Porth's Essentials of Pathophysiology, 4e

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Kidney and Urinary Tract Function

U N I T 7

TABLE 27-1 Action of Drug Groups on Bladder Function Function Drug Groups

Mechanism of Action

Detrusor Muscle Increased tone and contraction Cholinergic drugs

Stimulate parasympathetic receptors that cause detrusor contraction

β 2 -Adrenergic blockers

Block β 2

Inhibition of detrusor muscle relaxation during filling

receptors that produce detrusor muscle

relaxation

Anticholinergic drugs and drugs with an anticholinergic action

Block the muscarinic receptors that cause detrusor muscle contraction May interfere with influx of calcium to support contraction of detrusor smooth muscle

Decreased tone

Calcium channel blockers

Internal Bladder Sphincter Increased tone

α 1 -Adrenergic agonists

Activate α 1

receptors that produce contraction of the

smooth muscle of the internal sphincter Block contraction of the smooth muscle of the internal sphincter Decrease the tone of the external sphincter by acting at the level of the spinal cord or by interfering with release of calcium in the muscle fibers

α 1 -Adrenergic blockers

Decreased tone

External Sphincter Decreased tone

Skeletal muscle relaxants

stream are suggestive of outflow obstruction. Palpation and percussion provide information about bladder distention. Postvoid residual (PVR) urine volume provides information about bladder emptying. It can be esti- mated by abdominal palpation and percussion. Catheterization and ultrasonography can be used to obtain specific measurements of PVR. A PVR value of less than 50 mL is considered adequate bladder empty- ing, and more than 200 mL indicates inadequate blad- der emptying. 8 Pelvic examination is used in women to assess peri- neal skin condition, perivaginal muscle tone, genital atrophy, pelvic prolapse (e.g., cystocele, rectocele, uter- ine prolapse), pelvic mass, or other conditions that may impair bladder function (see Chapter 40). Bimanual examination (i.e., pelvic and abdominal palpation) can be used to assess PVR volume. Rectal examination is used to test for perineal sensation, sphincter tone, fecal impaction, and rectal mass. It is used to assess the con- tour of the prostate in men. Urine tests provide information about kidney func- tion and urinary tract infections. The presence of bacte- riuria or pyuria suggests urinary tract infection and the possibility of urinary tract obstruction. 9 Blood tests (i.e., blood urea nitrogen and creatinine) provide information about renal function. Bladder structures can be visualized indirectly by tak- ing x-ray films of the abdomen and through excretory urography, which involves the use of a radiopaque dye, computed tomographic (CT) scanning, magnetic reso- nance imaging (MRI), or ultrasonography. Urodynamic studies are used to evaluate bladder function and void- ing problems. Cystoscopy enables direct visualization of the urethra, bladder, and ureteral orifices.

SUMMARY CONCEPTS

■■ Although the kidneys control the formation of urine, its storage and periodic elimination depend on the coordinated activity of the smooth and striated muscle of the two functional units of the lower urinary tract—the urinary bladder, which serves as a storage reservoir; and the urethra and urethral sphincter, which function as an outlet for urine elimination. ■■ The bladder is composed of four layers: an outer serosal layer, a smooth muscle layer called the detrusor muscle, a submucosal layer of connective and elastic tissue, and an inner epithelial layer.The detrusor muscle is the muscle of micturition or passage of urine.The inner epithelial layer prevents substances in the urine from moving into the bloodstream and its mucin layer acts as defense against infection. ■■ Normal bladder function requires interaction between the sensory and motor components of both the autonomic nervous system (ANS), which controls the involuntary smooth muscle activity of the detrusor muscle, and the somatic nervous system, which controls voluntary skeletal muscle activity of the external sphincter.These functions are controlled by three ascending levels of nervous system control: the spinal cord reflex