Micturition

The urine that enters the calyces from the papillae is the final urine that will be excreted from the body, its composition having been determined by the functional characteristics of the nephrons. Urine is carried from the kidneys via the ureter to the bladder, where it is stored until it exits the body via the urethra (Fig. 5). The composition of the urine is altered little if at all by any of these structures. Nevertheless, the renal pelvis, ureters, bladder, and urethra are frequently the origin of urinary tract pathology. Obstruction of the urinary tract by calculi (stones) or tumors can lead to impaired renal function. The lower urinary tract is also frequently the site of infection, which may ascend the ureters and infect the kidneys. Finally, many neurologic and anatomic abnormalities can affect voiding of the

FIGURE 5 Schematic illustration of the bladder. The bladder wall is heavily invested with smooth muscle layers that provide the contractile force in voiding. The ureters enter the bladder on its posterior side, above the bladder neck in the region of the trigone called the ureterovesicular junction. The tension of the detrusor muscle in the bladder wall keeps the ureteral openings closed so that urine does not reflux toward the kidneys. When the pressure of a ureteral peristaltic wave exceeds the pressure in the bladder, this functional sphincter opens transiently, allowing urine to flow into the bladder. The muscle in the trigone region also forms part of the internal sphincter of the bladder neck. Beyond this, the urethra passes through the urogenital diaphragm, where voluntary muscle fibers form the external sphincter. Both the external and internal sphincters prevent urine movement out of the bladder until micturition is initiated.

FIGURE 5 Schematic illustration of the bladder. The bladder wall is heavily invested with smooth muscle layers that provide the contractile force in voiding. The ureters enter the bladder on its posterior side, above the bladder neck in the region of the trigone called the ureterovesicular junction. The tension of the detrusor muscle in the bladder wall keeps the ureteral openings closed so that urine does not reflux toward the kidneys. When the pressure of a ureteral peristaltic wave exceeds the pressure in the bladder, this functional sphincter opens transiently, allowing urine to flow into the bladder. The muscle in the trigone region also forms part of the internal sphincter of the bladder neck. Beyond this, the urethra passes through the urogenital diaphragm, where voluntary muscle fibers form the external sphincter. Both the external and internal sphincters prevent urine movement out of the bladder until micturition is initiated.

bladder (micturition) and lead either to incontinence or to urinary retention.

Urine that collects in the hollow structure of the renal pelvis is propelled through the ureter toward the bladder by peristaltic contractions of the renal pelvis and ureter, both of which are invested with smooth muscle in their walls. As in the case of other visceral smooth muscle, this action is enhanced by parasympa-thetic and inhibited by sympathetic innervation. The walls of the pelvis and ureters are also invested with pressure receptors that convey pain sensations to the central nervous system if pressure rises in these structures. This is the origin of the extreme pain associated with obstruction of the urinary tract by kidney stones. These receptors are also the sensory input to a reflex arc, the ureterorenal reflex, which produces a sympathetic efferent discharge to the kidney that reduces the rate of glomerular filtration.

As shown in Fig. 5, the ureters enter the bladder through the detrusor muscle in the trigone region. The normal tone of this muscle tends to occlude the ureter as it passes through the bladder wall, thus forming a functional valve referred to as the ureterovesicular valve, which prevents the backflow of urine from the bladder to the ureter. Each peristaltic wave along the ureter increases the pressure within the ureter so that the region within the bladder wall opens, allowing urine to flow into the bladder.

As urine enters the bladder, it distends. The volume of the bladder can increase from essentially zero after micturition to a maximum of about 500 mL in the adult. This is the largest volume change of any hollow structure in the body, and it is accompanied by thinning and an increase in the tension in the bladder wall that is proportional to the radius of the bladder according to the law of LaPlace. The increase in bladder wall tension as the bladder expands stimulates the firing of stretch receptors on afferent nerves. Expulsion of urine from the bladder is driven by contraction of the detrusor muscle. Normally this muscle is flaccid and allows the bladder to expand as it fills. At the tip of the trigone, the urethra exits from the bladder in a region called the bladder neck. Leakage of urine from the bladder during filling is prevented by contraction of the muscle fibers in what is referred to as the internal urethral sphincter. The internal sphincter is not a discrete anatomic structure but represents the combined effects of circular smooth and striated muscle fibers in the urethra to keep its lumen closed. The relative importance of different regions of the urethra in performing this sphincter function also differs between males and females. At the point where the urethra passes through the urogenital diaphragm, the striated muscles of the pelvic floor act as an external urethral sphincter. Voluntary or reflex contractions of these muscles increase the outflow resistance of the urethra and can prevent voiding even during strong constrictions of the detrusor muscle in the bladder wall.

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Get Rid of Gallstones Naturally

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