Renal Assessment and Micturition

Question 1

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

Answer 1

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

Question 2

What is the total GFR?

Answer 2

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

Question 3

Describe the measurement of GFR

Answer 3

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

Question 4

What is plasma clearance equation?

Answer 4

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

Question 5

What is the gold standard measurement of GFR?

Answer 5

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

Question 6

Explain insulin clearance

Answer 6

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

Question 7

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

Answer 7

125mls/min
Which is normal for a man

Question 8

What happens if substances are filtered and secreted?

Answer 8

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

Question 9

What happens if substances are filtered and reabsorbed?

Answer 9

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

Question 10

What does the magnitude of GFR correlate to?

Answer 10

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

Question 11

What is used in clinical practice instead of insulin?

Answer 11

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

Question 12

What is GFR equal to compared with plasma creatine levels?

Answer 12

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

Question 13

Is GFR and Pcr a linear relationship?

Answer 13

No because can half GFR before elevation of Pcr
Big caution

Question 14

What is GFR written as when using creatinine?

Answer 14

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

Question 15

What are factors affecting serum creatinine?

Answer 15

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

Question 16

What is normal GFR?

Answer 16

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

Question 17

What is the clearance of glucose?

Answer 17

Zero as all is normally reabsorbed

Question 18

What is the clearance of urea?

Answer 18

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

Question 19

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

Answer 19

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

Question 20

Describe para-hippuric acid (PAH)

Answer 20

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

Question 21

What is the clearance of penicillin?

Answer 21

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

Question 22

What does micturition mean?

Answer 22

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

Question 23

Describe the muscle of the bladder

Answer 23

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

Question 24

Describe the external and internal urethral sphincter

Answer 24

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

Question 25

Describe the bladder

Answer 25

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

Question 26

What does urethral and ureter obstruction lead to?

Answer 26

Urethral - bilateral renal problems
Ureter - unilateral renal problems

Question 27

Describe the pressure volume curve of the bladder

Answer 27

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

Question 28

What is normal urine production?

Answer 28

750mls to 2500mls in temperate climates

Question 29

What is the bladder like at rest?

Answer 29

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

Question 30

What is the motor innervation in control of micturition?

Answer 30

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

Question 31

Describe the sensory innervation of the bladder

Answer 31

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

Question 32

What is a summary of micturition?

Answer 32

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

Question 33

How is the external sphincter relaxed?

Answer 33

Inhibiting somatic motoneurons

Question 34

What happens in leaky babies?

Answer 34

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

Question 35

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

Answer 35

300-350mls

Question 36

How is delay of urination accomplished?

Answer 36

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

Question 37

Explain voluntary urination

Answer 37

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

Question 38

What is the difference between male and female urination?

Answer 38

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

Question 39

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

Answer 39

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

Question 40

Describe paraplegic patients voiding by pinching or stroking thighs

Answer 40

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