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The urinary system consists of the kidneys, collecting systems of the renal pelvis, ureters, urinary bladder, and urethra. Important sphincters are found at the ureterovesical junction, as the ureter passes obliquely through the bladder wall, and at the proximal urethra, as it passes through the urogenital septum. The urethral sphincter has an involuntary smooth muscle portion under parasympathetic and sympathetic control, and a voluntary striated muscle sphincter innervated via the lumbosacral plexus. The urinary system is designed to filter the blood at the glomerulus and to reabsorb and to secrete solutes and fluid across the renal tubules, and then to concentrate the urine in the medullary collecting ducts. The urine passes down the ureters to the bladder by gravity and peristaltic contractions. The bladder is a hollow muscular structure that actively relaxes with urinary distention keeping intravesical pressures low until capacity is reached. Further filling occurs by stretching of the bladder wall at rapidly increasing pressures. Continence is maintained by tonic contraction of the smooth muscle sphincter and active inhibition of detrusor muscle contraction as the bladder fills. Voiding is a complex process involving simultaneous, coordinated relaxation of the urethral sphincters and contraction of the previously relaxed detrusor muscle.

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The kidneys lie posteriorly, partially under the eleventh and twelfth ribs and lateral to L1–4 (see Fig. 9–2). They are enclosed in a tight capsule and lie retroperitoneally, surrounded by Gerota's fascia. The ureters course retroperitoneally, and descend over the psoas muscle and into the pelvis, where they 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 and runs through the urogenital diaphragm of the 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 prostatic secretions from the prostate and seminal vesicles. In the female, the 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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See also The Abdomen, Chapter 9; Chapter 11, The Female Genitalia and Reproductive System; and Chapter 12, The Male Genitalia and Reproductive System.

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Determination of the postvoid residual urine volume

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The adequacy of bladder emptying is measured by determining the volume of urine remaining in the bladder after a full voluntary voiding: the residual volume. First, have the patient empty his or her bladder completely. Residual volume can be estimated by ultrasonography devices or measured directly by passing a sterile urethral catheter into the bladder to collect the residual urine. The risk of infection increases sharply with residual volumes >100 cc.

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Key Symptom Discolored Urine

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Key Symptom Urethral Discharge

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