Renal assessment and micturition

Question 1

Why is it important to assess renal function?

Answer 1

Because of the kidney’s central role in homeostasis

Question 2

Which 2 clinical situations is the measurement of GFR particularly useful?

Answer 2

1. Disease progression of patients with renal disease. The underlying disease process results in nephron destruction and decreased nephron function.

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

2. Drug toxicity. Many drugs are removed from the body by excretion by filtration. When GFR falls, excretion falls - drug conc in plasma may rise causing toxicity

Question 3

Describe clearance tests

Answer 3

Measure the ability of the plasma to clear various substances

Clearance concerns the volume of plasma cleared and not a quantity of substance removed from the plasma

Question 4

What is the equation for the plasma clearance of X?

Answer 4

CX= [UX] V/[PX]

Ux = urine concentration of X

V = Urine flow rate

Px = Plasma concentration of X

Question 5

Which molecule is often used for plasma clearance tests?

Answer 5

Inulin

In clinical practice, inulin is no longer used, because too cumbersome, 51Cr-EDTA has been used instead, a suitable radioactive substance that is handled by the kidney in the same way as inulin.

Question 6

What makes inulin an effective measurer of GFR?

Answer 6

Freely filtered at the glomerulus

Not reabsorbed or secreted

Not metabolised by the kidney

Doesn’t interfere with normal renal function

Question 7

What is the ‘normal’ GFR in man?

Answer 7

125 mls/min

Depends on surface area, but values in women are about 10% lower, even after correction for surface area

GFR declines by
about 1ml/min/year after 30.

Question 8

How does secretion and reabsorption alter clearance rate?

Answer 8

Reabsorbed - lower clearance rate

Secreted - higher clearance rate

Question 9

GFR is too complex and expensive to measure, it takes several hours and an injection of isotope 51 Cr EDTA - what is the clinical replacement?

Answer 9

Plasma creatinine (a breakdown product of muscle creatine)

Question 10

What’s the big caution with using Creatinine

Answer 10

Plasma creatinine decreases as GFR increases

BUT

It is not a linear equation

The GFR can halve before there is an increase in creatinine

Creatinine plasma concentration is therefore taken into account alongside other variables to make up the estimated GFR

Question 11

What factors affect serum creatinine?

Answer 11

Muscle mass: athletes vs malnutrition

Dietary intake: creatinine supplements vs vegetarians

Drugs: Some lead to spurious increases as does ketoacidosis.

Question 12

What is normal GFR?

Answer 12

Normal GFR is approximately 100mls/min/1.73m2

Question 13

What is the clearance value of Glucose?

Answer 13

0 because normally all of it is reabsorbed

Question 14

What is the clearance of urea?

Answer 14

Less than that or inulin because 50% urea is reabsorbed

Question 15

What is used to measure real plasma flow (RPF)?

Answer 15

The organic anion para-amino-hippuric acid (PAH)

Question 16

What happens to para-amino-hippuric acid (PAH) in the kidney?

Answer 16

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.

Question 17

What is PAH clearance a measure of?

Answer 17

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

Question 18

How does urine flow from the kidney to the bladder?

Answer 18

Urine flows from the kidneys to the ureters via peristaltic contraction of the smooth muscle of the ureters, they enter the bladder at an oblique angle

Question 19

Why do the ureters enter bladder at an oblique angle?

Answer 19

Prevents reflux of urine

Question 20

Does the composition of urine does change once it leaves kidneys?

Answer 20

No

Question 21

What’s the name given to the smooth muscle found in the wall of the bladder?

Answer 21

Detrusor muscle

Question 22

When the smooth muscle of the bladder is relaxed what does the internal urethral ‘sphincter’ do?

Answer 22

The smooth muscle at the start of the urethra acts as an internal sphincter and passively contracts when the bladder is relaxed/fills

Question 23

What are the true and fake urethral sphincters?

Answer 23

Fake - internal sphincter

True - external sphincter

Question 24

What is the external sphincter made up of?

Answer 24

Skeletal muscle

Question 25

What control is the external sphincter under?

