Lecture 6: Physiology of Micturition and Assessment of Renal Function

Question 1

what is micturition?

Answer 1

• the process of eliminating water and electrolytes from the urinary system, commonly known as urinating.
• it has two distinct pahses: the storage/continence pahse, when urine is stored in the bladder; and the voiding phase, where urine is released through the urethra.

Question 2

describe the control of the storage/continence phase of micturition

Answer 2

• the storage phase of micturition is controlled at the highest level by continence centres of the brain.
• these in turn control the continence centres of the spinal cord.
• the storage of urine requires relaxation of the detrusor muscle of the bladder and simultaneous contraction of both the internal urethral sphincters (IUS) and external urethral sphincters (EUS).

Question 3

the bladder and internal urethral sphincters are under the control of which nervous system?

Answer 3

autonomic nervous system

Question 4

the external urethral sphincters (EUS) are under the control of which nervous system?

Answer 4

somatic nervous system
- can therefore be voluntarily opened or closed to control micturition.

Question 5

describe the pathway from the brain to the bladder, in controlling the storage/continence phase of micturition

Answer 5

• to stimulate storage, impulses from the cerebral cortex travel to the pons.
• from the pontine continence centre, signals are sent to sympathetic nuclei in the spinal cord (T10-L2), and finally to the detrusor muscle and IUS of the bladder via the hypogastric nerve (roots T10-L2).
• at the bladder, this stimulates: relaxation of the detrusor muscle via stimulation of beta-3-adrenoceptors in the fundus and body of the bladder.
• contraction of the IUS, via stimulation of alpha-1-adrenoceptors at the neck of the bladder.

Question 6

discuss the somatic innervation of the EUS during the storage phase

Answer 6

• impulses travel to the EUS via the pudenal nerve (S2-S4) to nicotinic (cholinergic) receptors on the striated muscle, resulting in its contraction > prevents any urine from leaking out.

Question 7

describe the treatment of incontinence secondary to neurological insults

Answer 7

• anticholinergics (e.g. oxybutynin, tolterodine) > reduce parasympathetic input to the bladder. Side effects such as dry mouth or constipation, increased risk of falls.
• beta-3-adrenoceptor agonists (e.g. Mirabegron): bind to beta-3 receptors on the detrusor muscle to cause relaxation, increasing the bladder’s capacity to store urine.
• other possible therapies: botulinum toxin A injection, sacral nerve stimulation, surgical procedures such as augmentation enterocystoplasty or urinary diversion.

Question 8

what is the urinary flow rate in a full bladder in men and women?

Answer 8

20-25ml/s in men
25-30ml/s in women

Question 9

what is the capacity of the bladder and at what volume is the voiding phase stimulated?

Answer 9

capacity varies from roughly 300-550ml
afferent nerves in the bladder wall signal the need to void the bladder at around 400ml of filling

Question 10

the passing of urine is under which control?

Answer 10

parasympathetic

Question 11

describe the voiding phase of micturition

Answer 11

• upon stretching of the bladder, afferent signals ascend through the spinal cord and project to the pontine micturition centre and cerebrum.
• upon the voluntary decision to urinate, neurons of the pontine micturition centre fire to excite the sacral preganglionic neurons.
• subsequent parasympathetic stimulation to the pelvic nerve > release of ACh > M3 receptors > contraction of detrusor muscle > increased intra-vesicular pressure.
• pontine micturition centre also inhibits Onuf’s nucleus > reduction in sympathetic stimulation to IUS > relaxation.
• finally, a concious reduction in voluntary contraction of EUS from the cerebral cortex > distension of urethra > passing of urine.

Question 12

what is urinary retention?

Answer 12

the inability to void the bladder i.e. being unable to urinate

Question 13

list the causes of urinary retention

Answer 13

• benign prostatic hyperplasia (BPH)
• nerve dysfunction
• infection e.g. UTI
• constipation
• drugs e.g. anticholinergics, antidepressants ad opioids.

Question 14

what are the complications of urinary retention?

Answer 14

• urinary incontinence
• nocturia (the need to urinate at night)
• hydronephrosis - high pressure in the bladder can push urine back up ureters into the kidneys > expansion of the renal pelvises.
• kidney failure
• sepsis
• bladder rupture

Question 15

what is the treatment for urinary retention in an acute setting?

Answer 15

• urinary catheterisation
• prostatic stenting
• suprapubic cystostomy

Question 16

urine remaining in the male urethra is expelled by contractions of which muscle?

Answer 16

bulbocavernosus muscle

Question 17

what do plasma clearance tests determine?

Answer 17

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

Question 18

how is GFR measured?

Answer 18

GFR = [urine production (ml/minute) x substance concentration in the urine (mg/L)] / substance concentration in plasma (mg/ml)

Question 19

in a healthy adult, the GFR is approximately…

Answer 19

120ml/min

Question 20

which substances are commonly used to measure GFR and why?

Answer 20

• creatinine and inulin
• they are inert, freely filtered and freely excreted substances and remain in a steady state in the bloodstream

Question 21

how is renal plasma flow measured?

Answer 21

• PAH is an organic acid that is completely filtered and secreted by the kidney.
• it is given by IV infusion and is used to measure renal plasma flow (PAH does not change renal plasma flow).
• Effective renal plasma flow (eRPF) = urine concentration of PAH x (urine flow rate/plasma concentration of PAH)

Question 22

the renal plasma flow is approx…

Answer 22

600 ml/min
- however, not all of this renal plasma flow can pass through the filtration barrier at once, approx 20% is filtered to join the filtrate (120ml/min). Over 24 hours, this equates to approx 173 litres of filtrate, majority is reabsobed leaving 1.2ml/min of urine output (1% of the filtrate).