Physiology of Micturition and assessment of renal function

Question 1

How is renal function assessed?

Answer 1

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

Plasma clearance tests

Question 2

What drugs can be dangerous if renal function drops?

Answer 2

Many of them!

Digitalis, antibiotics

Question 3

How is GFR measured?

Answer 3

Plasma clearance tests

Question 4

How are plasma clearance tests used to assess renal function

Answer 4

Plasma clearance of X - CX= [UX] V/[PX]                                     
o	Units are mls/min
o	UX = Urine concentration of X
o	V = urine flow rate
o	PX = plasma concentration of X

Question 5

What is the GFR in a normal healthy males and females?

Answer 5

GFR in “normal” man = 125mls/min; values in women are ≈ 10% lower

Question 6

Describe normal GFR decline as you age

Answer 6

GFR declines by ≈1ml/min/year after 30

Question 7

The clearance of what substance is sometimes used to estimate GFR?

Answer 7

Creatinine clearance: routinely used to estimate GFR.

Question 8

Why must estimating GFR with creatinine be done with caution?

Answer 8

Some confounding variables - there are formulae using serum creatinine value which can take into account confounding variables

Factors which affect creatinine
o Muscle mass: athletes vs malnutrition
o Dietary intake: creatine supplements vs vegetarians
o Drugs: Some lead to spurious increases as does ketoacidosis.

Question 9

What is normal GFR according to weight?

Answer 9

Normal GFR is approximately 100mls/min/1.73m2

Question 10

What is used to measure renal plasma flow?

Answer 10

• The organic anion para-amino-hippuric acid (PAH) is used to measure real plasma flow (RPF).
• 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.
• ∴PAH clearance is a measure of all the plasma flowing through the kidneys in a given time = renal plasma flow ≈ 660mls/min

Question 11

What is normal renal plasma flow?

Answer 11

660mls/min

Question 12

Describe the two sphincters of the urethra

Answer 12

• The internal urethral “sphincter” is not a true sphincter, but is where the smooth muscle at the start of the urethra acts as a sphincter when the smooth muscle is relaxed.
• The external urethral sphincter is a true sphincter, made up of skeletal muscle under voluntary somatic control.

Question 13

What is contraction of the detrusor muscle important in?

Answer 13

• Bladder is a bag of smooth muscle, arranged in spiral, longitudinal and circular bundles = detrusor muscle.
• Contraction of this muscle is mainly responsible for emptying the bladder during micturition.

Question 14

Where are the 2 vesicoureteric openings in the bladder?

Answer 14

Trigone of bladder

Question 15

Describe the vesicoureteric openings in the bladder?

Answer 15

– 1-2cm oblique passage through muscular wall

Question 16

What type of epithelium lines the bladder and urethra?

Answer 16

Transitional epithelium

Question 17

How can you tell between urethral and ureteric obstructions?

Answer 17

• Urethral obstructions → bilateral renal problems
• Ureter obstruction → unilateral renal problems

Question 18

What is normal daily urine production in temperate climates?

Answer 18

Normal daily urine production varies between 750ml-2500mls in temperate climates.

Question 19

What investigation is used to diagnose problems with conditions associated with urine control?

Answer 19

Cystometry, or cystometrogram, with a pressure flow study is part of urodynamic testing (or UDS). These tests measure how well the bladder functions. They help diagnose problems related to urine control. These can be incontinence, difficulty emptying the bladder, overactive bladder, obstructions or frequent infections.

Question 20

What conditions can be tested for with cystometry/cystometrograms?

Answer 20

Incontinence
Difficulty emptying bladder
Overactive bladder
Obstructions
Frequent infections

Question 21

What is cystometry/cystometrograms?

Answer 21

Cystometry, or cystometrogram, with a pressure flow study is part of urodynamic testing (or UDS). These tests measure how well the bladder functions.

Question 22

Describe the motor innervation of the bladder

Answer 22

o Rich parasympathetic supply (pelvic nerves) - ↑ activity →↑ contraction of detrusor muscle → ↑ Pressure within the bladder, S2-S4

o Sparse sympathetic supply (hypogastric nerves) - inhibits bladder contraction and closes the internal urethral “sphincter”. Sympathetic supply is not very important to bladder function but cutting the hypogastric nerve →↑ frequency of micturition. Main function is to prevent reflux of semen into the bladder during ejaculation. Arise from L1-L3

o Somatic motoneurones (pudendal nerves) - innervate the skeletal muscle that forms the external urethral sphincter, keeps the sphincter closed, even against strong bladder contractions. From spinal nerves S2-S4

Question 23

From what vertebral levels is the PS innervation of the bladder from?

Answer 23

S2-S4

Question 24

From what vertebral levels is the sympathetic innervation of the bladder from?

Answer 24

L1-3

Question 25

What is another name for the somatic motoneurones of the bladder, and from what vertebral levels do they arise?

Answer 25

Pudendal nerves - spina nerves S2-S4

Question 26

Describe the sensory innervation of the bladder and their role in urination

Answer 26

Stretch receptor afferents from the bladder wall.

As the bladder fills →↑ discharge in afferent nerves to spinal cord → via interneurones causes:
• Excitation of parasympathetic outflow
• Inhibition of sympathetic outflow
• Inhibition of somatic motoneurones to external sphincter
• Pathways to sensory cortex → sensation of fullness

Question 27

In an adult, how much urine is needed in the bladder to initiate the spinal reflex for micturition?

Answer 27

In an adult, the volume of urine in the bladder required to initiate the spinal reflex is ≈ 300- 350mls.

Question 28

How is urine remaining in the male and female urethra expelled?

Answer 28

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

In females, empties by gravity

Question 29

List 3 major types of abnormalities of micturition associated with spinal lesions

Answer 29

o Interruption of afferent nerves
o Interruption of both afferent and efferent nerves
o Interruption of facilitatory and inhibitory descending pathways from the brain.

Question 30

What is associated with changes in micturition caused by spinal lesions?

Answer 30

• In all 3 types, the bladder contracts but the contractions are generally insufficient to empty the bladder completely and urine is left in the bladder.
• Some paraplegic patients train themselves to initiate voiding by pinching or stroking their thighs → mild mass reflex
• 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.
• Returns some measure of “voluntary” control by allowing them to induce an intentional mass reflex.

Question 31

What is the normal urine flow rate?

Answer 31

Around 1ml/min