The urinary bladder is a muscular organ in the pelvis that stores normally sterile urine prior to micturition. In the healthy adult, the volume of the bladder is typically 300–500 mL. The bladder receives urine (at approximately 1 mL/min) from the kidneys via two muscular ureters (Chapter 36) that enter the bladder low down near the neck. The ureters drain into the bladder via vesico-ureteric junction, which ensures that flow is unidirectional and that there is no reflux of urine from the bladder to the kidneys, which could lead to infection. The bladder then stores urine prior to its elimination through the external sphincter and then via the urethra (Chapter 36).
The bladder is made up of the following.
- Detrusor muscle: this is smooth muscle in the body of the bladder. When relaxed, it enables the bladder to hold urine and when it contracts there is an increase in internal pressure and so urine is voided.
- Trigone muscle: this is triangular smooth muscle at the base of the bladder at the urethral opening.
- Urothelium: the inner transitional epithelial layer of the bladder, which may release mediators (such as ATP) to regulate tone.
Autonomic control (Figure 40.1)
The key innervation of the detrusor muscle is parasympathetic via acetylcholine acting at muscarinic M3 receptors, which leads to contraction and micturition. Sympathetic innervation results in noradrenaline acting at β2– and β3-adrenoceptors to relax the detrusor muscle, resulting in the bladder relaxing and storing urine. Noradrenaline also acts at α1-adrenoceptors to contract the neck of the bladder and urethra to facilitate the storage of urine.
Somatic control
The external sphincter is made up of skeletal muscle associated with the urogenital diaphragm and is regulated via the somatic nervous system such that acetylcholine acts at nicotinic receptors to cause contraction of the urethra.
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