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renal system > Physiology of Micturition > Flashcards

Physiology of Micturition Flashcards

1
Q

Why is it important to be able to assess renal function?

A

Because it plays a central role in homeostasis

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2
Q

When is the ability to measure GFR particularly useful?

A
  • In patients with renal disease

- When administering certain drugs

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3
Q

What is the total GFR equal to?

A

Sum of all filtration by functioning nephrons

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4
Q

What about the GFR would indicate progression of disease?

A

Reduction in GFR

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5
Q

In patients with renal disease, what does progression of the underlying disease process result in?

A

Nephron destruction and decrease in nephron function

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6
Q

Why may you need to adjust the dose of certain drugs in decreased renal function?

A

Many drugs eg digitalis and many antibiotics are removed from the body by excretion by filtration. When GFR falls, excretion falls so that [drug] in plasma may rise causing toxicity

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7
Q

What is used to measure renal function?

A

Plasma clearance tests

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8
Q

What do plasma clearance tests measure?

A

The ability of the kidney to clear the plasma of various substances

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9
Q

What does clearance relate to?

A

Clearance relates to a volume of plasma cleared NOT a quantity of substance removed from the plasma

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10
Q

What is the calculation for plasma clearance?

A

Plasma clearance is equal to urine concentration of x times the urine flow rate divided by the plasma concentration of x

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11
Q

What is the gold standard substance used to calculate plasma clearance?

A

Inulin (polyfructose)

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12
Q

How is inulin used to assess plasma clearance?

A
  • Loading IV dose of inulin
  • Allow time to equilibrate
  • Sample simultaneously plasma and urine (during a timed urine sample)
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13
Q

Why can inulin clearance be used to meaure GFR?

A

Inulin is freely filtered at the glomerulus and neither reabsorbed nor secreted. It is not metabolized by the kidney, nor does it interfere with normal renal function so inulin clearance is a measure of GFR.

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14
Q

What substances will have a lower clearance than inulin?

A

Substances filtered and reabsorbed will have a lower clearance than inulin, because [UX] will be less than if only filtered and [PX] higher. uese

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15
Q

What substances will have a higher clearance than inulin?

A

Substances filtered and secreted will have a higher clearance than inulin because [UX] will be higher and [PX] lower.

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16
Q

What is the normal GFR in man?

A

125mls/min

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17
Q

What does GFR correlated with?

A

Surface area

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18
Q

How does GFR differ between men and women?

A

Women have ~10% lower GFR than men even after corrected for surface area

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19
Q

How much does GFR decline each year?

A

~1ml/min/year after 30 years of age

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20
Q

Why is inulin no longer used in clinical practice?

A

It is too cumbersome

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21
Q

What is used in clinical practice instead of inulin now?

A

51Cr-EDTA is used instead, a suitable radioactive substance that is handled by the kidney in the same way as inulin.

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22
Q

What is routinely used to estimate GFR?

A

Creatinine clearance

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23
Q

What is creatinine?

A

Creatinine is endogenous (breakdown produce of muscle creatine

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24
Q

What do formulae using serum creatinine to estimate GFR take into account?

A

Confounding variables

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25
Q

What factors can affect serum creatinine?

A
  • Muscle mass: athletes vs malnutrition
  • Dietary intake: creatinine supplements vs vegetarians
  • Drugs: Some lead to spurious increases as does ketoacidosis
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26
Q

Why is GFR sometimes expressed as a percentage of normal?

A

Variability across range of adults, sexes for kidney function and size

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27
Q

What is the clearance of glucose?

A

0

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28
Q

Why is the clearance o of glucose 0?

A

It is normally all reabsorbed

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29
Q

Why is the clearance of urea less than that of inulin?

A

Some urea is reabsorbed

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30
Q

What is used to measure renal plasma flow?

A

The organic anion para-amino-hippuric acid (PAH)

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31
Q

How is PAH processed through the kidney?

A

PAH is freely filtered at the glomerulus and then the PAH remaining in the plasma is actively secreted into the tubule so that > 90% of plasma is cleared of its PAH content in one transit of the kidney.

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32
Q

What is renal plasma flow?

A

The plasma flowing through the kidneys in a given time

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33
Q

What is the normal renal flow rate?

A

~660mls/min

34
Q

Why does penicillin have a greater clearance than inulin?

A

It is filtered and secreted

35
Q

What is the renal handling of a substance which is freely filtered at the glomerulus and filtration is greater than excretion?

A

Net absorption of X

36
Q

What is the renal handling of a substance which is freely filtered at the glomerulus and excretion is greater than filtration?

A

Net secretion of X

37
Q

What is the renal handling of a substance which is freely filtered at the glomerulus and filtration and excretion are the same?

A

No net reabsorption or secretion

38
Q

What is the renal handling of a substance which is freely filtered at the glomerulus and clearance of X is less than inulin clearance?

A

Net reabsorption of X

39
Q

What is the renal handling of a substance which is freely filtered at the glomerulus and clearance of X is equal to inulin clearance?

A

X is neither reabsorbed nor secreted

40
Q

What is the renal handling of a substance which is freely filtered at the glomerulus and clearance of X is greater than inulin clearance?

A

Net secretion of X

41
Q

How does urine flow from the kidneys to the ureters?

A

Via peristaltic contraction of the smooth muscle of the ureters

42
Q

How do the ureters enter the bladder?

A

At an oblique angle to prevent reflux of urine

43
Q

What happens to the composition of urine once it leaves the kidneys?

A

It does not change

44
Q

What is the bladder?

A

Bladder is a bag of smooth muscle, arranged in spiral, longitudinal and circular bundles = detrusor muscle

45
Q

What is responsible or emptying the bladder during micturition?

