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*Urinary System > Physiology of Micturition & Assessment of Renal Function, Urea, Creatinine and GFR > Flashcards

Physiology of Micturition & Assessment of Renal Function, Urea, Creatinine and GFR Flashcards

1
Q

Why is ability to measure GFR particularly useful in patients with renal disease?

A

Total GFR = sum of all filtration by functioning nephrons so progression of disease would be indicated by the reduction in GFR

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2
Q

Why is measuring GFR particularly useful in drug dosing?

A

Many drugs e.g. digitalis and many abx are removed from the body by excretion by filtration; when GFR falls, excretion falls so that [drug] in plasma may rise causing toxicity therefore may need to adjust dose appropriate to decrease in renal function

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3
Q

What do plasma clearance tests measure?

A

The ability of the kidney to clear the plasma of various substances (it is the plasma that is important NOT the urine)

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4
Q

What does clearance relate to?

A

A volume of plasma cleared

(NOT a quantity of substance removed from the plasma)

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5
Q

Give the equation used to work out plasma clearance of X

A

CX = [UX] V/[PX]

units mls/min

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6
Q

Give the meaning for each of the components of CX = [UX] V/[PX]

A

UX = urine concentration of X

V = urine flow rate

PX = plasma concentration of X

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7
Q

What is the gold standard for clearance? What is`the process of testing?

A

INULIN CLEARANCE (polyfructose)

Loading IV dose of inulin, allow time to equilibriate, then sample simultaneously plasma and urine (during a timed urine sample)

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8
Q

Diagram of 100% inulin clearance

A
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9
Q

How is inulin a good measure of 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

= MEASURE OF GFR

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10
Q

What happens to substances that have a LOWER clearance than inulin?

A

Substances are filtered and reabsorbed

Because [UX] will be less than if only filtered and [PX] higher

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11
Q

What happens to substances that have a HIGHER clearance than inulin?

A

They will be filtered and secreted

Because [UX] will be higher and [PX] lower

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12
Q

What is the GFR in a normal man?

A

125ml/min

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13
Q

Magnitude of GFR correlates with surface area; even with correction for surface area, are GFR values in women equal or different to men?

A

Values in women are ~10% lower

(~112.5)

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14
Q

After the age of 30 how much does GFR decline by per year?

A

~1ml/min/year

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15
Q

Work out the GFR in a 21 yr old male with UIN = 285mg/dl, V = 1.1ml/min, PIN = 2.5mg/dl

A

125.4 GFR

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16
Q

Diagram showing: insulin and normal GFR; lower clearance substances reabsorbed; higher clearance substances secreted

A
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17
Q

In clinical practice inulin is no longer used, because too cumbersome, so what is used instead?

A

51Cr-EDTA

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18
Q

What is 51Cr-EDTA?

A

A radioactive substance that is handled by the kidney in the same way as inulin

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19
Q

GFR is now considered too complex and expensive to measure - takes several hrs and requires 51Cr-EDTA injection - what is now routinely used to estimate GFR instead?

A

CREATININE CLEARANCE !

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20
Q

Give the rationale for using creatinine instead of insulin to measure GFR

A

GFR = CIN = CCR

CCR = [UCR] V/ [PCR]

therefore GFR inversely proportional to 1/ [PCR]

so plasma creatinine can be used to estimate GFR

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21
Q

Is the relationship between GFR and creatinine described as linear?

A

NO!

GFR can halve before PCR elevates (if it was linear - halving of GFR would cause doubling of PCR)

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22
Q

Give 3 factors affecting serum creatinine

What does this mean for the measurement of GFR?

A

Muscle mass: athletes vs malnutrition

Dietary intake: creatinine supplements vs vegetarians

Drugs: some lead to spurious increases as does ketoacidosis

Flawed measurement but nevertheless useful

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23
Q

What is approximate normal GFR in mls/min/1.73m2?

A

100

(i.e. across range of adults and sexes, for kidney function and size, so may be expressed as a percentage of normal)

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24
Q

There are formulae using serum creatinine value which can take into account confounding variables; what can we find with these formulae?

A

estimated GFR (eGFR)

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25
Q

What is the clearance of glucose?

A

ZERO ! (all is normally reabsorbed)

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26
Q

What is the clearance of urea compared to that of inulin? Why?

A

Clearance of urea is less than that of inulin because some urea is reabsorbed

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27
Q

What is PAH?

A

An organic anion

Para-amino-hippuric acid (PAH)

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28
Q

What is PAH used to measure?

A

Renal plasma flow (RPF)

29
Q

How is PAH filtered? How much of plasma is cleared of its PAH content in one transit of the kidney?

A

It 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

30
Q

PAH clearance is therefore a measure of all the plasma flowing through the kidneys in a given time; what is the approximate normal renal plasma flow?

A

~660mls/min

31
Q

What is the clearance of penicillin compared with inulin?

A

Penicillin has a greater clearance than inulin because filtered and secreted

32
Q

Table summarising renal handling of solutes for any molecule X that is freely filtered at the glomerulus

A
33
Q

How does urine flow from the kidneys to the ureters?

A

Via peristaltic contraction of the smooth muscle of the ureters

(then enters bladder at an oblique angle to prevent reflux of urine)

34
Q

Does the composition of urine change once it leaves the kidneys?

