Physiology of Micturition & Assessment of Renal Function, Urea, Creatinine and GFR

Question 1

What kind of tests are used to measure renal function?

Answer 1

Plasma clearance tests

Question 2

Which is important in measuring renal function vis clearance tests, the plasma or the urine?

Answer 2

The plasma

Question 3

What is the gold standard of clearance test for measuring renal function? What are the problems with this method? What is commonly used instead?

Answer 3

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

Question 4

Is insulin reabsorbed and/or secreted?

Answer 4

It is neither, 100% should be freely filtered at the glomerulus

Question 5

Will substances that are filtered and reabsorbed, even in part, have a higher or lower clearance than insulin?

Answer 5

Lower

Question 6

What is the ‘normal’ GFR in a man?

Answer 6

125mls/min - across the demographic more likely to be 100mls/min

Question 7

What is a key physical determinant of the GFR?

Answer 7

Surface area

Question 8

What is a failure of the creatinine test that needs to be considered?

Answer 8

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

Question 9

What is eGFR?

Answer 9

The estimated GFR

Question 10

What are some factors which affect serum creatinine?

Answer 10

Muscle mass
Dietary intake
Drugs

Therefore flawed but useful measurement

Question 11

What is the normal clearance of glucose? Why?

Answer 11

Zero - All should be reabsorbed

Question 12

What is the normal clearance of insulin? Why?

Answer 12

100%, it should all be freely filtered and excreted

Question 13

Describe the normal clearance of urea

Answer 13

It is less than of insulin and more than of glucose, some urea is reabsorbed - around 50%

Question 14

What is para-amino-hippuric acid (PAH) used to measure?

Answer 14

The renal plasma flow (RPF)

Question 15

Describe the levels of filtration, secretion and reabsorption, if present, of PAH

Answer 15

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

Question 16

What is the normal renal plasma flow rate?

Answer 16

660mls/min

Question 17

Describe the renal handling of a solute for which filtration is greater than reabsorption

Answer 17

Not reabsorption of the solute

Question 18

Describe the renal handling of a solute for which the excretion is greater than the filtration

Answer 18

Net secretion of the solute

Question 19

Describe the renal handling of a solute for which filtration and excretion are the same

Answer 19

No net reabsorption or secretion

Question 20

Describe the renal handling of a solute for which clearance is less than insulin clearance

Answer 20

Net reabsorption of solute (clearance less than 100% therefore some is reabsorbed)

Question 21

Describe the renal handling of a solute for which the clearance is equal to that of insulin

Answer 21

Solute is neither reabsorbed not secreted (100% clearance so all is excreted first time)

Question 22

Describe the renal handling of a solute for which the clearance is greater than that of insulin

Answer 22

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

Question 23

What is micturition

Answer 23

Urination/voiding of the bladder

Question 24

Is the composition of urine altered after it leaves the kidneys?

Answer 24

No

Question 25

What is the smooth muscle of the bladder called, that is arranged in spiral, longitudinal and circular bundles?

Answer 25

Detrusor muscle

Question 26

What is mainly responsible for the emptying of the bladder?

Answer 26

Contraction of the detrusor muscle

Question 27

Are the internal and external urethral sphincters true sphincters?

Answer 27

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

Question 28

What is found in the trigone of the bladder?

Answer 28

To vesicoureteric openings - 1-2cm oblige passages through the muscular wall

Question 29

How may kidneys are affected in urethral obstruction?

Answer 29

Both - bilateral

Question 30

How many kidneys are affected in ureter obstruction?

Answer 30

One - unilateral

Question 31

What is the normal range of urine production per day?

Answer 31

750-2500mls

Question 32

Describe the factors which make up the motor innervation (both stimulatory and inhibitory) of micturition - (three)

Answer 32

1. Rich para supply from the pelvic nerves increase activity and contract the detrusor 2. 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 3. Somatic motoneurons innervate the skeletal muscle that forms the external sphincter, keeping it closed against strong bladder contractions

Question 33

Describe the factors which make up the sensory innervation of micturition

Answer 33

Stretch receptors afferents fro the bladder wall are triggered as the bladder fills and the discharge to the spinal cord, causing; 1. Excitation of para outflow 2. Inhibition of symp outflow 3. Inhibition of somatic motoneurons to the external sphincter 4. Pathways to sensory cortex relate sensation of fulness Essentially micturition is a spinal reflex which may or may not be influenced by higher centres

Question 34

What is the volume of urine required in the bladder to trigger the spinal reflex of micturition?

Answer 34

300-350mls

Question 35

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?

Answer 35

Pathways from many brain centres including the cortex and the brainstem do the following two things: 1. 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 2. 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

Question 36

What muscle expels remaining urine in the bladder at the end of micturition?

Answer 36

The bulbocavernous muscle

Question 37

What is a common cause of abnormalities of micturition?

Answer 37

Neural lesions - generally bladder contractions are still stimulated but not sufficiently to empty the bladder, leaving urine in the bladder in stasis