Your dog's body takes a beating throughout his life. Muscles are pulled, joints stressed, and organs scarred by infection. Cell structure breaks down, decreasing the efficacy of organs and tissues. All of these traumas cause abnormal cell development, which in turn create tumors and arthritic conditions. On the outside, he can appear as healthy and active as any younger dog, but inside his organs are not functioning as efficiently as when he was younger.

For example, if the kidneys begin to deteriorate, they can continue to function with only 40 percent of the tubules (the part of the kidney that breaks down nutrients from urea) working. Your aging dog will continue feeling fine and behaving normally. However, this can take a quick turn for the worse if a kidney disease continues to deteriorate. This can happen slowly or what seems like overnight. Until this happens, the only difference in your dog will be his need to urinate more often. Otherwise, there is no sign of a problem. As he ages, you need to ensure his complete health by adjusting his diet, exercise, and by keeping a close watch on his behavior. In fact, you'll notice many problems first through behavioral changes before his body shows the outward signs.

Changes in appetite, a lack of desire to move about, or overall grouchiness are usually symptoms of a deeper problem. Your dog's muscles will remain strong, provided he exercises. The more he does as a youngster, the more he can do as an oldster. You must keep in mind, however, that he cannot tell you he doesn't want to go those extra miles with you. All he wants is to be with you and please you, regardless of how he feels. His muscles may still be strong at this point, but his internal workings are no longer operating in prime condition.

Your dog can still remain physically healthy with a little less exercise - maybe two or three miles instead of five or maybe you can do the run on softer ground instead of hard concrete. The musculoskeletal system will usually exhibit arthritic changes as he turns into a senior dog. Arthritis is formed through changes in the joint bones, a reduction of cartilage, and a thickening of the synovial fluid between the joints. Often, inflammation can cause more irritation and lameness. Not only will the arthritic changes cause pain in the joints, but they will also cause atrophy in the muscles because your dog will not want to move around. The muscles begin to get loose and hang off the bones. This is most obvious along the spine, chest, and hind legs.

As the muscles atrophy, the skin will appear looser or baggy. Overall, your dog becomes a different dog as his senior years take over. He moves more slowly, picks at his meals, and may bump into things that he can't see. However, the biggest change will be in his behavior. As he ages, he may not only slow down, he will also become less excitable in general. He will still greet you with a wagging tail, but not jump on you or perform aerial leaps when you come home. When going out, he'll walk to the door and wait patiently as you search for his leash - no more racing in circles, barking excitedly and jumping about.