bladder wall and internal urethra sphincter causing contraction of the detrusor muscle and relaxation of the internal urethra sphincter muscle. Simultaneously, the micturition centre inhibits somatic motor neurons that innervate skeletal muscle in the external urethral sphincter.
Urethra
This is a small muscular tube leading from the internal urethral orifice into the floor of the urinary bladder to the exterior of the body. In the mare, the urethra is approx. 6-8 cm in length and opens into the vestibule within the vagina at the external urethral orifice where it is quite wide & distendable; a small horizontal fold of tissue overlies the urethral opening. The urethra of the male is much narrower, 12 cm, and begins at the neck of the bladder (pelvic urethra), exits through the pelvic outlet, turns ventrally around the ischiatic arch and travels distally to the penis (spongy urethra). The terminal end of the urethra is known as the urethral process which is surrounded by the urethra fossa.
Urine
Normal equine urine is viscous due to the large amounts of mucus secreted by the mucus glands within the urinary tract. It is also cloudy due to the presence of suspended calcium carbonate crystals. Urine contains nitrogen, salts, excess sugars, bile pigments and hormones. It is 96% water. An average 500 kg horse will void 5-15L of urine a day. This will vary based on amount of water consumed, feed, work & temperature.
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