Urine Transportation, Storage, and Elimination
From the collecting ducts urine drains through the papillary ducts into the minor calyces, which join to become major calyces that unite to form the renal pelvis. From the renal pelvis, urine drains into the ureters then into the urinary bladder. Urine is often discharged from the body through the single urethra.
Ureters
2 ureters transport urine from the renal pelvis of one kidney to the urinary bladder. Peristaltic contractions of the muscular walls of the ureters push urine toward the urinary bladder, but hydrostatic pressure and gravity also contribute. In an average sized horse, the ureter is approx. 70 cm in length. The ureter is lined by transitional epithelium with large numbers of mucus glands in its proximal part. There is no anatomical valve at the opening of each ureter into the urinary bladder. However, a physiological one is quite effective. As the urinary bladder fills with urine, pressure within it compresses the oblique openings into the ureters and prevents backflow of urine.
Urinary Bladder
This is a hollow distensible muscular organ. Urinary bladder capacity averages 4-5L. When the bladder is empty and contracted it recedes into the pelvic cavity & rests on the pubic bones as a ball of tissue approx. 8-10cm in diameter. In this position only the apex and small part of the body are covered with peritoneum, therefore the bladder is considered to be retroperitoneal. As it fills the bladder expands forward and hangs over the brim of the pelvis. When filled to capacity it rests on the ventral abdomen and may reach as far forward as the umbilicus. The smooth muscle of the wall of the bladder, the detrusor muscle is arranged in 3 layers; an oblique layer, transverse layer and a longitudinal layer. Loops of smooth muscle surrounding the neck of the bladder form the internal urethral sphincter muscle. This muscular layer is rich in blood vessels. The detrusor muscle is supplied by both sympathetic & parasympathetic nerve fibres. A mucus membrane of transitional epithelium lines the internal bladder. It rests in a thick submucosa which permits adequate stretch as the bladder fills.
The micturition Reflex
Micturition : discharge of urine from the urinary bladder.
This occurs via a combination of involuntary and voluntary muscle contractions. When the volume inside the bladder exceeds 2L, pressure increases dramatically and stretch receptors in its wall transmit nerve impulses into the spinal cord. The impulses then propagate to the Micturition Center in the sacral spinal cord segments S2 and S3 and trigger a spinal reflex called the Micturition Reflex. Parasympathetic impulses from this centre propagate to the urinary
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