A distended bladder may be palpable above the symphysis pubis. The bladder accommodates roughly 300 ml of urine filtered by the kidneys into the renal pelvis and the ureters. Bladder expansion stimulates contraction of bladder smooth muscle, the detrusor muscle, at relatively low pressures. Rising pressure in the bladder triggers the conscious urge to void.
Increased intraurethral pressure can overcome rising pressures in the bladder and prevent incontinence. Intraurethral pressure is related to such factors as smooth muscle tone in the internal urethral sphincter, the thickness of the urethral mucosa, and in women, sufficient support to the bladder and proximal urethra from pelvic muscles and ligaments to maintain proper anatomic relationships. Striated muscle around the urethra can also contract voluntarily to interrupt voiding.
Neuroregulatory control of the bladder functions at several levels. In infants, the bladder empties by reflex mechanisms in the sacral spinal cord. Voluntary control of the bladder depends on higher centers in the brain and on motor and sensory pathways between the brain and the reflex arcs of the sacral spinal cord. When voiding is inconvenient, higher centers in the brain can inhibit detrusor contractions until the capacity of the bladder, about 400 to 500 ml, is exceeded. The integrity of the sacral nerves that innervate the bladder can be tested by assessing perirectal and perineal sensation in the S2, S3, and S4 dermatomes (see p. 546).
Other structures sometimes palpable in the lower abdomen include the uterus enlarged by pregnancy or fibroids, which may also rise above the symphysis pubis, and the sacral promontory, the anterior edge of the first sacral vertebra. Until you are familiar with this normal structure, you may mistake its stony hard outlines for a tumor. Another stony hard lump that can sometimes mislead you, and may occasionally alarm a patient, is a normal xiphoid process.
The kidneys are posterior organs. Their upper portions are protected by the ribs. The costovertebral angle—the angle formed by the lower border of the
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