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Page 12
| Lower Urinary Tract |
| Urine collects slowly from the inner surface of each of the kidneys and drips into the funnel-like renal pelvis. When enough urine has gathered, a contraction squeezes it down into the ureter, and, from this point, as in all muscular ducts of the body, the small bolus of urine is milked down to the bladder. There may be several such urine masses descending towards the bladder at any one time. |
| The ureters have two methods of muscle activity. One, under sympathetic adrenergic control, constricts band-like and blocks urine passage. The other muscular urges are downward contractions under the stimulus of parasympathetic cholinergic nerves. As is so frequent in our bodies, the passage is orderly because it entails the cooperation of opposing forces. There must be enough urine and muscular energy to overcome the constricting muscles and force a bolus of urine downwards. This is the same action as that which allows food to pass downwards through the gut and bile through the biliary apparatus. Balance is the result of opposing energies. |
| There must be additional muscular contractions to squirt the bolus into the bladder, since both the two entrances and the exit of the bladder are under constricting sympathetic adrenergic control. So is the muscular basement of the bladder to which they are attached, the trigone muscle. The bladder expands as the urine collects, and this is controlled by the detrusor musclesunder parasympathetic nerve stimulusa similar balance between constriction and expansion as is found in the ureters. |
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