A puppy's bladder is a reservoir for collecting and holding urine until it can be expelled. As with humans, full control of the bladder takes time to develop; however, emptying the bladder is a reflex reaction. When the bladder stretches to a certain point, signals are sent to the spinal cord. Nerve centers in the spinal cord cause the urethral sphincter to relax, the bladder muscle to contract and urine to expel through the urethra.
As with humans, puppies gradually develop the ability to delay their reflex, or automatic, reaction to empty their bladder. According to the American Medical Association, "Stretching of the bladder is registered consciously (as discomfort) in brain centers, which, if desired, can then send signals suppressing the emptying reflex." As the bladder stretches further and further, the puppy's urge to pass urine becomes immediate and overwhelming.
Puppies learn to "hold" their urine as a result of house training, which normally does not begin until six weeks of age or older. Prior to that time, a puppy will squat and "pee" whenever the urge strikes. This is why establishing a routine for housebreaking, supervising him at all times, and confining him when you are unavailable is so important. In addition, picking up the puppy's water dish two hours before bed time will reduce night time accidents.
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Most puppies under six months cannot control their bladder for more than a few hours at a time. According to University of California's Davis College of Veterinary Medicine the frequency of elimination breaks should align with your puppy's age. The age/elimination guidelines are: six to 14 weeks/ eight to 10 times daily, 14 to 20 weeks/six to eight times daily, 20 to 30 weeks/four to six times daily, and 30 weeks and older/three to four times daily.
As with humans, puppies can have defective bladder function leading to incontinence and urine retention. Ectopic ureter(s), a birth defect, occurs when one or both ureters bypass the bladder and connect in another location, such as the urethra or vagina. This usually manifests in the puppy's dripping or dribbling urine, rather than passing urine normally. Siberian huskies are commonly affected with this birth defect as are miniature poodles, labrador retrievers, collies, Welsh corgis, wire-haired fox terriers and West Highland white terriers.
Bladder infections can be common in puppies as well, particularly those with ectopic ureter(s). The infection can cause a strong urge to urinate--oftentimes excessively and in inappropriate places--making it difficult for the puppy to be house trained. More importantly, longstanding bladder infections can cause bladder scarring, which can present serious health problems. Contact your vet for a complete health evaluation should your puppy exhibit signs for ectopic ureter(s) or infection.