7 Process of urination

Question 1

What should be mentioned?

Answer 1

Upper urinary tract
Lower urinary tract
Process in bladder: emptying and filling
Urination

Question 2

Upper urinary tract

Answer 2

The upper urinary tract consists of the calices, pelvis of the kidney and the ureters with the spiral smooth muscle fibers (syntitium formation).

• Rhythmical contraction of the calix forwards the urine into the renal pelvis. This goes on periodically in the case of basic level urine formation and becomes continuous only in case of increased urine formation. To ensure normal function of nephrons the pressure in the renal pelvis is low.
• Peristalsis of the ureter forwards the urine. Under normal urine formation speed (2ml/min) the wall of the ureter is shrunk, collapsed and only periodically opens, one “bolus” at a time is forwarded to the bladder.
• The ureter enters the bladder wall in such an angle, which automatically closes the way of urine backstream to the kidney from the filling bladder. There is no urinal reflux in normal case. In the case of difficulties of urination, the retrograde flow can occur, leads to danger of infection.

Question 3

Lower urinary tract

Answer 3

Consists of the urinary bladder and urethra.

• The wall of the bladder consists of special smooth muscle elements and elastic fibers, which react to stretch by relaxation.
• Shape of the bladder depends on the volume of urine. Ureter openings in trigonum vesicae, internal urethral orfice and sphinchter at other location. Its adaptable to the uptake of urine.
• During filling the muscles of the bladder are under sympathetic efferentation. The bladder wall is loosened, the sphincter is in a contracted stage.
• During urination the parasympathetic dominance prevails. The bladder wall is contracted, the sphincter dilates.

Question 4

Process in bladder: emptying

Answer 4

• The emptying of the bladder is controlled by the urinary centers in the pons, based upon the information arriving from the mechanoreceptors of the bladder.
• Execution is carried out via the lumbal (sympathetic) and sacral (parasympathetic), and the appropriate somatic motor system (abdominal muscles, perineum and outer sphincter).

Question 5

Process in bladder: filling

Answer 5

• Parasympathetic activity responsible for the contraction of the wall of the bladder(m. detrusor) is presynaptically inhibited by the sympathetic activity.
• The same synapathetic basic tone relaxes the detrusor muscles and contracts via the alpha1 receptors of the smooth muscles of the bladder neck.
• The outer urethral musculature consisting of striated muscles, as a consequence of increased AP release (n.pudendus, n-Ach-R) is also in a contracted state.

Question 6

Urination

Answer 6

Urine is produced continuously in the kidney. Its emptying from the organism is a periodical process. This is facilitated by the fact that the continuously produced urine is stored in the urinary bladder from which urination takes place.

Increased mechanoreceptor activity during urination (suddenly increasing AP frequency) affects the center in the pons, which increases significantly the parasympathetic activation and inhibits both the somatic and sympathetic activity:
• The bladder wall contracts, sphincters relax, urination begins.
• Center in the pons is connected to the higher cerebral areas (cortex, hypothalamus), which can exert a certain inhibition on its activity: in trained animal the retention of urine can be achieved under certain bladder tension earlier causing urination.