The respiratory system is an incredible series of passageways that works to take oxygen out of the air and deliver it to the bloodstream, where red blood cells then carry it to individual cells throughout the body. The respiratory system works with the cardiovascular system to remove carbon dioxide waste as well. Your dog's respiratory system starts with her nose as fresh air goes in, and it ends with her nose as carbon dioxide is breathed out.

Airways

What better place to start a trip around your dog's body than with the nose? Your dog's nose is an unparalleled scenting machine, capable of detecting much more than a human nose can. Even more importantly for your dog, it is the starting point of the respiratory system. While dogs can breathe through their mouths if necessary, such as when they are hot or tired, they generally breathe through their noses.

As air comes into her nose, your dog separates out all the fascinating smells (and perhaps even runs off to go follow one!). At the same time, her many nasal sinuses help to moisturize and warm the air to body temperature so her lungs won't get a shock from too-cold air. Dust, viruses, and bacteria can all enter the nose too, but the nasal passages are lined with fine hairs to catch debris and infection-fighting cells to trap any troublemakers.

From the nose, air travels into the larynx, the pharynx, and on to the trachea, or windpipe. The trachea is a tunnel of cartilage that leads from the upper airways into the lungs. The trachea should be composed of about three-quarters firm cartilage, with the remainder being soft tissue that stays taut but gives the trachea some flexibility. However, some breeds of dogs have tracheal collapse. In these cases, the cartilage is defective, or the soft tissue area too large or too loose. When the dog breathes in deeply, the soft area pulls down into the trachea or the tracheal rings themselves flatten. This reduces the area available for air to move through — picture squeezing a cardboard tube partially closed — and therefore the body receives less oxygen. This condition can be helped medically or surgically, depending on the exact problem.

Lungs

From the trachea, air moves into progressively smaller airways, called bronchi and bronchioli. These airways end in tiny areas called alveoli. Here, oxygen is added to the blood when your dog breathes in and carbon dioxide removed when your dog breathes out. Lungs contain miles of these passageways along with many blood vessels, all to make sure your dog's cells get the oxygen they need. Your dog's lungs have two main lobes on the left side and four lobes on the right side. To breathe, your dog needs the assistance of a specialized muscle called the diaphragm, which separates the chest from the abdomen. This muscle helps to pull air into the lungs and expel carbon dioxide.

Like most of your dog's respiratory system, the lungs have cells to fight infection as well as fine hairs called cilia that separate out debris and try to push it up and out. Your dog can only get the most benefit from breathing if the air she breathes is clean. Smog and secondhand cigarette smoke can all lead to respiratory problems in dogs.