A

Contraction of the detrusor muscle

46
Q

What is the internal urethral ‘sphincter’?

A
  • It is not a true sphincter

- It is where the smooth muscle at the start of the urethra acts as a sphincter when the smooth muscle is relaxed.

47
Q

What is the external urethral sphincter?

A

A true sphincter, made up of skeletal muscle under voluntary somatic control

48
Q

What is the bladder overlain with?

A

Peritoneum

49
Q

Where does the bladder lie?

A
  • Lies in the midline posterior to the pubic bones

- Lies anterior to reproductive system and rectum

50
Q

What is the bladder made of?

A

Smooth muscle (detrusor muscle)

51
Q

What lines the bladder?

A

Transitional epithelium

52
Q

What are the 3 points of the trigone of the bladder?

A
  • 2 Vesicoureteric openings

- Urethral opening

53
Q

What are the Vesicoureteric openings?

A

1-2cm oblique passage through muscular wall

54
Q

What will a urethral obstruction result in?

A

Bilateral renal problems

55
Q

What will ureter obstruction result in?

A

Unilateral renal problems

56
Q

What is the characteristic shape of the pressure-volume curve of the bladder?

A

There is a long flat segment as the initial increments of urine enter the bladder and then a sudden sharp rise as the micturition reflex is triggered.

57
Q

What does normal daily urine production vary between?

A

750ml-2500mls in temperate climates

58
Q

What are the 3 components of motor innervation to the bladder?

A
  • Rich parasympathetic supply (pelvic nerves)
  • Sparse sympathetic supply (hypogastric nerves)
  • Somatic motorneurones (pudendal nerves)
59
Q

What affect does the parasympathetic system have on the bladder?

A
  • Increase in parasympathetic activity leads to an increase in contraction of the detrusor muscle
  • Consequent increased of pressure within the bladder, S2-S4
60
Q

What does cutting the hypogastric nerve result in?

A

Increased frequency of micturition

61
Q

What role does the sparse sympathetic supply (hypogastric nerve) play in the control of micturition?

A

-Inhibit bladder contraction and closes the internal urethral “sphincter”

62
Q

What is the main function of the hypogastric nerves?

A

Main function is to prevent reflux of semen into the bladder during ejaculation L1-L3.

63
Q

What role do the somatic motorneurones (pudendal nerves) play in the control of micturition?

A

Innervate the skeletal muscle that forms the external urethral sphincter, keeps the sphincter closed, even against strong bladder contractions S2-S4

64
Q

What type of reflex is micturition?

A

Micturition is basically a spinal reflex which may or may not be influenced by higher centres.

65
Q

What sensory innervation is involved in the control of micturition?

A

-Stretch receptor afferents from the bladder wall. As the bladder fills there is increased discharge in afferent nerves to the spinal cord via interneruones

66
Q

What occurs as a result of the increased discharge of afferent nerves to the spinal cord via internerones as the bladder fills?

A
  • Excitation of parasympathetic outflow
  • Inhibition of sympathetic outflow
  • Inhibition of somatic motoneurones to external sphincter
  • Pathways to sensory cortex which leads to sensation of fullness
67
Q

What stimulates the stretch receptors in the bladder?

A

As the bladder, fills it becomes distended and the stretch receptors are increasingly stimulated

68
Q

The stretch receptors output becomes great enough to cause bladder contraction via:

A
  • Stimulation of the parasympathetic systems

- Relax the external sphincter by inhibiting the somatic motonerones

69
Q

What volume of urine is required in the bladder to initiate the spinal reflex in adults?

A

~300-350mls

70
Q

Who is the local spinal reflex particularly visible in?

A
  • Babies as their higher brain connections have yet to be established
  • Adults with spinal cord transection after the initial period of spinal shock
71
Q

How is delay of micturition achieved?

A

It is accomplished by descending pathways from many brain centres, including cortex and brainstem

72
Q

How do the descending pathways delay micturition?

A

Inhibiting the parasympathetic system
-Stimulation of the somatic nerves to the external sphincter thus, over-ridding the input from the bladder stretch receptors

73
Q

Voluntary initiation of micturition involves descending pathways which:

A
  • Stimulate the parasympathetic system

- Inhibit the somatic motor neurones thus summating with the stretch receptor effects

74
Q

What does potty training involve?

A

The setting up the controlling descending pathways

75
Q

What is one of the main initial events in voluntary urination?

A

Relaxation of the muscles of the pelvic floor which may cause a sufficient downward tug on the detrusor muscle to initiate its contraction

76
Q

How can urine flow down the urethra be or flow once urination has began be interrupted?

A

Perineal muscles and external sphincter can be contracted voluntarily

77
Q

How does the female urethra empty after urination?

A

By gravity

78
Q

How is urine remaining in the male urethra after urination expelled?

A

Expelled by contractions of the bulbocavernosus muscle.

79
Q

What are the 3 major types abnormalities of micturition due to neural lesions?

A
  • Interruption of afferent nerves
  • Interruption of both afferent and efferent nerves
  • Interruption of facilitatory and inhibitory descending pathways from the brain
80
Q

What occurs in all 3 types of abnormalities of micturition due to neural lesions?

A

The bladder contracts but the contractions are generally insufficient to empty the bladder completely and urine is left in the bladder

81
Q

How can some paraplegic patients train themselves to initiate voiding?

A

]By pinching or stroking their thighs initiating a mild mass reflex

82
Q

Why does pinching or stroking their thighs sometimes initiate voiding in paraplegic patients?

A
  • After spinal section, afferent stimuli irradiate from one reflex centre to another.
  • When a relatively minor noxious stimulus is applied to the skin it may irradiate to autonomic centres and evoke bladder or rectal voiding.

renal system (26 decks)

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