Physiology of Micturition and Assessment of Renal Function

Question 1

Why is it important to be able to asses renal function?

Answer 1

It has a central role in homeostasis

Question 2

In what situations is the ability to measure GFR useful?

Answer 2

1) In patients with renal disease to measure the disease progression
2) Many drugs are removed by excretion at the kidneys. If GFR falls then the excretion falls and the plasma concentration of the drug may rise causing toxicity

Question 3

How is the renal function measured?

Answer 3

Plasma clearance tests: these measure the ability of the kidney to clear the plasma of various substances

Question 4

What is the equation for plasma clearance?

Answer 4

Cx = [Ux]V/[Px]
Ux - urine concentration of x
V = urine flow rate
Px = plasma concentration of x

Question 5

Which test is the gold standard for measuring renal clearance?

Answer 5

Inulin clearance. IV dose, given time to equilibrate and then plasma and urine are measured simultaneously

Question 6

What happens to inulin in the kidneys?

Answer 6

It is freely filtered but not reabsorbed or secreted

Question 7

What will happen to the clearance of substances that are filtered and reabsorbed?

Answer 7

The clearance will be decreased

Question 8

What is the normal GFR in males and is it different in females?

Answer 8

125ml/min

In females it is 10% lower

Question 9

Why is inulin no longer used in clinical practice?

Answer 9

It is too cumerbersome and time consuming

Question 10

What is used instead of inulin to estimate GFR?

Answer 10

Creatinine (clearance is similar to inulin)

Question 11

What is eGFR?

Answer 11

Estimated GFR

Question 12

What factors affect serum creatinine?

Answer 12

Muscle mass (more muscle = more creatinine), dietary intake (creatinine) supplements vs vegetarians) and drugs (some cause increases as does ketoacidosis)

Question 13

What is the clearance of glucose in a healthy kidney?

Answer 13

Zero - normally all glucose is reabsorbed

Question 14

How does the clearance of urea compare to the clearance of inulin?

Answer 14

Less - some urea is reabsorbed (50%)

Question 15

Which substance is used to measure real plasma flow and why?

Answer 15

PAH - 90% of the plasma is cleared of its PAH in one transit of the kidneys

Question 16

How does the clearance of penicillin clearance compare to inulin clearance?

Answer 16

It is greater because the penicillin is filtered and secreted

Question 17

What causes the urine to flow from the kidneys to the ureters?

Answer 17

Peristaltic contraction of the smooth muscle of the ureters

Question 18

Does the urine composition change after it has left the kidneys?

Answer 18

No

Question 19

Which muscle is responsible for emptying the bladder during micturition?

Answer 19

Detrusor muscles (smooth muscle arranged in spirals, longitudinal and circular bundles)

Question 20

Which sphincter is a true sphincter, internal or external urethral?

Answer 20

External

Question 21

Where does the bladder lie?

Answer 21

In the midline posterior to the pubic bones

Question 22

What renal problems arise from urethral obstructions?

Answer 22

Bilateral problems

Question 23

What renal problems can arise from ureter obstruction?

Answer 23

Unilateral renal problems

Question 24

What is the normal daily urine production in temperate climates?

Answer 24

750-2500ml

Question 25

What is the nerve control of micturition?

Answer 25

Parasympatetic control (pelvic nerves) - contraction of detrusor muscle
Sympathetic (hypogastric nerves) - inhibit bladder control and close the internal urethral sphincter
Somatic Motoneurones: innervate the skeletal muscle keeping the external urethral sphincter closed

Question 26

What is the sensory innervation of micturition?

Answer 26

Stretch receptors in the bladder wall

Spinal reflex controls micturition - may or may not be influenced by higher centres

Question 27

In which groups of people would there be no input fro the higher centres in micturition?

Answer 27

‘Leaky’ babies and adults with spinal cord transection

Question 28

How does the pathway from the brain delay micturition?

Answer 28

Inhibiting the parasympathetic and stimulates the somatic nerves

Question 29

How does voluntary initiation work?

Answer 29

Stimulates the parasympathetic and inhibits the somatic motor neurones

Question 30

Which muscles are involved in the initial events of micturition?

Answer 30

Muscles of the pelvic floor

Question 31

What are the 3 types of neural lesions that can cause abnormal micturition?

Answer 31

Interruption of afferent nerves, interruption of both afferent and efferent nerves and interruption of facilitatory and inhibitory descending pathways from the brain