Porth's Essentials of Pathophysiology, 4e

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Disorders of the Bladder and Lower Urinary Tract

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referred to as the internal urethral sphincter, are a continuation of the detrusor muscle. They run down obliquely behind the proximal urethra, forming the pos- terior urethra in males and the entire urethra in females. When the bladder is relaxed, these circular muscle fibers are closed and act as a sphincter. When the detrusor muscle contracts, the sphincter is pulled open as the shape of the bladder changes as urine is forced into the urethra. Another muscle important to bladder function is the external sphincter, a circular muscle composed of stri- ated skeletal muscle fibers that surrounds the urethra distal to the base of the bladder 1,2 (see Fig. 27-1). The external sphincter operates as a reserve mechanism to stop micturition when it is occurring and to maintain continence in the face of unusually high bladder pres- sure. The skeletal muscle of the pelvic floor also contrib- utes to the support of the bladder and the maintenance of continence. Neural Control of Bladder Function To maintain continence, or retention of urine, the blad- der must function as a low-pressure storage system, with the pressure in the bladder being lower than that in the urethra. To ensure that this condition is met, the increase in intravesical pressure that accompanies bladder filling is almost imperceptible. 5,6 Abnormal sustained eleva- tions in intravesical pressures often are associated with vesicoureteral reflux (i.e., backflow of urine from the bladder into the ureter) and the development of ureteral dilation (see Chapter 25, Fig. 25-7). Although the pres- sure in the bladder is maintained at low levels, sphincter pressures remains high, preventing loss of urine as the bladder fills. Micturition, or urination, involves the activity of both sensory and motor neurons. When the bladder is distended to 150 to 250 mL in the adult, the first sen- sation of fullness is transmitted to the spinal cord and then to the cerebral cortex, and at 350 to 450 mL there is a definite sense of bladder fullness. 5 During the act of micturition, the detrusor muscle of the bladder fun- dus and bladder neck contract down on the urine and the ureteral orifices are forced shut. The bladder neck is widened and shortened, and the external sphincter relaxes as urine moves out of the bladder. Descent of the diaphragm and contraction of the abdominal muscles raise intra-abdominal pressure and aid in the expulsion of urine from the bladder. Normal bladder function requires coordinated inter- actions between the sensory and motor components of both the autonomic nervous system (ANS), which controls involuntary smooth muscle activity, and the somatic nervous system, which controls voluntary skel- etal muscle activity. 5–7 The motor components of the neural reflex that causes bladder emptying is controlled by the parasympathetic division of the ANS, and the relaxation and storage functions of the bladder is con- trolled by the sympathetic division. The somatic nervous system innervates the skeletal muscles of the external

Epithelium when bladder is empty

Epithelium when bladder is full

Detrusor muscle

Ureters

Trigone

Internal sphincter

External sphincter

Bladder Structure The bladder, also known as the urinary vesicle , is com- posed of four layers: an outer serosal layer covers the upper surface of the bladder and is continuous with the peritoneum; beneath it is an external network of smooth muscle fibers called the detrusor muscle; this is followed by a submucosal layer formed largely of connective and elastic tissue; and an innermost layer of transitional epi- thelium, often referred to as the urothelium . 1,3 Stretching of the bladder wall as a result of distention with urine is accomplished by flattening of its numerous mucosal folds and thick transitional epithelium (see Fig. 27-1). The urothelium is essentially impermeable to urine solutes and water. The tonicity and composition of the urine often is quite different from that of the blood, and the urothelium acts as an effective barrier to prevent the passage of water and other urine elements between the bladder and the blood. 4 The surface of the urothelium is covered with a mucin layer that is thought to act as a nonspecific anti-adherence factor and as a defense mech- anism against infection. The urothelium not only acts as barrier between the bladder contents and the underlying bladder tissues but also as a sensory organ by transmit- ting physical and chemical information to afferent sen- sory neurons in the underlying smooth muscle fibers. 4 It has been suggested that conditions such as overac- tive bladder and interstitial cystitis are associated with alterations in the function of this sensory system. The detrusor muscle is the muscle of micturition or urination. When it contracts, urine is expelled from the bladder. 1,2 Muscles in the bladder neck, sometimes FIGURE 27-1. Diagram of the bladder, showing the detrusor muscle, ureters, trigone area, and urethral orifice. Note the folding of the epithelial (urothelial) cells when the bladder is empty and flattening of the cells when the bladder is full and the wall is stretched.