Bladder physiology and micturition

Bladder filling & emptying

Bladder filling

• Empty bladder
  • 0 - intravesicular pressure
• 30-50 ml urine in bladder
  • pressure: 5-10cm of water
• 200-300 ml urine in bladder
  • pressure rise minimum
• more than 300ml urine
  • rapid increase in pressure

First sensation of bladder filling is experienced at a volume of 100 – 150 ml in an adult. Then the 1st desire to void/urinate is when the bladder contains about 150-250 ml of urine. A person becomes uncomfortably aware of a full bladder when the volume is 350-400 ml. The volume of urine that normally initiates a reflex contraction is about 300-400 ml. An increase in volume to 700 ml creates pain and loss of control!

Cystometrogram

• A plot of intravesical pressure vs volume of fluid in the bladder
  • Ia - initial slight rise in pressure
  • Ib - long nearly flat segment
    • Law of Laplace
    • Pressure in a spherical viscus is equal to twice the wall tension divided by the radius
    • P = 2T / r
    • Due to its ability to stretch, T (Tension) increase will cause r (radius) increase as well, therefore P (pressure) will not increase much.
    • This gives us the ability to fill the bladder beyond 300ml
  • II - sudden sharp rise in pressure ( micturition reflex triggered)

Micturition Reflex

A single complete cycle of:

• Progressive & rapid increase of pressure
• A period of sustained pressure
• Return of the pressure to the basal tone of the bladder

Receptor

• Sensory stretch receptors in bladder wall
  • receptors in the posterior urethra are highly sensitive to stretch when the area begins to fill with urine at the higher bladder pressures

Afferents

• through pelvic nerves

Center

• Spinal cord
• Pons
• Medulla
• Hypothalamus
• Cortex

Efferents

• through pelvic nerves

Effector organ

• Detrusor muscle
• Internal & external sphincters

Micturition Reflex - Process

• Sensory signals from the bladder stretch receptors sent to spinal cord
• Parasympathetic efferent fibers discharge
  • produce ‘micturition contractions’
  • initially die out immediately, but with increasing pressure in bladder these become stronger & last longer & more frequently
  • Reflex will not empty the bladder always
    • may become inhibited for a period of few minutes to one hour before another reflex occurs
    • ‘self-regenerative’
• Urination occur when
  • powerful micturition reflex
  • cortical inhibition removed
    • relax external sphincter
  • relaxation of pelvic floor muscles
  • Increase intra-abdominal pressure
    • by contracting abdominal muscles
  • Relaxation of internal & external sphincters

Higher Centers for Micturition

The higer centers keep the micturition reflex partially inhibited, except when desired/during urination. It can prevent micturition, even if the micturition reflex occurs, by tonic contractions of the external sphincter until it is convenient to urinate. When it is time to urinate, thecortical centers can facilitate the sacral micturition centers to help initiate the external  urinary sphincter so that urination can occur.

Cerebral lesions (like tumors, Parkinsons, vascular accident) are known to affect the perception of bladder sensation and can result in voiding dysfunction (loss of control/dribbling)

• Facilitatory areas
  • Pons
  • Posterior hypothalamus
• Inhibitory areas
  • Cerebral cortex
  • Midbrain

Micturition in young children

• micturition is purely reflex
  • occurs whenever the bladder is sufficiently distended
    • myelination not complete
• About 2 1/2 years old, it begins to come under cortical control
• About 3 years old, complete control is achieved
  • cortex takes over control