51 Urine Transport, Storage, and Elimination
Urine Transport
The difference in hydrostatic pressure between the glomerular capsule (10 mm Hg) and the renal pelvis (practically no pressure at all) creates a pressure gradient that forces filtrate to flow from the glomerular capsule through the tubules and into the renal pelvis. In contrast, no pressure gradient exists to propel the flow of urine through the ureters and into the urinary bladder. Instead, urine flow at this point is controlled by peristaltic contractions in the circular smooth muscle of the ureter walls. These peristaltic waves vary in frequency from once every few seconds to once every two to three minutes. Their frequency is increased by parasympathetic stimulation and decreased by sympathetic stimulation.
Urine Storage
Much more time is spent storing urine than micturating (urinating). When urine accumulates in the bladder, distension of the bladder walls activate stretch receptors. These receptors transmit signals through visceral afferent fibers to the sacral area of the spinal cord. The signals initiate spinal storage reflexes, which enhance sympathetic inhibition of the bladder’s detrusor muscle and maintain the internal sphincter in a closed position. The reflexes also stimulate pudendal motor fibers, which cause the external urethral sphincter to contract, preventing the urine from escaping.
Urine Elimination
As previously noted, micturition (also called urination or voiding) is the process of emptying urine from the bladder and is the result of involuntary and voluntary muscle contractions. When the amount of urine accumulated in the bladder reaches about 200 milliliters (7 ounces) (though this amount varies from person to person), afferent impulses are sent to the sacral region of the spinal cord that initiate a reflexive relaxation of the internal urethral sphincter and contraction of the detrussor muscle. The afferent signals that stimulate the urge to urinate are also sent to the brain. This allows the person to relax their external uretheral sphincter, made of skeletal muscle, so that micturition will occur. If we do not void immediately, the reflexive responses will initially weaken, but as more volume is added to the bladder, these responses will come back more strongly, creating a more urgent need to void.
When we are ready to empty the bladder—a decision executed by the cerebral cortex—the micturition reflex is set in motion. Afferent impulses activate the micturition center of the brain. This signal integrates with parasympathetic signals of the spinal cord to allow the external sphincter to relax, thus releasing urine from the bladder.
If we need to delay micturition, the reflex bladder contractions will taper off and stop within about one minute, and urine will continue accumulating. The addition of another 200 to 300 milliliters (7 to 10 ounces) of urine will prompt another micturition reflex. If voiding is still not possible, the reflexes will again subside. When more than 500 to 600 milliliters (about 20 ounces) of urine accumulates, urination will occur whether we want to or not. After micturition, about 10 milliliters (0.33 ounces) of urine will remain in the bladder.