Physiology of Micturition & Assessment of Renal Function, Urea, Creatinine and GFR Flashcards
What kind of tests are used to measure renal function?
Plasma clearance tests
Which is important in measuring renal function vis clearance tests, the plasma or the urine?
The plasma
What is the gold standard of clearance test for measuring renal function? What are the problems with this method? What is commonly used instead?
51Cr-EDTA - handled by the kidney in the same way as insulin
It is expensive and takes several hours
Creatinine clearance is used to estimate GFR
Is insulin reabsorbed and/or secreted?
It is neither, 100% should be freely filtered at the glomerulus
Will substances that are filtered and reabsorbed, even in part, have a higher or lower clearance than insulin?
Lower
What is the ‘normal’ GFR in a man?
125mls/min - across the demographic more likely to be 100mls/min
What is a key physical determinant of the GFR?
Surface area
What is a failure of the creatinine test that needs to be considered?
The relationship between the GFR and the creatinine levels is not linear, and it requires a substantial drop in GFR for a change to be seen in the creatinine levels - around 50% rate may be lost
What is eGFR?
The estimated GFR
What are some factors which affect serum creatinine?
Muscle mass
Dietary intake
Drugs
Therefore flawed but useful measurement
What is the normal clearance of glucose? Why?
Zero - All should be reabsorbed
What is the normal clearance of insulin? Why?
100%, it should all be freely filtered and excreted
Describe the normal clearance of urea
It is less than of insulin and more than of glucose, some urea is reabsorbed - around 50%
What is para-amino-hippuric acid (PAH) used to measure?
The renal plasma flow (RPF)
Describe the levels of filtration, secretion and reabsorption, if present, of PAH
Freely filtered at the glomerulus and then any PAH remaining in the plasma is actively secreted into the tubule so that 90% is cleared in one transit of the kidney. There is no reabsorption
What is the normal renal plasma flow rate?
660mls/min
Describe the renal handling of a solute for which filtration is greater than reabsorption
Not reabsorption of the solute
Describe the renal handling of a solute for which the excretion is greater than the filtration
Net secretion of the solute
Describe the renal handling of a solute for which filtration and excretion are the same
No net reabsorption or secretion
Describe the renal handling of a solute for which clearance is less than insulin clearance
Net reabsorption of solute (clearance less than 100% therefore some is reabsorbed)
Describe the renal handling of a solute for which the clearance is equal to that of insulin
Solute is neither reabsorbed not secreted (100% clearance so all is excreted first time)
Describe the renal handling of a solute for which the clearance is greater than that of insulin
This would be the case where 100% of the solute in the glomerulus is filtered and excreted, but there is also some which is not filtered but leaves via the efferent arteriole to then be actively secreted into the tubule, so there is clearance from the blood in addition to clearance of the amount of solute filtered at the glomerulus e.g. penicillin
What is micturition
Urination/voiding of the bladder
Is the composition of urine altered after it leaves the kidneys?
No
What is the smooth muscle of the bladder called, that is arranged in spiral, longitudinal and circular bundles?
Detrusor muscle
What is mainly responsible for the emptying of the bladder?
Contraction of the detrusor muscle
Are the internal and external urethral sphincters true sphincters?
The internal is not - made of smooth muscle and is contracted passively, the external is ad is made up of skeletal muscle under voluntary somatic control
What is found in the trigone of the bladder?
To vesicoureteric openings - 1-2cm oblige passages through the muscular wall
How may kidneys are affected in urethral obstruction?
Both - bilateral
How many kidneys are affected in ureter obstruction?
One - unilateral
What is the normal range of urine production per day?
750-2500mls
Describe the factors which make up the motor innervation (both stimulatory and inhibitory) of micturition - (three)
- Rich para supply from the pelvic nerves increase activity and contract the detrusor
- Sparse symp supply inhibits bladder contraction and closes internal sphincter - this supply is not very important to bladder function, the main function is to prevent reflux of semen into the bladder
- Somatic motoneurons innervate the skeletal muscle that forms the external sphincter, keeping it closed against strong bladder contractions
Describe the factors which make up the sensory innervation of micturition
Stretch receptors afferents fro the bladder wall are triggered as the bladder fills and the discharge to the spinal cord, causing;
- Excitation of para outflow
- Inhibition of symp outflow
- Inhibition of somatic motoneurons to the external sphincter
- Pathways to sensory cortex relate sensation of fulness
Essentially micturition is a spinal reflex which may or may not be influenced by higher centres
What is the volume of urine required in the bladder to trigger the spinal reflex of micturition?
300-350mls
What are the two mechanisms which allow a delay of the spinal reflex and micturition to allow for continence and then voluntary stimulation of urine flow?
Pathways from many brain centres including the cortex and the brainstem do the following two things:
- Inhibit para and stimulate the somatic nerves to external sphincter, over-riding the input from the bladder stretch receptors to allow the sphincter to remain closed
- Voluntary stimulation of para an inhibition the somatic motoneurons to the external sphincter to relax it in combination with the bladder stretch signals
These two mechanisms balance to allow adults to achieve continence
What muscle expels remaining urine in the bladder at the end of micturition?
The bulbocavernous muscle
What is a common cause of abnormalities of micturition?
Neural lesions - generally bladder contractions are still stimulated but not sufficiently to empty the bladder, leaving urine in the bladder in stasis
