Physiology of Micturition and Assessment of Renal Function Flashcards
When would renal function need to be assessed?
-In patients w renal disease
-To check drug level in body*
->*this is because most drugs are removed from the body by excretion by filtration and if there is a fall in GFR, drugs may accumulate causing toxicity
What could assessing renal function tell us about someone with renal disease?
Progression of the disease as results in nephron descruction and loss of function
What is total GFR?
Sum of all filtration by functioning nephrons
Progression of renal disease would be indicated by what GFR?
Decrease in GFR
What type of tests are used to measure renal function?
Plasma clearing tests
->show how much of a substance can be cleared from the plasma.
What is the gold standard for measuring GFR?
Inulin clearance
This involves polyfructose loaded via IV of inulin, allowing time to equilibrate and then sampling plasma and urine simultaneously
To measure GFR, what must the substance used to measure this be?
The substance used must not be one that is secreted or reabsorbed
What is the normal GFR is a ‘normal’ healthy man?
125mls/min
->values for women about 10% lower
GFR is now not commonly measured in clinical practice- why?
Too complex and expensive
Takes several hours and requires injections of isotope 51Cr EDTA
If EFR is taken in clinical practice, inulin is no longer used, what is used instead?
51Cr-EDTA
What alternative is used routinely as a replacement for measuring GFR?
Creatinine clearance is used to estimate GFR
What is creatinine?
Breakdown product of muscle
There is a relationship between GFR and plasma creatinine concentration. However, what % of GFR is lost before there is any change in plasma creatinine concentration?
50% reduction in GFR before seeing any increase in plasma creatinine concentration
->may be worth looking a graph to show this - google like GFR and plasma creatinine rate graph
What are some factors which affect serum creatinine?
Muscle mass
Dietary intake e.g. supplements, vegetarians
Drugs
What is normal GFR?
100mls/min/1.73 meter squared of body surface area
…duh
What is the usual clearance of glucose by the kidneys?
Zero as normally all of it is reabsorbed
RECAP- in which part of the system is water reabsorbed?
Proximal tubule
What is the clearance of inulin like?
Inulin is completely filtered out at the glomerulus but neither secreted or reabsorbed by the tubules
What is the clearance of urea like?
Less than that of inulin as urea is not fully reabsorbed
What % of urea is reabsorbed in the proximal tubule?
50%
->if filtration rate is 100mL/min and 50% of urea is reabsorbed, urea clearance is 50mL/min
What organic anion can be used to measure renal plasma flow?
PAH- para-amino-hippuric acid
How can PAH be used to measure real plasma flow?
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
->idk if this makes sense, imma be honest with you, the next few flashcards might not make a lot of sense cos I’m doing this after having brownies w Oli at Kilau…you’ll remeber
What has a greater clearance- inulin or penicillan?
Penicillin
->more penicillin is excreted than was filtered
Okay so bear with me;
Let’s use molecule X in these examples. It can be literally whatever you fancy. Molecule X gets freely filtered at the glomerulus.
What happens if filtration of molecule X is greater than the excretion?
There will be a net reabsorption of molecule X
Okay so bear with me;
Let’s use molecule X in these examples. It can be literally whatever you fancy. Molecule X gets freely filtered at the glomerulus.
What happens if excretion of molecule X is greater than filtration?
There will be a net secretion of molecule X
Okay so bear with me;
Let’s use molecule X in these examples. It can be literally whatever you fancy. Molecule X gets freely filtered at the glomerulus.
What happens if filtration and excretion of molecule X are the same?
No net reabsorption or secretion of molecule X
Okay so bear with me;
Let’s use molecule X in these examples. It can be literally whatever you fancy. Molecule X gets freely filtered at the glomerulus.
What happens if clearance of molecule X is less than inulin clearance?
Net reabsorption of molecule X
Okay so bear with me;
Let’s use molecule X in these examples. It can be literally whatever you fancy. Molecule X gets freely filtered at the glomerulus.
What happens if clearance of molecule X is equal to inulin clearance?
