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12. Year 2: Urinary System COPY > Reabsorption and Secretion > Flashcards

Reabsorption and Secretion Flashcards

1
Q

Does filtration or reabsorption occur at glomerular capillaries?

A

Only filtration

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2
Q

Does filtration or reabsorption occur at the peritubular capillaries?

A

Reabsorption

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3
Q

Why is oncotic pressure greater in pertibular capillaries than in glomerular cappilaries?

A
  • Since about 20% of plasma has filtered into Bowman’s capsule in the glomerulus (filtration fraction) the blood remaining in efferent arteriole and then peritubular capillaries has higher concentration of plasma proteins and \ ­Pp
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4
Q

Where does most reabsorption occur?

A

Proximal convoluted tubule

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5
Q

What are the 2 different mechanisms of reabsorption?

A
  • Reabsorbed by carrier mediated transport systems
  • Reabsorption of sodium ions
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6
Q

What are examples of substances reabsorbed by carrier mediated transport systems?

A
  • Glucose
  • Amino acids
  • Organic acids
  • Sulphate ions
  • Phosphate ions
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7
Q

What is the maximum capacity of carriers expressed as?

A

TM

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8
Q

What is TM due to?

A

Saturation of the carriers

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9
Q

What happens if TM is exceeded?

A

Excess substrates enters the urine

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10
Q

What do carrier proteins enable?

A

Large substances like glucose to cross the membrane

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11
Q

What is the capacity of reabsorption of large molecules like glucose limited by?

A

Number of carriers

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12
Q

What is renal threshold?

A

Plasma threshold at which saturation occurs

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13
Q

Plasma glucose levels up to what will be reabsorbed?

A

10mmole/L

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14
Q

What happens to glucose beyond the plasma level of 10mmoles/L?

A

Glucose appears in the urine, so if plasma [glucose] is 15mmoles/L, then 10 is reabsorbed and 5 is excreted

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15
Q

Why is TM set way above normal glucose levels?

A

Ensures that all valuable nutrients are normally reabsorbed

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16
Q

What is the medical term for the appearance of glucose in the urine?

A

Glycosuria

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17
Q

What causes glucosuria?

A

Failure of insulin, not failure of the kidney

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18
Q

What are examples of substances that are regulated by TM?

A

Sulphate

Phosphate

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19
Q

Is glucose regulated by TM?

A

No, insulin and counter-regulatory hormones do

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20
Q

How are things, such as sulphate and phosphate, regulated by TM?

A

Because TM is set at a level whereby the normal [plasma] causes saturation, anything above will be excreted therefore achieving plasma regulation:

  • Is also subject to PTH regulation for phosphate, PTH causes decreased reabsorption
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21
Q

As well as TM, what else is phosphate regulated by?

A
  • Is also subject to PTH regulation for phosphate, PTH causes decreased reabsorption
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22
Q

How does PTH impact phosphate?

A

PTH causes decreased reabsorption

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23
Q

Are sodium ions most abundant in ICF or ECF?

A

ECF

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24
Q

What percentage of sodium is reabsorbed?

A

99.5%

25
Q

What percentage of sodium reabsorption occurs in proximal tubule?

A

65-75%

26
Q

How much sodium is filtered each day?

A
  • 180L/day x 142mmol/L = 25560 mmoles/day
27
Q

By what mechanism is sodium reabsorbed?

A
  • Not reabsorbed by a TM mechanism but by active transport which establishes a gradient for sodium across the tubule wall
28
Q

Explain the process of sodium reabsorption?

A
  1. Active sodium pumps are located on the basolateral surfaces where the is a high density of mitochondria
  2. This decreases [Na] in epithelial cells, increasing the gradient for Na ions to move into the cells passively across the luminal membrane
29
Q

Does brush border of proximal tubule cells have a greater or lesser permeability to Na than other membranes?

A

Greater:

  • Due to enormous surface area offered by microvilli and the large number of sodium ion channels which facilitate this passive diffusion of Na
30
Q

Why is reabsorption of Na so important?

A

It is key to reabsorption of other components of the filtrate

31
Q

How does reabsorption of Na impact the reabsorption of other components of 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+
  • Active transport of Na out of tubule followed by Cl creates an osmotic force, drawing water out of the tubule
  • H2O removed by osmosis from tubule fluid concentrates all other substances left in tubule creating outgoing concentration gradients
32
Q

What does the rate of reabsorption of non-actively absorbed solutes (ones that are absorbed due to action of Na reabsorption) dependent on?

