Heart Anatomy

What the Heart Is and Where It Sits

The heart is a hollow, muscular pump about the size of your own closed fist. In most adults it weighs between 250 and 350 grams. It sits inside the chest in a space called the mediastinum, nestled between the two lungs, behind the breastbone, and resting on the diaphragm.

A common misconception is that the heart is directly in the centre of the chest. In reality it tilts slightly to the left, which is why you feel your heartbeat more strongly on the left side. The pointed bottom tip of the heart, called the apex, points down and to the left; the wider, flatter top is called the base.

The heart is wrapped in a tough, double-layered protective sac called the pericardium. The inner layer hugs the heart's surface, while the outer layer anchors the heart loosely to the chest wall. A small amount of fluid between the two layers reduces friction as the heart beats.

You can explore all the body's major systems interactively with the Body Systems Explorer or check your resting heart rate with the Heart Rate Calculator.

The Four Chambers

The heart is divided into four hollow rooms, called chambers. They are arranged in two side-by-side pairs: the right side and the left side. A thick wall of muscle called the septum runs down the middle, keeping blood on the right side completely separate from blood on the left.

Each side has an upper chamber and a lower chamber:

• Right atrium — the upper chamber on the right. It receives oxygen-depleted blood returning from the body through two large veins.
• Right ventricle — the lower chamber on the right. It receives blood from the right atrium and pumps it to the lungs.
• Left atrium — the upper chamber on the left. It receives oxygen-rich blood coming back from the lungs.
• Left ventricle — the lower chamber on the left. It is the most muscular chamber of all, and it pumps freshly oxygenated blood out to the whole body.

The two atria (plural of atrium) act as receiving rooms. They collect incoming blood and pass it down to the ventricles below. The ventricles are the main pumping chambers; they have much thicker walls because they need to push blood further and with more force.

The left ventricle works especially hard because it must pump blood all the way through the body's full circuit of blood vessels — sometimes called the systemic circulation. Its walls are roughly three times thicker than those of the right ventricle, which only needs to push blood the short distance to the nearby lungs.

The Four Valves — One-Way Doors

Blood must always move in one direction: into a chamber, then out of it, and never backwards. Four valves enforce this rule by opening to let blood through and snapping shut to stop it from flowing the wrong way. Think of them as one-way doors with flaps, called leaflets or cusps.

The valves fall into two groups based on their location:

• Atrioventricular (AV) valves — sit between the atria and the ventricles.
  • The tricuspid valve (right side) has three leaflets.
  • The mitral valve (left side) has two leaflets, which is why it is also called the bicuspid valve.
• Semilunar valves — sit at the exits of the ventricles.
  • The pulmonary valve guards the exit from the right ventricle into the pulmonary artery heading to the lungs.
  • The aortic valve guards the exit from the left ventricle into the aorta, the body's largest artery.

The familiar lub-dub sound of a heartbeat is caused by valves closing. The first sound ("lub") is the AV valves closing as the ventricles begin to contract. The second sound ("dub") is the semilunar valves closing as the ventricles relax. A doctor listening with a stethoscope can detect abnormal valve sounds (called murmurs) that may signal a valve not opening or closing properly.

The Heart Wall — Three Layers of Muscle and More

The wall of the heart is not just one solid slab of muscle. It has three distinct layers, each with its own job:

• Epicardium — the outermost layer, which is actually the same as the inner layer of the pericardium sac. It contains fat and the blood vessels that feed the heart muscle itself.
• Myocardium — the thick, muscular middle layer. This is the part that actually contracts and relaxes to pump blood. The word "myocardium" literally means "heart muscle" (from the Greek myo for muscle and kardia for heart).
• Endocardium — the smooth innermost lining. It covers the inside of all four chambers and also covers the valves. Its smooth surface helps blood flow through without catching or clotting.

The myocardium is made of a special type of muscle tissue called cardiac muscle. Unlike skeletal muscle (which you use to move your limbs and which you consciously control) or smooth muscle (which lines organs like the stomach), cardiac muscle is involuntary — it works automatically — and it is highly resistant to fatigue. It must keep contracting without rest for your entire lifetime.

The Heart's Electrical System

A pump needs a timing system, and the heart has its own built-in electrical network that tells it exactly when to beat. This network is made of specialised heart cells that can generate and carry electrical signals.

The process begins at the sinoatrial (SA) node, sometimes called the natural pacemaker. This small cluster of cells sits in the upper wall of the right atrium. It fires an electrical signal about 60 to 100 times per minute at rest. That signal spreads across both atria, causing them to contract and push blood into the ventricles.

