Functions of the urinary bladder
Urinary bladder provides an expandable reservoir for urine, which is continuously flowing from pelvis of the kidney through ureters.
Micturition
Micturition is the process in which the urinary bladder empties when it becomes filled with urine.
Passage of Urine from the kidney to the bladder
Urine is secreted continuously though at a varying rate and it passes through the collecting ducts into the pelvis of the kidney and carried to the urinary bladder by the ureters. The ureters contain muscles capable of contraction, which helps in propulsion of urine along the tube into bladder. Each ureter is innervated by both sympathetic and parasympathetic nerves. As urine collects in the pelvis, pressure increases which initiates peristaltic contraction beginning from pelvis and spreading down along the ureters to force urine towards bladder.
Parasympathetic stimulation increases frequency and sympathetic stimulation decreases frequency of peristalsis of ureters.
Filling and emptying of bladder (Micturition Reflex)
At the junction of ureter with bladder, an ureterovesicular valve is present which prevents reflux of urine from bladder. The urinary bladder possess the power of accommodation to increase in its contents without increase in internal pressures up to about 150 mm H2O. So, as urine enters the bladder, its walls becomes distended. Out flow of urine into the urethra is prevented by sphincters at the neck of the bladder (internal and external sphincters).
When the pressure in the bladder reaches 150 mm water, contraction of bladder wall begins, relaxation of sphincter occurs and urination or micturition occurs. Contraction of abdominal muscles and contraction of diaphragm assist the emptying of the bladder. The urinary reflex may be assisted by voluntary effect; may also be opposed voluntarily. This is accompanied by inhibition of spinal centres of micturition and by contraction of external sphincter which surrounds the external part of the urethra. The desire to urinate arises from stimulation of receptors in the wall of the bladder by stretch and contraction of musculature.
Nerves from sympathetic and parasympathetic divisions of ANS supply the bladder. The preganglionic sympathetic fibres leave from spinal cord in 2nd to 4th lumbar nerves to the posterior mesentric ganglion and postganglionic fibres reach the bladder through hypogastric nerves.
In all mammals, parasympathetic nerves cause contraction of whole bladder and they are main motor nerves. They are inhibitory to internal sphincter.
Complete emptying of the bladder depends upon maintenance of bladder muscle contraction and sphincter relaxation. This is achieved by two reflex systems.
Receptors are present in the bladder walls that are stimulated during contraction of urinary bladder. Another reflex arises from receptors in the wall of urethra. Urine flow through urethra helps to maintain bladder contraction and relaxation of external sphincter.
The tone of the bladder decreases as the bladder becomes emptied of its contents and this is accompanied by contraction of sphincter. The cessation of micturition involves voluntary control and voluntary regulation.