A

Nope

35
Q

How is the structure of the bladder described?

A

‘A bag of smooth muscle’, arranged in spiral, longitudinal and circular bundles = detrusor muscle

36
Q

What is the detrusor muscle mainly responsible for?

A

The emptying of the bladder during micturition

37
Q

The internal urethral sphincter is NOT a true sphincter, what is it?

A

Where the smooth muscle at the start of the urethra acts as a sphincter when the smooth muscle is relaxed

38
Q

Is the external urethral sphincter a true sphincter?

A

YES; made up of skeletal muscle under voluntary somatic control

39
Q

Describe the anatomical position of the bladder

A
  • Lies in midline posterior to pubic bones
  • Lies anterior to reproductive system and rectum
40
Q

What type of muscle and epithelium is the bladder made up of?

A

Smooth muscle (detrusor muscle)

Lined transitiona epithelium

41
Q

What are the 1-2cm oblique passages through the muscular wall of bladder form ureter called?

A

Vesicoureteric openings

42
Q

What is the ‘trigone’ of the bladder?

A
  • 2 vesicoureteric openings
  • Urethral opening
43
Q

Does urethral obstruction lead to unilateral or bilateral renal problems?

A

Bilateral renal problems

44
Q

Does ureter obstruction lead to bilateral or unilateral renal problems?

A

Unilateral renal problems

45
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

46
Q

Graph of cystometrogram showing pressure-volume curve

A
47
Q

Motor supply of bladder consists of:

A
  1. Rich parasympathetic supply - pelvic nerves S2-S4
  2. Sparse sympathetic supply - hypogastric nerves L1-L3
  3. Somatic motoneurones - pudendal nerves S2-S4
48
Q

Describe the function of the parasympathetic supply to the bladder

A

When activity is increase = contraction of detrusor muscle (starts of micturition process) = increases pressure within bladder

49
Q

Describe the function of the sympathetic supply to the bladder

A

Inhibits bladder contraction and closes the internal urethral ‘sphincter’

(not v important for bladder function but if u cut the hypogastric nerve = increased frequency of micturition; main function is to prevent reflux of semen into bladder during ejaculation)

50
Q

Describe the function of the somatic motoneurones innervating the bladder

A

Innervate the skeletal muscle that forms the external urethral sphincter, keeps the sphincter closed, even against storng bladder contractions

51
Q

What is the sensory innervation to the bladder wall?

A

Stretch receptor afferents from the bladder wall

52
Q

Describe the function of the sensory innervation of the bladder via stretch receptors

A

As bladder fills = increased dischage in afferent nerves to spinal cord -> via interneurones:

  • Excitation of parasympathetic flow
  • Inhibition of sympathetic flow
  • Inhibition of somatic motoneurones to external sphincter
  • Pathways to sensory cortex -> sensation of fullness

(micturition is basically a spinal reflex which may or may not be influenced by higher centres)

53
Q

Bladder at rest

A

Bladder during micturition

54
Q

Describe operation of the local spinal reflex

A

As bladder fills, it becomes distended and stretch receptors are increasingly stimulated until their output becomes great enough to cause bladder contraction via stimulation of parasympathetic and relax the external sphincter by inhibiting somatic motoneurones

55
Q

In ‘leaky’ babies, why does the micturition reflex operate at the level of the local spinal reflex?

A

Because the higher brain connections still have to be established

Also the case in adult patients with spinal cord transection after the initial period of spinal shock

56
Q

In an adult what is the volume of urine in the bladder required to initiate the spinal reflex?

A

~300-350mls

57
Q

Summary control of micturition

A
58
Q

How is delay of micturition accomplished?

A

By descending pathways from many brain centres, inc cortex and brainstem

59
Q

What do the descending pathways from e.g. cortex and brainstem do to accomplish delay of micturition?

A

Inhibit the parasympathetic and stimulate somatic nerves of external sphincter = overrides input from bladder strecth receptors

60
Q

How is volunatary initiation of micturition accomplished?

A

Involves descending pathways which stimulate the parasymaptehtic and inhibit somatic motor neurones thus summating the stretch receptor effects

(potty training = setting up these pathways)

61
Q

How do the muscles of the pelvic floor play a role in voluntary urination?

A

Relaxation of muscle of pelvic floor is one of the initial events of voluntary initiation - this may cause a sufficient downward tug on detrusor muscle to initiate its contraction

62
Q

What can be contracted voluntarily to prevent urine flow down the urethra/interrupt flow once urination begins?

A

Perineal muscles and external sphincter

63
Q

How is remaining urine expelled in males vs females after urination?

A

Female urethra empties by gravity

Urine remaining in male urethra is expelled by contractions of the bulbocavernosus muscle

64
Q

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

A
  1. Interruption of afferent nerves
  2. Interruption of both afferent and efferent nerves
  3. Interruption of facilitatory and inhibitory descending pathways from the brain
65
Q

In all 3 types of micturition abnormalities due to neural lesions, what happens?

A

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

66
Q

Control of micturition diagram w delay and initiation

A
67
Q

What is the mild mass reflex?

A

Used in some paraplegic patients to train themselves to initiate voiding by pinching or stroking their thighs

68
Q
A

*Urinary System (26 decks)

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