The signal then reaches the atrioventricular (AV) node, which sits at the junction between the atria and ventricles. The AV node briefly delays the signal — giving the ventricles time to fill with blood — before sending it onwards.

From the AV node, the signal travels down a pathway called the Bundle of His, then splits into right and left bundle branches, and finally spreads through a network of fibres called the Purkinje fibres. This rapid spread makes both ventricles contract almost simultaneously, producing one strong, coordinated pump stroke.

An electrocardiogram (ECG or EKG) is a medical test that records this electrical activity as a wave pattern. The familiar spiky waves on an ECG printout correspond directly to the different stages of atrial and ventricular activity in each heartbeat.

The Coronary Arteries — Feeding the Heart Itself

The heart pumps blood to the rest of the body, but it also needs its own constant blood supply to keep its muscle cells alive. This supply comes from two main coronary arteries that branch off from the aorta just above the aortic valve.

• The left coronary artery quickly splits into the left anterior descending (LAD) artery, which supplies the front and bottom of the left ventricle, and the left circumflex artery, which wraps around the left side of the heart.
• The right coronary artery curves around the right side of the heart and supplies the right ventricle and parts of the left ventricle.

These arteries branch into smaller and smaller vessels that weave through the myocardium, delivering oxygen and nutrients. After delivering their oxygen, the blood drains into veins on the heart's surface and collects in the coronary sinus, a short vessel that empties directly into the right atrium.

If one of the coronary arteries becomes narrowed or blocked — a condition called coronary artery disease — the heart muscle it supplies is starved of oxygen. A complete blockage can cause a heart attack (medically known as a myocardial infarction). This is why coronary health is a central focus of cardiovascular medicine. If you ever experience chest pain or symptoms you are unsure about, always seek medical advice rather than trying to self-diagnose.

The Heartbeat Cycle — Systole and Diastole

One complete heartbeat is called the cardiac cycle. It has two main phases that repeat continuously:

Diastole (the relaxation phase): The heart muscle relaxes. Blood flows from the body and lungs into the atria, then passes through the open AV valves into the relaxed ventricles. The heart is filling up.

Systole (the contraction phase): First the atria contract, topping up the ventricles with any remaining blood. Then the ventricles contract powerfully. The AV valves snap shut (preventing backflow into the atria), the semilunar valves are forced open, and blood is ejected — from the right ventricle into the pulmonary artery toward the lungs, and from the left ventricle into the aorta toward the body.

Once the ventricles have ejected their blood, they relax again, the semilunar valves close (preventing backflow from the arteries), and diastole begins once more. The whole cycle at a resting heart rate of around 70 beats per minute takes less than one second.

The fraction of blood ejected from the left ventricle with each beat is called the ejection fraction. In a healthy heart this is typically 55 to 70%. Doctors measure ejection fraction using ultrasound (echocardiography) to assess how well the heart is pumping.

Heart Rate and Cardiac Output

Your heart rate is simply the number of times the heart beats per minute. At rest, a normal range for adults is about 60 to 100 beats per minute. Athletes who do a great deal of endurance training often have resting heart rates well below 60 — sometimes as low as 40 — because their hearts have become more efficient and pump more blood with each stroke.

Cardiac output is the total volume of blood the heart pumps per minute. It is calculated by multiplying heart rate by stroke volume (the amount of blood ejected per beat). At rest the heart pumps roughly 5 litres per minute — approximately the total blood volume of an adult. During intense exercise this can rise to 20 or even 25 litres per minute.

Several factors influence heart rate in everyday life, including physical activity, stress, temperature, caffeine, dehydration, and medications. The autonomic nervous system fine-tunes heart rate constantly: the sympathetic branch speeds it up (the "fight or flight" response) and the parasympathetic branch slows it down (the "rest and digest" response).

You can use the Heart Rate Calculator to understand your target heart rate zones, and the Pulse Zone Calculator to plan activity intensity. For a broader look at how this fits into exercise, see our guide to blood circulation.

Looking After Your Heart — What the Science Says

The heart is an extraordinarily resilient organ, but it is not indestructible. Years of certain lifestyle factors can gradually affect its health. Educational resources like this one can help you understand the biology, but for personal health decisions and any symptoms, a healthcare professional is always the right source of advice.

In general, mainstream science consistently finds that regular physical activity, not smoking, maintaining a healthy blood pressure and cholesterol level, and managing chronic stress are all associated with better long-term cardiovascular health. These are population-level observations, not individual prescriptions.

Understanding the structure of your heart is a great starting point. From here, you might explore how blood circulates through the body, read about the respiratory system that works alongside the heart, or take the anatomy quiz to test what you have learned. The heart and blood category has more guides on related topics.