is key to the reabsorption of the other components of the filtrate

21
Q

How does negative ions cross the proximal tubular membrane?

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+

22
Q

The active transport of Na+ out of the tubule followed by Cl- creates what?

A

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

23
Q

By H2O being removed by osmosis, what happens to the substances left in the tubule?

A

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

24
Q

the rate of re-absorption of these non-actively reabsorbed solutes depends on what things?

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

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

25
Q

How permeable is the tubular membrane in response to urea?

A

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

26
Q

can all substances be reabsorbed if a concentration gradient is established?

A

So despite a concentration gradient being established favouring their reabsorption, they cannot gain access through the tubule membrane so that all that is filtered stays in the tubule and passes out in the urine

27
Q

what is it that establishes the gradient in which other things pass down?

A

It is the active transport of Na+ that establishes the gradients down which other ions, H2O and solutes pass passively

28
Q

what happens if active transport is decrease of sodium?

A

Anything which decreases active transport eg decreased BF = disruption of renal function

29
Q

why is sodium important for carrier mediated transport systems?

A

Importance of active transport of Na+ also for carrier mediated transport systems

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

30
Q

High [Na+] in the tubule ________ and low [Na+] ______ glucose transport

Na+ reabsorption also linked to______ ion reabsorption (A/B)

A

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

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

31
Q

the third renal process is tubular secretion, what is it?

A

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

32
Q

what 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

33
Q

are carrier mechanisms specific?

A

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

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

All of these substances are secreted at the proximal tubule

34
Q

sudy this diagram - shows how secretion fits into everything

A
35
Q

is K+ important?

A

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

Normal ECF[K+] around 4mmoles/l

36
Q

What happens if K+ is too high or too low?

A

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

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

37
Q

We are normally in K+ balance, even though a normal diet contains more than enough K+ for the body’s needs. Beware the killer banana!

How is K+ managed?

A

Renal handling of K+ is complex. K+ filtered at the glomerulus is reabsorbed, 1°ily at the proximal tubule

38
Q

What are changes in K+ excreiton due to?

A

due to changes in its secretion in the distal parts of the tubule. Any increase ­in renal tubule cell [K+] due to increased ingestion will = K+ secretion, while any decrease in intracellular [K+] = reduced secretion

39
Q

K+ secretion is also regulated by what?

A

K+ secretion is regulated by the adrenal cortical hormone aldosterone

40
Q

how does aldosterone affect K+ secretion?

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

41
Q

Aldosterone also stimulates ___ reabsorption at the distal tubule but by a different reflex pathway

A

Na+

42
Q

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

A