Renal Assessment and Micturition

Question 1

Name two situations which is useful to measure GFR?

Answer 1

1. In Pt with renal disease, progression of underlying disease results in nephron destruction and decrease nephron function -> decrease GFR
2. Many drugs (digitals and antibiotics) are removed from body by excretion by filtration.
   When GFR falls, excretions decreases so that [drug] in plasma may rise causing toxicity -> therefore may need to adjust does appropriate to decrease in renal function

Question 2

How does progression of renal disease effect GFR?

Answer 2

Total GFR = sum of all filtration by functioning nephrons so progression of disease would be indicated by the reduction in GFR.

Question 3

Name a test used to measure renal function

Answer 3

Plasma clearance tests

Question 4

How do plasma clearance tests work?

Answer 4

They measure the ability of the kidney to clear the plasma of various substances

Clearance related to the volume of plasma cleared NOT a quantity of substance removed from the plasma

Question 5

What is the equation use to work out plasma clearance of a substance?

Answer 5

Plasma clearance of X, CX= [UX] V/[PX] : units are mls/min

UX = Urine concentration of X, 
V = urine flow rate, 
PX = plasma concentration of X

Question 6

What is another name for inulin?

Answer 6

Polyfructose

Question 7

What is the gold standard measurement of plasma clearance?

Answer 7

Using inulin

Give loading IV dose of inulin, allow time to equilibrate and then sample simultaneously plasma and urine (during a times urine sample)

Question 8

Why is using inulin the gold standard for measuring plasma clearance and thus GFR?

Answer 8

Inulin is freely filtered at the glomerulus and neither reabsorbed nor secreted.

It is not metabolised by the kidney, nor does it interfere with normal renal function so inulin clearance is a measure of GFR.

Question 9

What is the effect of plasma clearance of a substance in comparison to inulin if it is reabsorbed?

Answer 9

Will have a lower clearance than inulin, because [UX] will be less than if only filtered (as some has been reabsorbed) and [PX] higher (as some has been reabsorbed).

Question 10

What is the effect of plasma clearance of a substance in comparison to inulin if it is secreted as well?

Answer 10

Substances filtered and secreted will have a higher clearance than inulin because [UX] will be higher and [PX] lower.

Question 11

How does GFR decline with age?

Answer 11

Declines by 1ml/min/year after 30yrs old

Question 12

What is a new substance used to measure GFR in clinical practice rather than inulin?

Answer 12

51Cr-EDTA - radioactive substance that is handled by the kidney in the same way as inulin

Question 13

As GFR measurement takes hours, what substance can be measured to estimate GFR?

Answer 13

Creatinine

Question 14

Name three factors affecting serum creatinine

Answer 14

• Muscle mass: athletes vs malnutrition
• Dietary intake: creatine supplements vs vegetarians
• Drugs: Some lead to spurious increases as does ketoacidosis.

Question 15

What is the normal GFR?

Answer 15

100mls/min/1.73m^2

Question 16

What is the plasma clearance of glucose?

Answer 16

0 because all is normally reabsorbed

100mL, 100% glucose reabsorbed and non-excreted -> Glc clearance = 0mL/min

Question 17

What is the plasma clearance of urea?

Answer 17

Less than inulin as some is reabsorbed

100mL, 50% of urea reabsorbed and 50% of urea excreted -> urea clearance = 50mL/min

Question 18

What substance is used to measure renal plasma flow (RPF)?

Answer 18

Para-amino-hippuric acid (PAH)

Question 19

Why is para-amino-hippuric acid sed to measure renal plasma flow?

Answer 19

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.

Therefore, PAH clearance is a measure of all the plasma flowing through the kidneys in a given time = renal plasma flow 660mls/min

Question 20

Why does penicillin have a greater clearance than inulin?

Answer 20

Because it is filtered AND secreted

Question 21

What does it indicate about the renal handling of X if filtration > excretion?

Answer 21

Net reabsorption of X

Question 22

What does it indicate about the renal handling of X if excretion > filtration?

Answer 22

Net secretion of X

Question 23

What does it indicate about the renal handling of X if filtration = excretion?

Answer 23

No net reabsorption or secretion

Question 24

What does it indicate about the renal handling of X if clearance of X is less than inulin?

Answer 24

Net reabsorption of X

Question 25

What does it indicate about the renal handling of X if clearance of X = inulin clearance?

Answer 25

X is neither reabsorbed nor secreted

Question 26

What does it indicate about the renal handling of X if clearance of X is greater than inulin clearance?

Answer 26

Net secretion of X

Question 27

How does ruin flow from kidneys through the ureters?

Answer 27

Peristaltic contraction of smooth muscle of ureters

Question 28

What angle do the ureters enter the bladder at and what is the significance of this?