Answer 25

Voluntary somatic control

Question 26

The bladder lies anterior to which structures?

Answer 26

The reproductive system and rectum

Question 27

What makes the trigone?

Answer 27

The 2 vesicoureteric openings and the urethral opening makes a smooth triangle

Question 28

Unilateral renal problems occur when which structure is obstructed?

Answer 28

Ureter

Question 29

Bilateral renal problems occur when which structure is obstructed?

Answer 29

Urethra

Question 30

Describe the pressure-volume curve of the bladder?

Answer 30

Has a long flat segment as the initial increments of urine enter the bladder

Then a sudden sharp rise as the micturition reflex is triggered (urgency)

Question 31

How does the external urethral sphincter stay contracted?

Answer 31

Due to higher CNS input causing tonic discharge of impulses to the bladder

Question 32

Motor control of micturition

Answer 32

Rich parasympathetic supply (pelvic nerves) increase activity => increased contraction of detrusor muscle =>
increased pressure within the bladder, S2-S4

Sparse sympathetic supply (hypogastric nerves)

Somatic motoneurones (pudendal nerves), innervate the skeletal muscle that forms the external urethral sphincter, keeps the sphincter closed, even against strong bladder contractions S2-S4.

Question 33

Main function of sympathetic supply (hypogastric nerves) in the bladder?

Answer 33

Main function is to prevent reflux of semen into the bladder during ejaculation L1-L3.

Question 34

Describe ensory innervation to the bladder by stretch receptors

Answer 34

Stretch receptor afferents from the bladder wall. As the bladder fills there is increased discharge in afferent nerves to spinal cord via interneurones

Question 35

Which nerves are responsible for contraction of the detrusor muscle?

Answer 35

S2 - S4

Increases the pressure within the bladder

Question 36

How does the hypogastric nerve influence micturition?

Answer 36

Inhibits bladder contraction and closes the internal urethral sphincter

Cutting the hypogastric nerve causes increase in frequency of micturition

Main function is to prevent reflux of semen into the bladder during ejaculation- L1-L3

Question 37

What happens when stretch receptor afferents start increasing discharge to spinal cord via interneurones?

Answer 37

Excitation of parasympathetic outflow

Inhibition of sympathetic outflow

Inhibition of somatic motoneurones to external sphincter

Pathways to sensory cortex - sensation of fullness

Micturition is basically a spinal reflex which may or may not be influenced by higher centres.

Question 38

Why do babies wet themselves?

Answer 38

Micturition reflex serves to stimulate parasympathetic contraction and relaxes the external sphincter by inhibiting somatic motoneurones

Higher brain connections have not yet been established

This is also the case in adult patients with spinal cord transection after the initial period of spinal shock.

Question 39

What quantitiy of urine in the bladder is enough to initiate the spinal reflex?

Answer 39

300 - 350 mls

Question 40

Which parts of the brain are involved in delay of voiding?

Answer 40

Descending pathways from many brain centres including the cortex and the brainstem

Question 41

What is the effect of descending pathways which cause delay?

Answer 41

They inhibit the parasympathetic and:

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

–

Question 42

What does voluntary initiation of micturition involve

Answer 42

Descending pathways that stimulate the parasympathetic and:

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

Question 43

How are pelvic floor muscles involved in micturition?

Answer 43

Relaxation of pelvic floor muscles can cause sufficient downward tug on the detrusor muscle to initiate its contraction

Question 44

Which muscles cannot be contracted voluntarily? Why?

Answer 44

Perineal muscles and external sphincter muscles

To prevent urine flow flowing down the urethra or interrupting the flow once urination begins

Question 45

After urination, how does the female urethra empty?

Answer 45

By gravity

Question 46

How is urine remaining in the male urethra removed after urination?

Answer 46

It is expelled by contractions of the bulbocavernosus muscle

Question 47

3 major abnormalities of micturition due to neural lesions

Answer 47

Interruption of afferent nerves

Interruption of both afferent and efferent nerves

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 48

What is a mild mass reflex?

Answer 48

Some paraplegic patients train themselves to initiate voiding by pinching or stroking their thighs creating a mild mass reflex

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.