Physiology 7 - RFTs & Micturation

Question 1

What is normal GFR?

Answer 1

~125mls/min/1.73m2 surface area

Question 2

Whats GFR important for?

Answer 2

Monitoring progression of renal disease

Monitoring renal function for drug dosing

Question 3

How do we measure GFR?

Answer 3

Through Plasma clearance (Cx) of a specific substance where:
• Cx= [Ux] V/[Px]

Ux is urine conc of x
V is urine flow rate
Px is plasma conc of x

Question 4

What substance do we routinely use to measure GFR?

Answer 4

Inulin is gold standard but needs to be injection IV

Creatinine is used to measure eGFR because you should produce a fairly constant amount of it, therfore plasma creatinine is all thats needed to tell how well the kidney is excreting it

Question 5

How do we measure renal plasma flow and what is normal?

Answer 5

~660ml/min

Measured using clearance of PAH (Para-amino-hippuric) acid.
PAH is freely filtered and secreted so plasma clearance is >90%

Question 6

Dsecirbe the structure of the bladdeR?

Answer 6

1) Found ant to repro system & rectum
2) Found post to pubic bones
3) Mostly smooth muscle (Detrusor)
4) lined with transitional epithelium
5) Trigone contains 2 vesicouretic openings and 1 urethral opening

Question 7

How do we prevent reflux up the vesicouretic opening?

Answer 7

They pass obliquely through the bladder wall so pressure of urine closes off the opening preventing backflow

Question 8

describe the graph of intravesicle pressure vs volume of urine in bladdeR?

Answer 8

ITs a sigmoid curve
I.e. it rises fast at first and you get the urge to void at ~100mls

Then pressure remains fairly constant till around 400mls when the pressure rises fast, along with the sense of urgency

Question 9

What volume of urine in the bladder triggers spinal reflex micturition?

Answer 9

300-350mls

Question 10

Describe the afferent sensory supply of micturition?

Answer 10

Stretch receptors in bladder wall discharge triggering the spinal reflex:

• Excitation of parasymp outflow
• Inhibition of sympathetic & somatic outflow

Also conveys the sensation of fullness to the brain

Question 11

Describe the motor supply of micturition?

Answer 11

Parasymp pelvic nerves (S2-4) - Contract detrusor

Symp Hypogastric nerves (L1-3) - Relax detrusor and contract Internal urethral sphincter

Somatic pudendal nerves (S2-4) contract external urethral sphincter

Question 12

Sympathetic supply is much weaker than parasympathetic, what is its main function?

Answer 12

To close the internal urethral sphincter so you don’t reflux semen into your bladder

Question 13

How does voluntary delay/initiation of micturition work?

Answer 13

Your higher brain centres can inhibit parasympathetic outflow (S2-4) and stimulate somatic outflow (S2-4) to delay or vice versa to initiate micturition

Question 14

How do we clear the last urine left in our urethra?

Answer 14

In women its expelled by gravity

In men its by the bulbocavernosus muscle

Question 15

What kind of neural lesions can cause abnormal micturition?

Answer 15

Neural lesions can:

1) interrupt afferent nerves
2) Interrupt afferent & efferent
3) interrupt descending pathways

Net result from any of them is too weak bladder contraction so you dont fully empty it

Question 16

What is a mass refleX?

Answer 16

For patients with a spinal section (paraplegia) some afferent stimuli can irradiate from one reflex centre to another.
Some paraplegics can control bladder/rectal voiding by pinching their thigh to initiate a mild mass reflex that irradiates to the autonomic centres.

Question 17

Summary of micturition control

Answer 17

Mainly a spinal reflex through L1-3 afferents detecting stretch in the bladder and S2-4 parasymp efferents causing contraction of the bladder and relaxation of the internal urethral sphincter.
Can be overridden by descending pathways