Urinary System--The two major organs of the urinary system are the kidney and the bladder. Both are affected by aging.
Kidney--Chronic kidney (renal) failure is a common problem in older dogs and often begins to show at about seven years of age. Some studies put kidney failure as the second most common non-accidental death in dogs. As the kidney ages, its weight and volume decrease with resultant loss in capacity to concentrate or dilute urine as appropriate. The kidney, with the lungs, plays a critical role in the maintenance of blood pH (acid-base balance).
Bladder--The bladder stores urine received from the kidneys and empties through the urethra. In older dogs, it is common for the sphincters to fail to operate properly for one reason or another, with the dog becoming incontinent. Bladder and urinary tract infections are common causes of incontinence. Spayed older bitches are especially prone to incontinence. Older dogs are also prone to bladder stones (uroliths) and may require special diets to maintain urine pH at the correct level.
Colon (Large intestine) Rectum, and Anus--Colitis is a common old age problem in dogs, usually stemming from recurring bouts with intestinal parasites, such as whip worms and hookworms. In dogs with limited fluid intake and limited activity, constipation and even colon impaction may be common. For older dogs of breeds where spinal problems are common, such as German Shepherd Dogs and Dachshunds, paralysis can lead to frequent constipation. Frequent constipation may further be complicated by rectal prolapse where the rectum may protude through the anus as a result of frequent straining to defecate. Older dogs are prone to anal sac disease, and in some dogs, the condition becomes chronic rather than just recurring. If stools are too soft, feces may be retained with resulting infection.