M - Urinary System (Liam Lennox) > Reabsorption and Secretion > Flashcards

Reabsorption and Secretion Flashcards

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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?

25
What percentage of sodium reabsorption occurs in proximal tubule?
65-75%
26
How much sodium is filtered each day?
* 180L/day x 142mmol/L = 25560 mmoles/day
27
By what mechanism is sodium reabsorbed?
* Not reabsorbed by a TM mechanism but by active transport which establishes a gradient for sodium across the tubule wall
28
Explain the process of sodium reabsorption?
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
Does brush border of proximal tubule cells have a greater or lesser permeability to Na than other membranes?
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
Why is reabsorption of Na so important?
It is key to reabsorption of other components of the filtrate
31
How does reabsorption of Na impact the reabsorption of other components of filtrate?
* 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
What does the rate of reabsorption of non-actively absorbed solutes (ones that are absorbed due to action of Na reabsorption) dependent on?
* Amount of water removed, which will determine extent of the concentration gradient * The permeability of the membrane to any particular solute
33
How much urea is reabsorbed?
* Tubule membrane is only moderately permeable to urea, so only about 50% is reabsorbed
34
What is an example of a substance that the tubular membrane is impermeable to?
* 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
What establishes the concentration gradients that allows various components of filtrate to be reabsorbed?
Active transport of Na
36
As well as creating concentration gradients for other substances, what else is Na important for?
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
What are some examples of molecules that share the same carrier molecule as Na?
Glucose Amino acids
38
How does concentration of Na in tubule impact glucose transport?
* High [sodium] in tubule facilitates and low [Na] inhibits glucose transport
39
What transporters does glucose use to be reabsorbed?
1) SGLT from tubule lumen to proximal tubule cell 2) GLUT from proximal tubule cell to interstitial fluid
40
What transporters does Na reabsorption use?
1) SGLT from tubule lumen to proximal tubule cell 2) Na-K-ATPase from proximal tubule cell to interstitial fluid
41
What does tubular secretion do?
Transports substances from peritubular capillaries into tubule lumen and provide a second route into tubule
42
What is tubular secretion for?
Structures which are protein bound since filtration at glomerulus is very restriced, also for potentially harmful substances meaning they can be eliminated rapidly
43
Are carrier mechanisms used for tubular secretion specific or non-specific?
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
What carrier mechanism can substances such as penicillin, aspirin and PAH use for tubular secretion?
Organic acid mechanism which secretes lactic and uric acid
45
What carrier mechanism can substances such as morphine and atropine use for tubular secretion?
Organ base mechanism for choline, creatinine etc
46
Where does tubular secretion occur?
Proximal tubule
47
Is K more concentrated in ICF or ECF?
ICF
48
What is normal ECF [K+]?
About 4mmol/L
49
When does hyperkalaemia occur?
When ECF [K] increases to 5.5mmol/L
50
What are consequences of hyperkalaemia?
* Decreases resting membrane potential of excitable cells and eventually ventricular fibrillation and death
51
When does hypokalaemia occur?
When ECF [K] falls below 3.5mmol/L
52
What are consequences of hypokalaemia?
* Increases resting membrane potential ie hyperpolarises muscle, cardiac cells leading to arrhythmias and eventually death
53
Explain the renal handling of K?
* 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
What are changes in K excretion due to?
Changes in its secretion in distal parts of the tubule
55
How does intracellular K levels impact K secretion?
* 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
What is K secretion regulated by?
ICF K levels Adrenal cortical hormone aldosterone
57
Explain how aldosterone regulates K secretion?
* 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
Where is H+ actively secreted from?
Are actively secreted from tubule cells (not the peritubular capillaries) into the lumen for acid/base balance
M - Urinary System (Liam Lennox)
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