How does the heart work? Your heart is a muscular pump about the size of your fist, with four chambers. The right side sends oxygen-poor blood to your lungs. The left side pumps oxygen-rich blood out to the rest of your body. Four valves keep blood moving one way, and an electrical signal times every beat.
This is general information, not medical advice. Talk to your doctor or pharmacist about your own situation.
The National Heart, Lung, and Blood Institute (NHLBI, part of the U.S. National Institutes of Health) calls the heart the center of the circulatory system, a network of arteries, veins and capillaries that carries blood to and from every part of your body. Blood delivers oxygen and nutrients, and it carries carbon dioxide back to the lungs so you can breathe it out.
Where it is: Cleveland Clinic says it sits in the front of your chest, slightly behind and to the left of your breastbone, between your lungs.
How big it is: roughly the size of your fist. Cleveland Clinic says an average adult heart weighs about 10 ounces.
Four chambers: two upper chambers, the right and left atria, receive blood. Two lower chambers, the right and left ventricles, pump it out.
Four valves: one-way doors between the chambers and the big arteries.
An electrical system: it sets the rate and rhythm of each heartbeat.
NHLBI describes the route like this. Follow one drop of blood around a full loop:
Back from the body. Oxygen-poor blood returns through two large veins, the superior and inferior vena cava, into the right atrium.
Into the right ventricle. It passes through the tricuspid valve into the right ventricle.
Out to the lungs. The right ventricle pumps it through the pulmonary valve into the pulmonary artery, which leads to the lungs.
Picking up oxygen. In the lungs, the blood takes on oxygen and drops off carbon dioxide.
Back to the heart. The pulmonary veins carry oxygen-rich blood to the left atrium.
Into the left ventricle. It passes through the mitral valve into the left ventricle.
Out to the body. The left ventricle pumps it through the aortic valve into the aorta, the body's main artery, and on to every organ.
Round again. After delivering oxygen and nutrients, the blood heads back toward the heart through the veins, and the loop repeats.
It helps to think of the heart as two pumps side by side. The right side runs a short loop to the lungs and back, often called the pulmonary circulation. The left side runs a long loop to the whole body, often called the systemic circulation.
That's why the left ventricle works hardest. NHLBI says it generates the high pressure needed to pump blood to your whole body, and Cleveland Clinic notes it is slightly larger than the right ventricle. One exception to the usual rule: arteries normally carry oxygen-rich blood, but Cleveland Clinic points out that the pulmonary arteries are the only arteries that carry oxygen-poor blood, because they're taking it to the lungs to be refilled.
NHLBI says valves control the flow of blood so it moves in the right direction. Each has flaps, called cusps or leaflets, that open and close with every beat so blood can't flow backward.
Tricuspid valve: between the right atrium and right ventricle.
Pulmonary valve: between the right ventricle and the pulmonary artery.
Mitral valve: between the left atrium and left ventricle.
Aortic valve: between the left ventricle and the aorta.
Your heartbeat is your heart contracting to push blood to your lungs and body. NHLBI says your heart's electrical system decides how fast it beats, and the signal travels from the top of the heart to the bottom with each beat:
The spark. The signal starts in a group of pacemaker cells called the sinoatrial (SA) node, in the right atrium.
The atria squeeze. The signal spreads across the atria, which pump blood down into the ventricles.
A short pause. The signal reaches the atrioventricular (AV) node, where it slows down slightly so the ventricles can finish filling.
The ventricles squeeze. Cleveland Clinic describes the next stretch of wiring: the bundle of His, the left and right bundle branches, and the Purkinje fibers, which make the ventricles contract and pump blood out.
Reset. The ventricles relax, and the cycle starts again at the SA node.
The familiar "lub-dub" sound comes from valves closing. NHLBI says the "lub" is the valves between the atria and ventricles closing, and the "dub" is the aortic and pulmonary valves closing. Your nervous system and hormones speed your heart up or slow it down, for example to send more oxygen to your muscles when you exercise.
Heart muscle needs its own oxygen supply. NHLBI explains that the coronary arteries branch off the aorta and run over the heart's surface. The left coronary artery splits into the circumflex artery and the left anterior descending artery, and the right coronary artery supplies the right atrium and parts of both ventricles. Coronary veins then return the used blood to the right atrium.
NHLBI lists these heart attack warning signs:
chest pain, heaviness or discomfort in the center or left side of the chest
pain or discomfort in the arms, back, shoulders, neck, jaw or above the belly button
shortness of breath, at rest or with light activity
sweating a lot, or feeling unusually tired, for no clear reason
nausea and vomiting, light-headedness or sudden dizziness, or a rapid or irregular heartbeat
Call 911 (or your local emergency number) right away, even if you aren't sure it's a heart attack. NHLBI advises against driving yourself or having someone drive you, because emergency crews can start treatment on the way. Symptoms can also be mild, come and go, or build slowly. For heart symptoms that worry you but don't feel like an emergency, book an appointment with your doctor.
The right atrium, right ventricle, left atrium and left ventricle. The atria collect blood coming in, and the ventricles pump it out.
An electrical signal that starts in the SA node, the heart's natural pacemaker, and spreads from the top of the heart to the bottom.
NHLBI says that at rest, a heart rate of 60 to 100 beats per minute is normal. To check yours, rest two fingers on the inside of your wrist, below your thumb, count the beats for 30 seconds and double the number. Ask your doctor what's normal for you.
No. The pulmonary arteries carry oxygen-poor blood from the heart to the lungs. Other arteries carry oxygen-rich blood to the body.
Body Unpacked: Health Atlas is a free iPhone app for learning how food and the body connect. Its Physiology Lab steps through three teaching models, one of them on breathing and circulation, and its connected atlas has 249 sourced topics at three depth levels. It's built for learning, not diagnosis, with no account and no ads, and the content works without a connection.
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