Molecule X is neither reabsorbed or secreted
->this is the same in inulin clearance, no reabsorption or secretion
Okay so bear with me;
Let’s use molecule X in these examples. It can be literally whatever you fancy. Molecule X gets freely filtered at the glomerulus.
What happens if clearance of molecule X is greater than inulin clearance?
Net secretion of molecule X
How does urine flow from the kidneys to the ureters?
Via peristaltic contraction of the ureter smooth muscle
Why do the ureters enter the bladder at an oblique angle?
Prevents the reflux of urine
Does the composition of urine change at all after leaving the kidney?
No
What type of muscle is the bladder composed of?
Detrusor muscle
->Smooth muscle arranged in spiral, longitudinal and circular bundles
Micturition?
Action of urinating
How many sphincters does the bladder have?
Internal urethral sphincter- NOT a true sphincter
External urethral sphincter
What type of muscle is the internal urethral ‘sphincter’ made of?
Smooth muscle
->this acts as a sphincter when the smooth muscle is relaxed but is not a true sphincter
What type of muscle is the external urethral sphincter made of?
Skeletal muscle
What type of control is the external urethral sphincter under?
Voluntary somatic control
Where does the bladder in relation to the reproductive system and rectum?
Lies anterior
What is the bladder overlain with?
Peritoneum
What type of epithelium is the bladder lined with?
Transitional epithelium
If there is urethral obstruction, how does this affect the renal system?
Bilateral renal problems
If there was ureter obstruction, how would this affect the renal system?
Unilateral renal problems
What is normal daily urine production?
Varies between 750ml-2500mls in temperate climates
Which sphincter of the bladder stays contracted?
External sphincter
->internal sphincter is passively contracted
What is the motor innervation in the control of micturition?
Rich parasympathetic supply from the pelvic nerves
What does the rich parasympathetic supply from the pelvic nerves lead to?
Increased contraction of the detrusor muscle leading to increased pressure within the bladder
There is sparse sympathetic supply which inhibits the bladder contraction and closes the internal urethral ‘sphincter’. What nerves provide this innervation?
Hypogastric nerves
Sympathetic supply is not very important to the bladder. However, what would happen if the hypogastric nerve was cut or damaged?
Increased frequency of micturition
What role does the somatic motoneurons have in micturition?
Innervate the skeletal muscle that forms the external urethral sphincter
This keeps the bladder closed even against strong bladder contractions
The stretch receptor of the bladder afferents from the bladder wall.
As the bladder fills, there is increased discharge in afferent nerves to the spinal cord.
What happens in terms of different innervation pathways e.g. parasympathetic, sympathetic
Excitation of parasympathetic outflow
Inhibition of sympathetic outflow
Inhibition of somatic motoneurones to external sphincter
Pathways to sensory cortex lead to sensation of fullness
What happens to the stretch receptors as the bladder fills?
The stretch receptors become increasingly stimulated until their output becomes great enough to cause bladder contraction
In bladder contraction, the external sphincter relaxes to allow urination. What ensures this sphincter relaxes?
Inhibition of somatic motoneurons
In an adult, what is the volume of urine required in the bladder to initiate the spinal reflex responsible for urination?
300-350mls
How is delay of urination achieved?
Inhibition of parasympathetic
Stimulation of somatic nerves to the external sphincter which overrides the bladder stretch response
How does voluntary initiation of urination occur?
Inhibition of the somatic motor neurons thus summating w the stretch receptors
One of the initial stages of voluntary urination involves the relaxation of which muscles?
Pelvic floor
->this can cause a sufficient downward tug on the detrusor muscle to trigger contraction
Which muscles can be contracted voluntarily to prevent urine flow which can stop flow once urination begins?
Perianal muscles
External sphincter
After urination, how does the female urethra empy?
Gravity
After urination, how is any urine remaining in the male urethra expelled?
By contractions of the bulbocavernous muscle
There are three major abnormalities to urination due to neural lesions.
What do these cause?
- Interruption of afferent nerves
- Interruption of both afferent and efferent nerves
- Interruption of faciliatory and inhibitory descending pathways from the brain
->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.
