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Overview and Physiology of the Urinary System

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The kidneys, collecting systems of the renal pelvis, ureters, urinary bladder, and urethra constitute the urinary system. There are two sphincters, one involuntary and the other having both voluntary and involuntary components. The sphincter at the ureterovesical junction is formed by the oblique passage of the ureter through the bladder wall; detrusor muscle tone compresses the ureter preventing reflux of urine from the bladder. The other sphincter is at the bladder outlet as it passes through the urogenital septum. The urethral sphincter has an involuntary smooth muscle portion under parasympathetic and sympathetic control, and voluntary striated muscle innervated via the lumbosacral plexus. The kidney filters the blood at the glomerulus, reabsorbs and secretes solutes and fluid in the renal tubules, and concentrates the urine in the medullary collecting ducts. The urine passes down the ureters to the bladder by gravity and peristaltic contractions to fill the hollow urinary bladder. Active relaxation of detrusor muscle in the bladder wall maintains low pressure within the bladder until its capacity is reached. Further filling occurs by stretching the bladder wall at rapidly increasing pressures. Continence is maintained by tonic contraction of the smooth muscle sphincter and active inhibition of detrusor contraction. Voiding is a complex process involving simultaneous, coordinated relaxation of the urethral sphincters and contraction of the detrusor muscle.

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Anatomy of the Urinary System

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The kidneys lie posteriorly, partially under the 11th and 12th ribs and lateral to L1–4 (Fig. 9-2). They are enclosed in a tight capsule and lie retroperitoneally, surrounded by Gerota fascia. The ureters course retroperitoneally descending over the psoas muscle and into the pelvis where they first run laterally and then anteriorly to enter the inferior portion of the bladder on either side of the midline. The bladder lies anteriorly in the pelvis behind and below the symphysis pubis. The urethra exits the bladder through the urogenital diaphragm formed by pelvic floor muscles to enter the male prostate and penis or the female perineum. In males, the proximal urethra is surrounded by the prostate gland and receives secretions from the prostate and seminal vesicles. The female urethra is quite short. Only the urethral meatus is visible on physical examination and normally the deeper structures cannot be identified by palpation.

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Physical Examination of the Urinary System

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See also The Abdomen, Chapter 9; The Female Genitalia and Reproductive System, Chapter 11; and The Male Genitalia and Reproductive System, Chapter 12.

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Determination of the Post-Void Residual Urine Volume
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The adequacy of bladder emptying is measured by the volume of urine remaining in the bladder after a full voluntary voiding: the residual volume. The post-void residual volume can be estimated by ultrasonography or measured directly by passing a urethral catheter into the bladder. The risk for bladder infection increases sharply with ...

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