The Urinary Bladder

 The urinary bladder is a muscular sac lined by transitional epithelium that stores urine and periodically releases it to the outside. As urine accumulates in the urinary bladder, it stretches the bladder wall. Eventually a spinal reflex is activated which stimulates the smooth muscle in the bladder wall to contract. The animal feels this as the urge to urinate. If urination at that time is not appropriate, a skeletal muscle sphincter around the neck of the bladder can be consciously contracted to prevent urination.