Answer 28

Olique angle which prevents reflex of urine

Question 29

Contraction of what muscle is responsible for emptying of the bladder during micturition?

Answer 29

Detrusor muscle

Question 30

Is the internal urethral sphincter a true sphincter?

Answer 30

No, but it is where smooth muscle at the start of the urethra acts as a sphincter when the smooth muscle is relaxed

Question 31

Is the external urethral sphincter a true sphincter?

Answer 31

Yes, made up of skeletal muscle under voluntary control

Question 32

Are the symptoms of urethral obstruction bilateral or unilateral?

Answer 32

Bilateral

Question 33

Are symptoms of ureter obstructions bilateral or unilateral?

Answer 33

Unilateral

Question 34

What is a cystometrogram?

Answer 34

Pressure flow study

Question 35

Describe the pressure-volume curve of the bladder (cystometrogram)

Answer 35

There is a long flat segment as the initial increments of urine enter the bladder and then a sudden sharp rise as the micturition reflex is triggered.

Question 36

What is the normal daily urine production?

Answer 36

Varies between 750-2500mls

Question 37

Describe the motor innervation which controls micturition

Answer 37

• Parasympathetic increases contraction of detrusor muscle via S2,3,4 -> increase pressure of bladder
• Sympathetic (hypogastric L1-3) inhibit bladder contraction and closes internal urethral ‘sphincter’ -> mainly to prevent reflux of semen into bladder during ejactulation
• Somatic motor neurones (pudendal nerves S2-4) innervate skeletal muscle of external urethral sphincter, keeps sphincter closed

Question 38

What is the consequence of cutting the hypogastric nerve?

Answer 38

Increase frequency of micturition

Question 39

Describe the sensory innervation which initiates micturition

Answer 39

Stretch receptor afferents from bladder wall: as bladder fills -> increase discharge in afferents to spinal cord -> via interneurones - >

a) excitation of parasympathetic outflow
b) inhibition of sympathetic outflow
c) inhibition of somatic motoneurones to external sphincter
d) pathways to sensory cortex -> sensation of fullness

Question 40

Describe the local spinal reflex of micturition

Answer 40

As the bladder fills, it becomes distended and the stretch receptors are increasingly stimulated, until their output becomes great enough to cause bladder contraction via:

Stimulation of the parasympathetic and relax the external sphincter by inhibiting the somatic motoneurones.

Question 41

In what individuals is micturition controlled by the local spinal reflex?

Answer 41

In “leaky” babies as higher brain connections have to be established,

In adult Pts with spinal cord transection after initial period of shock. Vol of urine in bladder for reflex in 300-350mls

Question 42

How is delay in micturition established to override local spinal reflex (prevent incontinence)?

Answer 42

Descending pathways from many brain centres, including cortex and brainstem, which:

Inhibit the parasympathetic and stimulate the somatic nerves to the external sphincter, thus over-ridding the input from the bladder stretch receptors.

Question 43

What does voluntary initiation of micturition involve?

Answer 43

Stimulate the parasympathetic inhibit the somatic motor neurones thus summating with the stretch receptor effects.

• Potty training involves the setting up of these controlling pathways.

Question 44

Other than stimulation and inhibition of nerves, what else can help initiate micturition?

Answer 44

One of initial events in voluntary urination is relaxation of the muscles of the pelvic floor and this may cause a sufficient downward tug on the detrusor muscle to initiate its contraction.

Question 45

What muscles involved in micturition can be contracted voluntarily?

Answer 45

Perineal muscles and external sphincter, preventing urine flow flowing down the urethra or interrupting the flow once urination begins.

Question 46

After urination, how does the female urethra empty?

Answer 46

By gravity

Question 47

After urination, how does the male urethra empty?

Answer 47

Contractions of the bulbocavernosus muscle.

Question 48

Name three abnormalities of mictuirtion

Answer 48

1. Interruption of afferent nerves
2. Interruption of both afferent and efferent nerves
3. Interruption of facilitatory 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.

Question 49

What is the mild mass reflex?

Answer 49

Some paraplegic patients train themselves to initiate voiding by pinching or stroking their thighs

Question 50

How does the mild mass reflex work?

Answer 50

• After spinal section, afferent stimuli irradiate from one reflex centre to another.
• When a relatively minor noxious stimulus is applied to the skin it may irradiate to autonomic centres and evoke bladder or rectal voiding.
• Returns some measure of “voluntary” control by allowing them to induce an intentional mass reflex.

Question 51

What is the normal GFR?

Answer 51

125mls/min

Question 52

If creatinine clearance is 50ml/min, is this in its normal range?

Answer 52

No, as it is used to calculate eGFR, so would expect it to be 125mls/min

Question 53

Is a urine flow rate of 2.5mls/min above normal?

Answer 53

Yes;

2.5 x 60 x 24 = 3600ml/day

Normal range is 750ml-2500mls