W05: Micturition, Assessment, Renal, Urea, Creatinine, GFR

Question 1

Discuss the physiology of micturition

Answer 1

⇧para pelvic nerves activity = ⇧detrusor contraction
= ⇧pressure = S2-S4

sympathetic supply (hypogastric) inhibit bladder contraction and closes the internal sphincter

somatic neurones (pudendal nerves) = skeletal sphincter

Question 2

Describe the micturition reflex.

Answer 2

1) stretch receptors on bladder wall discharges to SC
2) para excitation
3) inhibition of symp and somatic
4) sensory cortex = sensation of fullness

*S2-S4 - ext urethral
higher centres can override symp and para

Question 3

Explain how voluntary control of micturition may be achieved

Answer 3

• relaxation of pelvic floor muscles to initiate detrusor contraction
• perineal muslces and external sphincter can be contracted to prevent flow once it begins
• female urethra empties by gravity
• urine in male urethra expelled by contractions of bulbocavernosus muscle

Question 4

Significance of GFR

Answer 4

nephron destruction = ⇩GFR
+
pharmacological excretion compromise with ⇩GFR = plasma tox. THEREFORE dose-adjustments

gold standard: INULIN CLEARANCE (not eabs)

Question 5

equation for GFR

Answer 5

C = [U] V/P

if substance is not reabs or secreted from plasma, then C[x] = GFR

• reabsorption will increase [P] and decrease U
• secretion will decrease [P] and increase U

Question 6

GFR in practice and use of eGFR

Answer 6

utilises creatinine clearance to estimate
GFR = C(in) = C(cr)
GFR = 1/P(cr) (proportional to classic GFR eq.)

*non-linear = GFR can halve before P(cr) can elevate

eGFR:

Question 7

Factors affecting serum cr

Answer 7

muscle mass: athletes v malnutrition

diet: cr suppl. v vegetarians
drugs: increases (seen in ketoacidosis)

Question 8

Describe the mechanisms used for the secretion of organic acids such as PAH.

Answer 8

measures real plasma flow (RPF)

• freely filtered
• actively secreted into tubule
• gives indication of plasma being cleared in one transit of kidney

= ~660ml/mins

Question 9

Describe the principles involved in renal replacement therapy and state the social, economic and psychological implications of dialysis and renal transplantation.

Answer 9

a

Question 10

Glc clearance

Answer 10

Tm results in reabsorption therefore 0mL/min

Question 11

Urea clearance

Answer 11

50% reabsorbed and 50% excreted @ prox tubule therefore 50ml/min

Question 12

Penicillin clearance

Answer 12

Filtered, no reabs, +SECRETION
= 150ml/min

thus penicillin has a greater clearance than inulin

Question 13

Bladder anatomy & function

Answer 13

Bladder is a bag of smooth muscle, arranged in spiral, longitudinal and circular bundles = detrusor muscle.
posterior to pubic bones and anterior to repro and rectum
* transitional epithelium

detrusor contraction = emptying and micturition

• internal sphincter (smc junction)
• external true sphincter

Question 14

Pressure-Volume curve of bladder

Answer 14

flat segment progresses to sudden sharp rise (d/t micturition reflex)

Question 15

principles of management of disorders related to micturition, including retention and incontinence.

Answer 15

NEURAL LESIONS

• afferent nerve interruption
• both afferent and efferent
• interruption of descending control pathways

> mild mass reflex utilises irradiating stimuli from one reflex centre to another stiulating autonomic centres and evoking voiding