What is normal GFR?
~125mls/min/1.73m2 surface area
Whats GFR important for?
Monitoring progression of renal disease
Monitoring renal function for drug dosing
How do we measure GFR?
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
What substance do we routinely use to measure GFR?
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
How do we measure renal plasma flow and what is normal?
Measured using clearance of PAH (Para-amino-hippuric) acid.
PAH is freely filtered and secreted so plasma clearance is >90%
Dsecirbe the structure of the bladdeR?
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
How do we prevent reflux up the vesicouretic opening?
They pass obliquely through the bladder wall so pressure of urine closes off the opening preventing backflow
describe the graph of intravesicle pressure vs volume of urine in bladdeR?
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
What volume of urine in the bladder triggers spinal reflex micturition?
Describe the afferent sensory supply of micturition?
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
Describe the motor supply of micturition?
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
Sympathetic supply is much weaker than parasympathetic, what is its main function?
To close the internal urethral sphincter so you don’t reflux semen into your bladder
How does voluntary delay/initiation of micturition work?
Your higher brain centres can inhibit parasympathetic outflow (S2-4) and stimulate somatic outflow (S2-4) to delay or vice versa to initiate micturition
How do we clear the last urine left in our urethra?
In women its expelled by gravity
In men its by the bulbocavernosus muscle
What kind of neural lesions can cause abnormal micturition?
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
What is a mass refleX?
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.
Summary of micturition control
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