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Structure of the Urinary Tract
1. The renal pelvis and ureter In most domestic species, the ureter begins in a common expansion, the renal pelvis, into which all papillary ducts open. In cattle however, the ureter is formed by a coming together of the short passages that lead from the calices that enclose the individual renal papillae. The remaining tubular part of each ureter has a fairly even calibre. It follows a course along the abdominal roof. On reaching the pelvic cavity, the ureter bends medially to enter the genital fold in the male or the broad ligament in the female. This carries the ureter over the dorsal surface of the bladder, into which it opens near the neck. In the male, the ureter passes dorsally to the corresponding deferent duct. The ureters can therefore be divided into abdominal and pelvic portions. The ureter penetrates the bladder wall obliquely. If a ureter does not open in the correct position, it is described as being ectopic.
The wall of the renal pelvis and ureter is composed of an external adventitia, a middle muscularis and an internal mucosa. The muscle coat is well developed and aids movement of urine into the bladder through peristalsis. There is an inner longitudinal and middle circular layer of muscle. The outer layer of longitudinal muscle is found near the bladder only. The internal mucosa is composed of transitory epithelium and a lamina propria.
2. The urinary bladder The bladder is a distensible storage organ. When the bladder enlarges it becomes pearshaped, presenting a cranial apex, an intermediate body and a caudal neck that narrows to the internal urethral orifice at the junction with the urethra. Once the bladder is filled past a threshold level, pressure rises sharply, initiating an urge to urinate. The inner surface of the bladder is highly folded when it is empty, but is stretched into smoothness when it fills. However two folds remain even when the bladder is filled.
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