The average cat's heart, weighing less than one-tenth of an ounce, will beat more than 350 million times during a 13-year lifetime. The heart has four chambers with valves that regulate blood flow. The major veins bringing blood to the heart are called the vena cavas. Blood first enters the heart's right atrium, then passes through the tricuspid valve and flows into the right ventricle. The right ventricle contracts and sends blood into the pulmonic artery through the pulmonic valve. Blood then goes to the lungs where it becomes oxygenated. Oxygenated blood from the pulmonic vein enters the heart's left atrium, then passes into the left ventricle via the mitral valve. When the left ventricle contracts, blood shoots out of the heart through the aorta and circulates throughout the rest of the body.
The tricuspid and mitral valves regulate blood flow through the heart. In cats, deformities of these valves are the most common congenital cardiac malformations. A murmur, or squishing noise, indicates blood is leaking out of the valves during heart contraction. Instead of forming a tight seal, blood escapes around the valve. By listening to the heart with a stethoscope, vets can often detect valve problems. A murmur's intensity and volume does not correlate with the severity of valve damage.
Murmurs are common in older cats. Some kittens are born with murmurs and outgrow them; others have murmurs their entire lives without problems. Though uncommon in cats, some murmurs may progress and lead to congestive heart failure.