Measurement of Body Water and Micturition

Question 1

What is the capacity of the urinary bladder?

Answer 1

moderately full - 500 ml

max. - 800 ml

Question 2

Describe the parasympathetic innervation of the bladder?

Answer 2

• Via pelvic nerves from sacral segments (S2 to S4)
• Innervate the bladder wall
• Pelvic nerves also transmit sensory fibres – from receptors that detect bladder fullness, pain and temperature sensation

Question 3

Describe the sympathetic innervation of the bladder?

Answer 3

• Via hypogastric nerves from the lumbar (L1 and L2) segments of spinal cord
• Innervate smooth muscles of the bladder neck via α-adrenergic receptors

Question 4

Describe the somatic innervation of the bladder?

Answer 4

• via pudendal nerves
• originates from motor neurones from segments S2 to S4
• innervate and control voluntary skeletal muscle of external sphincter

Question 5

Describe the innervation of the detrusor muscle during filling and micturition?

Answer 5

parasympathetic - causes contraction

1. during filling - inhibited
2. during micturition - stimulated

Question 6

Describe the innervation of the internal urethral sphincter during bladder filling and micturition?

Answer 6

sympathetic - causes contraction

1. during filling - stimulated
2. during micturition - inhibited

Question 7

Describe the innervation of external urethral sphincter during filling and micturition?

Answer 7

somatic motor - causes contraction

1. during filling - stimulated
2. during micturition - inhibited

Question 8

How does urine move into the bladder?

Answer 8

by peristaltic waves originating in the pelvis through to ureter

Question 9

What prevents the backflow of urine into the ureter?

Answer 9

ureterovesical valve

Question 10

Describe the state of the muscles of the urinary system during filling?

Answer 10

internal sphincter muscle is tonically contracted while detrusor muscle is flaccid

Question 11

What is micturition?

Answer 11

The process of emptying of urine from the bladder
involving:
1. contraction of detrusor muscle, muscles of abdominal wall and pelvic floor
2. relaxation of external urethral sphincter

Question 12

What is continence?

Answer 12

the ability of the bladder to retain the urine
- Continence is maintained by the influence of descending fibres from the pons (in the brain stem) that suppress the firing of parasympathetic efferents to the detrusor muscle

Question 13

What is incontinence?

Answer 13

the inability of the bladder to retain urine

Question 14

Describe the micturition reflex?

Answer 14

1. As the radius of the bladder increases due to bladder filling (from ≈ 200 ml), the wall tension increases and activates stretch receptors
2. The resulting afferent firing is conveyed to the sacral spinal cord via the pelvic nerve
3. Efferent impulses from the sacral spinal cord are sent back to the bladder through the parasympathetic nerve fibres
4. As bladder continues to fill micturition reflexes become more frequent and cause greater contractions of detrusor muscle
5. Once the micturition reflex becomes powerful enough it causes another reflex, which passes through the pudendal nerves to the external sphincter to inhibit it
   - if this inhibition is more potent than the voluntary control from the brain then, urination will occur
   - If not urination will not occur until the bladder fills even further and the micturition reflex becomes more powerful

Question 15

What is the function of the external sphincter?

Answer 15

Contraction of the external sphincter prevents urinary leakage during the bladder pressure waves

Question 16

Describe the spinal reflex of the micturition reflex?

Answer 16

• Parasympathetic stimulation causes bladder to contract
• Internal sphincter opens
• External sphincter relaxes due to inhibition

Question 17

Describe the facilitation/inhibition of micturition by the brain?

Answer 17

• Micturition reflex is an autonomic spinal reflex but can be inhibited or facilitated by centres in the brain
• Located in the pons and several centres in the cerebral cortex
• Higher centres keep the micturition reflex inhibited except when micturition is desired

Question 18

What is the principle for measuring body fluid volumes?

Answer 18

indicator-dilution principle

Question 19

What is the method for measuring body fluid volumes?

Answer 19

1. Administer a known amount (A) of a substance that will get distributed in a particular body fluid compartment
2. Determine the final steady state concentration (C) of this indicator in blood
3. Volume of distribution Vd of the indicator =
   Amount of indicator injected (A)/ Concentration in blood after mixing (C)

Question 20

What is the technique for measuring red cell volume?

Answer 20

indicator dilution principle

Question 21

Describe the technique for measuring red cell volume?

Answer 21

1. Use Chromium 51 labeled RBC as the indicator
2. Inject a known amount of Cr 51 labeled RBC intravenously
3. Allow them to mix with RBC in blood
4. Measure the fraction of RBC tagged with Cr 51

Question 22

What is the technique for determining body fat?

Answer 22

bioelectric impedance technique

Question 23

Describe the principle behind determining body fat?

Answer 23

• Body fluids conduct electricity well
• But fat is anhydrous and therefore is a poor conductor of electricity
• The resistance to flow of a small current between points on the body is proportional to fat mass