A Beginner's Guide to the Feline Cardiovascular System

In this guide

Understanding how a cat's heart and blood vessels work makes it much easier to grasp what goes wrong when heart disease develops.

The Basics

The cardiovascular system is made up of the heart and blood vessels — arteries and veins. The heart's job is simple in concept: pump blood. Its right side sends blood to the lungs to pick up oxygen, while its left side pumps oxygen- and nutrient-rich blood out to the rest of the body, collecting waste products like carbon dioxide on the return trip. Structurally, the heart is a hollow, muscular organ divided into four chambers: two upper chambers, the left and right atria, and two lower chambers, the left and right ventricles. The muscle tissue itself is called the myocardium.

A set of valves keeps blood moving in a single direction through the heart. The atrioventricular valves sit between the atria and ventricles, while the semilunar valves sit between the heart and the major arteries leaving it. In the left ventricle, blood enters through the mitral valve and exits through the aortic valve; in the right ventricle, it enters through the tricuspid valve and exits through the pulmonary valve.

How Blood Moves Through the Heart

Blood returning from the body enters the right atrium via the two largest veins, the venae cavae. As the right ventricle relaxes, blood flows through the tricuspid valve to fill it; once nearly full, the right atrium contracts to push in the remaining blood, and the right ventricle then contracts, sending blood through the pulmonary valve to the lungs. There, it picks up oxygen and releases carbon dioxide before returning via the pulmonary veins to the left atrium. The same relax-and-fill, contract-and-push sequence then happens on the left side: blood fills the left ventricle through the mitral valve, and a final powerful contraction sends it out through the aortic valve into the aorta — the body's largest artery — for distribution everywhere except the lungs.

Heartbeats, Rate and Sound

Each heartbeat has two phases: diastole, when the ventricles relax and fill with blood, and systole, when they contract to pump it out. What you hear as the classic "lub-dub" through a stethoscope is actually the sound of these valves closing — the mitral and tricuspid valves for one half of the beat, the aortic and pulmonary valves for the other.

The heart's rhythm originates from the sinoatrial node, its natural pacemaker, which fires rhythmic electrical impulses that trigger each contraction. At rest, a cat's sinoatrial node fires more than 120 times a minute. Heart rate and blood pressure are inversely linked: as blood pressure rises, heart rate tends to fall, and vice versa. In heart failure, this feedback system can misfire, pushing heart rate inappropriately high in a way that further strains the heart.

Murmurs, Arrhythmias and Pulse

Heart murmurs are vibrations audible through a stethoscope, usually caused by turbulent blood flow or vibrating valve structures. Vets describe them by timing, intensity and location — and not every murmur signals disease; innocent murmurs are sometimes picked up in perfectly healthy kittens under three months old.

Arrhythmias are irregularities in the rate, rhythm or origin of the heartbeat. Many are harmless and need no treatment, though some can cause fainting or even sudden death. Common patterns include a rate that's too slow (bradycardia), too fast (tachycardia), premature beats, irregular rhythms, or unexpected pauses. If a vet detects something unusual through a stethoscope, further tests like an ECG or echocardiogram may follow.

Finally, a pulse — the rhythmic expansion of an artery you can feel under your fingertips — is typically checked at the femoral artery in a cat's thigh, with a jugular pulse sometimes noted in the neck of healthy cats too. A pulse that's absent, unusually strong, or unusually weak can each point toward a specific underlying heart condition.

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