The cardiovascular system includes your dog's heart (the “cardio” part) and the blood vessels (the “vascular” part). There are two types of blood vessels — the arteries, which move blood away from the heart, and the veins, which move blood back to the heart. The cardiovascular system works with the lungs to bring oxygen to cells and take away carbon dioxide, as well as to carry nutrients to the cells and remove wastes.
A dog's heart is very similar to a human heart. The heart has four chambers — a right and left atrium and a right and left ventricle. The chambers on the right side receive blood from the body and send it out to the lungs, to be enriched with oxygen. Blood returns to the heart from the lungs on the left side, and the strong left ventricle then pumps the oxygen-rich blood out to the body.
A puppy may be born with a defective heart. The chambers may be malformed or missing, the blood vessels in and out of the heart too small or in the wrong place, or the heart valves between the chambers may not be right. Older dogs can develop heart failure due to infections, stress, dietary problems, or genetic predispositions. There are now many medications to help dogs with heart problems, and even pacemakers, but canine heart transplants are not an option yet.
Dogs have literally miles of blood vessels. These include the large arteries that travel to the abdomen, head, and heart, along with the tiny capillaries that join the arteries and veins. Arteries are the muscular blood vessels that move oxygen and nutrients from the heart to the rest of the body. When you feel a pulse, you are feeling that action along an artery. The veins bring blood that has been depleted of oxygen back to the heart and lungs.
If your dog cuts an artery, the blood will spurt due to the muscle action passed along from the heart. The blood will be bright red from the fresh load of oxygen it carries. The blood from a cut into a vein runs rather than spurts and is darker in color as it carries carbon dioxide, not oxygen.
Blood itself is made up of various components. Red blood cells carry oxygen; white blood cells fight infection; and platelets help with blood clotting, along with the fluid called plasma. Many substances are transported through the bloodstream. Hormones, nutrients, waste products, and even medications are delivered to cells via the blood. Disorders with the different blood components can cause several different kinds of problems. Too few red blood cells can result in anemia, while too few white blood cells may lead to infections. Dogs that don't have enough platelets or clotting factors can have bleeding disorders, such as von Willebrand's disease. These problems can be side effects of other diseases, or they may occur as congenital defects.