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ZA - Renal System > Reabsorption and Secretion > Flashcards

Reabsorption and Secretion Flashcards

1
Q

What occurs in glomerular capillaries?

A

Filtration

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2
Q

Why is the peritubular capillary hydrostatic pressure (Ppc) low?

A

Because the hydrostatic pressure is lost trying to overcome the frictional forces of the long and thin efferent arterioles

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3
Q

Why is oncotic pressure higher in the peritubular capillaries?

A

Due to loss of 20% of plasma which concentrates the plasma proteins

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4
Q

If oncotic pressure is higher than the hydrostatic pressure in the peritubular capillaries, what process does this favours?

A

Reabsorption

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5
Q

What is reabsorbed within the tubule after filtration?

A

99% of H2O
100% of glucose
99.5% of Na+
50% of urea

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6
Q

Where is most reabsorption taking place?

A

In the proximal convoluted tubule

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7
Q

What is a mechanism of reabsorption?

A

Carrier mediated transport systems

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

If Tm is exceeded then excess substrate is released into the urine

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8
Q

What sort of things are reabsorbed by carrier proteins?

A 
Glucose
Amino Acids
Organic acids
Sulphate
Phosphate
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9
Q

Explain briefly how carrier mediated transport systems work.

A

Molecule to be transported enters the carrier system and binds to a binding site.

Once bound it closes the carrier on the entry side and opens it on the exit side

Molecule is released and reaborped absorbed and carrier returns back to original shape

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10
Q

What is the renal threshold?

A

The plasma concentration at which saturation of carriers occurs

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11
Q

In man for plasma glucose up to BLANK - all will be reabsorbed, but beyond this level of plasma [glucose], it appears in the BLANK

A

10 mmoles/l

Urine

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12
Q

So this makes 10 mmoles/l the…

A

Renal plasma threshold for glucose.

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13
Q

So if plasma [glucose] is 15 - what will be reabsorbed and what will be excreted?

A

10 reabosped and 5 excreted

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14
Q

Does the kidney regulate blood glucose?

A

No - done by endocrine system

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15
Q

What is normal conc. of glucose ?

A

5 mmoles/l

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16
Q

Why is the renal plasma threshold for glucose set way higher than the actual normal plasma glucose levels?

A

Makes sure that all of the glucose is reabsorbed and not wasted as its too valuable

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17
Q

The appearance of glucose in the urine of a diabetic patient is due to the failure of…?

A

Insulin not the kidneys

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18
Q

What is the Tm like for amino acids? What regulates AA levels

A

Also set so high that urinary excretion does not occur,

Regulated by insulin and counter-regulatory hormones.

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19
Q

What DOES the kidney actually regulate plasma levels of?

A

Phosphate and sulphate ions

Tm is set at a level that causes normal plasma concentrations to cause saturation and excretion

This maintains normal levels at a tight control

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20
Q

What ions are most abundant in the ECF?

A

Na+

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21
Q

What % of Na ions is reabsorbed?

A

99.5%

22
Q

What part of the kidney reabsobes the most Na ions and where does this happen?

A

65-75% in the proximal tubule

23
Q

How is Na reabsorbed?

A

Not by Tm mechanism but by active transport pumps establishing a gradient across the tubule wall

24
Q

Explain how Na is pumped back into the body.

A

Na flows from the tubule lumen into the proximal tubule cells down its electrochemical gradient

Once in the proximal tubule cell - a Na/K-ATPase on the basolateral side of the cell pumps it out into the interstitial fluid

25
Q

How does Na get into the proximal tubule cell if cell membranes are NOT permeable to Na?

A

The brush border of the proximal tubule cells has a higher permeability to Na+ ions

This is partly because of the enormous surface area offered by the microvilli and the large number of Na+ ion channels,

26
Q

Why is Na reabsorption so important?

A

It is key to the reabsorption of other components of the filtrate

27
Q

How do negative ions like Cl- get reabsorbed?

A

Diffuse passively across the proximal tubular membrane down the electrical gradient created by the active transport of Na+.

28
Q

What does the reabsorption of these ions do water?

A

Creates an osmotic force which draws H2O out of the tubules

29
Q

When water is drawn out of the tubules, how does this help reabsorption of further ions and solutes?

A

Less water = more concentrated ions which creates outgoing conc. gradients

30
Q

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

A
  1. Amount of H2O removed determining the conc. gradents

2. Permeability of the membrane to a particular solute

31
Q

Why is only 50% or urea reabsorbed?

A

Tubule membrane is only moderately permeable to it

32
Q

Name some substances that cannot pass through the tubule at all.

A

Insulin

Mannitol

33
Q

Recap - what one ion establishes the gradients for water and all other solutes to follow?

A

Na

34
Q

What would a decrease in blood flow do to renal function?

A

Would decrease active transport which would disrupt all renal function not just Na ions

35
Q

Why is Na active transport important for maintaining carrier proteins?

A

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

So high Na in tubules helps transport of glucose and low inhibits

36
Q

What does secretion do?

A

Transports substances from the peritubular capillaries after filtration - providing a 2nd route into the tubule

37
Q

Why do we need secretion if we have filtration at the glomerulus?

A

Filtration is very restrictive so secretion is important for protein bound substances

Also for potentially harmful substances helping remove them faster

38
Q

What is the secretion mechanism?

A

Non-specific Tm limited carrier mediated secretion

39
Q

What sort of things are secreted?

A

Large number of endogenous substances but also exogenous like drugs

40
Q

What does it mean that these carrier mechanisms are not specific?

A

Organic acid mechanism, which secretes lactic and uric acid can also be used for substances such as penicillin, aspirin etc

Similarly organic base mechanisms can be used for choline, creatinine or morphine and atropine

41
Q

What is the major cation in the cells of the body?

A

K+

42
Q

Normal ECF conc of K+?

A

4 mmoles/l

43
Q

When does hyperkalaemia occur?

A

Over 5.5

44
Q

When does hypokalaemia occur?

A

Under 3.5

45
Q

What does hyperK cause?

A

Reduced resting membrane potential of excitable cells

= ventricular fibrillation and death

46
Q

What does hypoK cause?

A

Increased RMP - hyperpolarizing muscle, and cardiac cells leading to arrhythmias and eventually death

47
Q

What happens to K+ as it passes through the renal system?

A

Filtered at glomerulus
Reabsorbed at proximal tubule
Re-secreted at the distal tubule as needed

Changes in total excretion depends on how much is secreted back into the distal tubule = an increase in renal tubule cell K+ conc. due to increase ingestion leads to increased secretion and excretion

48
Q

What else regulates K+ secretion? How?

A

Aldosterone

Increase in K+ conc. in ECF stimulates aldosterone release which circulates to kidneys causing stimulation of renal tubule to secrete K+

49
Q

What effect does aldosterone have on Na?

A

stimulates Na+ reabsorption at the distal tubule

50
Q

How are H+ ions secreted?

A

Actively from the tubule cells, not the peritubular capillaries, into the lumen

ZA - Renal System (27 decks)

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