Renal Assessment and Micturition Flashcards

1
Q

What are 2 clinical situations where ability to measure GFR is particularly useful?

A

In patients with renal disease - nephron destruction and decreased nephron function
Many drugs - if GFR falls then excretion falls so can cause toxicity

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2
Q

What is the total GFR?

A

Sum of all filtration by functioning nephrons so progression of disease would be indicated by a reduction

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3
Q

Describe the measurement of GFR

A

Plasma clearance testes are used to measure renal function - ability of kidney to clear the plasma of various substances
Plasma is what is important not urine, and volume of plasma cleared not quantity of substance

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4
Q

What is plasma clearance equation?

A

Plasma clearance of X, CX = (UX)V/ (PX)mls/min
UX - urine conc. of X
V - urine flow rate
PX - plasma conc. of X

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5
Q

What is the gold standard measurement of GFR?

A

Insulin clearance, polyfructose, loading IV dose of insulin
Allow time to equilibrate and then sample simultaneously plasma and urine

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6
Q

Explain insulin clearance

A

Insulin is freely filtered at glomerulus and neither reabsorbed nor secreted
Not metabolised by kidney and does not interfere with renal function so insulin clearance is a measure of GFR

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7
Q

What is the GFR of 21 year old with UIN = 285mg/dl
V = 1.1mls/min
PIN = 2.5mg/dl?

A

125mls/min
Which is normal for a man

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8
Q

What happens if substances are filtered and secreted?

A

Higher clearance than insulin because UX will be higher and PX will be lower

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9
Q

What happens if substances are filtered and reabsorbed?

A

Lower clearance than insulin as UX will be lower and PX will be higher

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10
Q

What does the magnitude of GFR correlate to?

A

Surface area
Values in women are 10% lower even after correlation of surface area
GFR declines by 1ml/min/year after 30

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11
Q

What is used in clinical practice instead of insulin?

A

51 Cr-EDTA - suitable radioactive substance
Takes several hours
Creatine clearance used to estimate GFR

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12
Q

What is GFR equal to compared with plasma creatine levels?

A

GFR is proportional to 1/plasma creatine conc.
Used to estimate GFR

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13
Q

Is GFR and Pcr a linear relationship?

A

No because can half GFR before elevation of Pcr
Big caution

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14
Q

What is GFR written as when using creatinine?

A

eGFR
Formulae using serum creatinine value which takes into account cofounding variables

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15
Q

What are factors affecting serum creatinine?

A

Muscle mass - athletes vs malnutrition
Dietary intake - creatinine supplements vs vegetarians
Drugs - some can lead to ketoacidosis

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16
Q

What is normal GFR?

A

Approx. 100mls/min/1.73m2
Can also be written as a percentage of normal function

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17
Q

What is the clearance of glucose?

A

Zero as all is normally reabsorbed

18
Q

What is the clearance of urea?

A

Less than that of insulin because some urea is reabsorbed
50% excreted and 50% reabsorbed

19
Q

What is para-amino-hippuric acid (PAH) used to measure?

A

Real plasma flow - RPF
All plasma flowing through kidneys at a given time
660mls/min

20
Q

Describe para-hippuric acid (PAH)

A

PAH is freely filtered at glomerulus and then PAH remaining in plasma is secreted into tubule so more 90% of plasma is cleared of PAH content

21
Q

What is the clearance of penicillin?

A

Greater than insulin because filtered and secreted
None is reabsorbed
150ml/min

22
Q

What does micturition mean?

A

Urinating/ bladder voiding
Urine flows from kidneys to ureters via peristaltic contraction of smooth muscle and enter bladder at oblique angle

23
Q

Describe the muscle of the bladder

A

Smooth muscle bag arranged in spiral, longitudinal and circular bundles - detrusor muscle
Contraction of this muscle is mainly responsible for urination

24
Q

Describe the external and internal urethral sphincter

A

Internal - not true sphincter but smooth muscle at start of urethra acts as a sphincter when smooth muscle is relaxed
External - true, made of skeletal muscle under voluntary somatic control

25
Q

Describe the bladder

A

Overlain with peritoneum
Lies in midline posterior to pubic bones
Lies anterior to reproductive system
Smooth muscle
Lined transitional epithelium
Trigone of bladder
Vesicoureteral openings

26
Q

What does urethral and ureter obstruction lead to?

A

Urethral - bilateral renal problems
Ureter - unilateral renal problems

27
Q

Describe the pressure volume curve of the bladder

A

Characteristic shape - long flat segment where initial increments of urine enter bladder then sharp rise as micturition reflex is triggered

28
Q

What is normal urine production?

A

750mls to 2500mls in temperate climates

29
Q

What is the bladder like at rest?

A

High CNS input - Tonic discharge of motor neuron fibres which keeps external sphincter contracted
Internal sphincter is passively contracted

30
Q

What is the motor innervation in control of micturition?

A

Rich sympathetic supply increases activity which increases contraction of detrusor muscle - S2-S4
Sparse sympathetic supply (hypogastric nerves) which inhibit bladder contraction and close internal sphincter
Somatic motoneurons (pudendal nerves) innervate skeletal muscle of external sphincter

31
Q

Describe the sensory innervation of the bladder

A

Stretch receptor afferents from bladder wall
As bladder fills then increase discharge in afferent nerves to spinal nerves via interneurons - excitation of parasympathetic, inhibition of sympathetic, inhibition of somatic to external sphincter, and pathways to sensory cortex

32
Q

What is a summary of micturition?

A

Stretch receptors fire
Parasympathetic neurons fire and motor neurons stop firing
Smooth muscle contracts
Internal sphincter passively pulled open and external sphincter relaxes

33
Q

How is the external sphincter relaxed?

A

Inhibiting somatic motoneurons

34
Q

What happens in leaky babies?

A

Micturition reflex operates at this level because higher brain connections have to be established
Also in adult patients with spinal cord transection

35
Q

What is the adult volume of urine in bladder required to imitate spinal reflex?

A

300-350mls

36
Q

How is delay of urination accomplished?

A

Descending pathways from many brain centres including cortex and brainstem which inhibit parasympathetic -stimulate somatic nerves to external sphincter to over ride stretch receptors
Voluntary initiation - stimulate somatic motor neurons

37
Q

Explain voluntary urination

A

Relaxation of muscles of pelvic floor and may cause downward tug on detrusor muscles to initiate contraction
Perineal muscles and external sphincter can be contracted voluntarily

38
Q

What is the difference between male and female urination?

A

After urination, female urethra empties by gravity
Urine remaining in males is expelled by contraction of bulb cavernous muscle

39
Q

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

A

Interruption of afferent nerves
Interruption of both afferent and efferent nerves
Interruption of faciliatory and inhibitory descending pathways from the brain
Bladder contracts in all but not sufficient enough

40
Q

Describe paraplegic patients voiding by pinching or stroking thighs

A

Causes mild mass reflex
After spinal section, afferent stimuli irradiate from one reflex to another
It may irradiate to autonomic centres and evoke bladder or rectal voiding