A
  • Amount of water removed, which will determine extent of the concentration gradient
  • The permeability of the membrane to any particular solute
33
Q

How much urea is reabsorbed?

A
  • Tubule membrane is only moderately permeable to urea, so only about 50% is reabsorbed
34
Q

What is an example of a substance that the tubular membrane is impermeable to?

A
  • Some substances such as insulin and mannitol the tubular membrane is impermeable
    • So despite concentration gradient favouring their reabsorption, they cannot gain access through membrane so that all that is filtered stays in the tubule and passes out in urine
35
Q

What establishes the concentration gradients that allows various components of filtrate to be reabsorbed?

A

Active transport of Na

36
Q

As well as creating concentration gradients for other substances, what else is Na important for?

A

Sodium is also important for carrier mediated transport systems:

  • Substances such as glucose, amino acids share the same carrier molecule as sodium (symport)
  • High [sodium] in tubule facilitates and low [Na] inhibits glucose transport
  • Na reabsorption is also linked to HCO3- ion reabsorption
37
Q

What are some examples of molecules that share the same carrier molecule as Na?

A

Glucose

Amino acids

38
Q

How does concentration of Na in tubule impact glucose transport?

A
  • High [sodium] in tubule facilitates and low [Na] inhibits glucose transport
39
Q

What transporters does glucose use to be reabsorbed?

A

1) SGLT from tubule lumen to proximal tubule cell
2) GLUT from proximal tubule cell to interstitial fluid

40
Q

What transporters does Na reabsorption use?

A

1) SGLT from tubule lumen to proximal tubule cell
2) Na-K-ATPase from proximal tubule cell to interstitial fluid

41
Q

What does tubular secretion do?

A

Transports substances from peritubular capillaries into tubule lumen and provide a second route into tubule

42
Q

What is tubular secretion for?

A

Structures which are protein bound since filtration at glomerulus is very restriced, also for potentially harmful substances meaning they can be eliminated rapidly

43
Q

Are carrier mechanisms used for tubular secretion specific or non-specific?

A

Carrier mechanisms are not very specific:

  • So eg organic acid mechanism which secretes lactic and uric acid can also be used for substances such as penicillin, aspirin and PAH
  • Similarly, organic base mechanism for choline, creatinine etc can be used for morphine and atropine
44
Q

What carrier mechanism can substances such as penicillin, aspirin and PAH use for tubular secretion?

A

Organic acid mechanism which secretes lactic and uric acid

45
Q

What carrier mechanism can substances such as morphine and atropine use for tubular secretion?

A

Organ base mechanism for choline, creatinine etc

46
Q

Where does tubular secretion occur?

A

Proximal tubule

47
Q

Is K more concentrated in ICF or ECF?

A

ICF

48
Q

What is normal ECF [K+]?

A

About 4mmol/L

49
Q

When does hyperkalaemia occur?

A

When ECF [K] increases to 5.5mmol/L

50
Q

What are consequences of hyperkalaemia?

A
  • Decreases resting membrane potential of excitable cells and eventually ventricular fibrillation and death
51
Q

When does hypokalaemia occur?

A

When ECF [K] falls below 3.5mmol/L

52
Q

What are consequences of hypokalaemia?

A
  • Increases resting membrane potential ie hyperpolarises muscle, cardiac cells leading to arrhythmias and eventually death
53
Q

Explain the renal handling of K?

A
  • K filtered at the glomerulus is reabsorbed, mainly at the proximal tubule
  • Changes in K excretion are due to changes in its secretion in distal parts of the tubule
  • Any increase in renal tubule cells [K+] due to increased ingestion will cause K secretion, while a decrease in intracellular [K] causes reduced secretion
54
Q

What are changes in K excretion due to?

A

Changes in its secretion in distal parts of the tubule

55
Q

How does intracellular K levels impact K secretion?

A
  • Any increase in renal tubule cells [K+] due to increased ingestion will cause K secretion, while a decrease in intracellular [K] causes reduced secretion
56
Q

What is K secretion regulated by?

A

ICF K levels

Adrenal cortical hormone aldosterone

57
Q

Explain how aldosterone regulates K secretion?

A
  • Increase in [K] in ECF bathing the aldosterone secreting cells stimulates aldosterone release which circulates to kidneys to stimulate increase in renal tubule K secretion
  • Aldosterone also stimulates Na reabsorption at the distal tubule but by a different reflex pathway
58
Q

Where is H+ actively secreted from?

A

Are actively secreted from tubule cells (not the peritubular capillaries) into the lumen for acid/base balance

12. Year 2: Urinary System COPY (30 decks)

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