Renal Assessment and Micturition Flashcards
What are 2 clinical situations where ability to measure GFR is particularly useful?
In patients with renal disease - nephron destruction and decreased nephron function
Many drugs - if GFR falls then excretion falls so can cause toxicity
What is the total GFR?
Sum of all filtration by functioning nephrons so progression of disease would be indicated by a reduction
Describe the measurement of GFR
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
What is plasma clearance equation?
Plasma clearance of X, CX = (UX)V/ (PX)mls/min
UX - urine conc. of X
V - urine flow rate
PX - plasma conc. of X
What is the gold standard measurement of GFR?
Insulin clearance, polyfructose, loading IV dose of insulin
Allow time to equilibrate and then sample simultaneously plasma and urine
Explain insulin clearance
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
What is the GFR of 21 year old with UIN = 285mg/dl
V = 1.1mls/min
PIN = 2.5mg/dl?
125mls/min
Which is normal for a man
What happens if substances are filtered and secreted?
Higher clearance than insulin because UX will be higher and PX will be lower
What happens if substances are filtered and reabsorbed?
Lower clearance than insulin as UX will be lower and PX will be higher
What does the magnitude of GFR correlate to?
Surface area
Values in women are 10% lower even after correlation of surface area
GFR declines by 1ml/min/year after 30
What is used in clinical practice instead of insulin?
51 Cr-EDTA - suitable radioactive substance
Takes several hours
Creatine clearance used to estimate GFR
What is GFR equal to compared with plasma creatine levels?
GFR is proportional to 1/plasma creatine conc.
Used to estimate GFR
Is GFR and Pcr a linear relationship?
No because can half GFR before elevation of Pcr
Big caution
What is GFR written as when using creatinine?
eGFR
Formulae using serum creatinine value which takes into account cofounding variables
What are factors affecting serum creatinine?
Muscle mass - athletes vs malnutrition
Dietary intake - creatinine supplements vs vegetarians
Drugs - some can lead to ketoacidosis
What is normal GFR?
Approx. 100mls/min/1.73m2
Can also be written as a percentage of normal function
What is the clearance of glucose?
Zero as all is normally reabsorbed
What is the clearance of urea?
Less than that of insulin because some urea is reabsorbed
50% excreted and 50% reabsorbed
What is para-amino-hippuric acid (PAH) used to measure?
Real plasma flow - RPF
All plasma flowing through kidneys at a given time
660mls/min
Describe para-hippuric acid (PAH)
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
What is the clearance of penicillin?
Greater than insulin because filtered and secreted
None is reabsorbed
150ml/min
What does micturition mean?
Urinating/ bladder voiding
Urine flows from kidneys to ureters via peristaltic contraction of smooth muscle and enter bladder at oblique angle
Describe the muscle of the bladder
Smooth muscle bag arranged in spiral, longitudinal and circular bundles - detrusor muscle
Contraction of this muscle is mainly responsible for urination
Describe the external and internal urethral sphincter
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
Describe the bladder
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
What does urethral and ureter obstruction lead to?
Urethral - bilateral renal problems
Ureter - unilateral renal problems
Describe the pressure volume curve of the bladder
Characteristic shape - long flat segment where initial increments of urine enter bladder then sharp rise as micturition reflex is triggered
What is normal urine production?
750mls to 2500mls in temperate climates
What is the bladder like at rest?
High CNS input - Tonic discharge of motor neuron fibres which keeps external sphincter contracted
Internal sphincter is passively contracted
What is the motor innervation in control of micturition?
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
Describe the sensory innervation of the bladder
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
What is a summary of micturition?
Stretch receptors fire
Parasympathetic neurons fire and motor neurons stop firing
Smooth muscle contracts
Internal sphincter passively pulled open and external sphincter relaxes
How is the external sphincter relaxed?
Inhibiting somatic motoneurons
What happens in leaky babies?
Micturition reflex operates at this level because higher brain connections have to be established
Also in adult patients with spinal cord transection
What is the adult volume of urine in bladder required to imitate spinal reflex?
300-350mls
How is delay of urination accomplished?
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
Explain voluntary urination
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
What is the difference between male and female urination?
After urination, female urethra empties by gravity
Urine remaining in males is expelled by contraction of bulb cavernous muscle
What are 3 major types of abnormalities of micturition due to neural lesions?
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
Describe paraplegic patients voiding by pinching or stroking thighs
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
