Heart facts for kids
The heart is a powerful muscle found in every animal that has a backbone. It's about the size of your fist. Its main job is to pump blood all around your body. The heart does this by making regular contractions, which means it squeezes the blood out to other parts of the body.
The words cardiac and cardio both mean "about the heart." So, if you see a word with cardio or cardiac in it, you know it has something to do with the heart! For example, myocardium means heart muscle. The word 'myo' comes from the Greek word for muscle, 'mys', and 'cardium' comes from the Greek word for heart, 'kardia'.
Contents
Heart's Structure
The human heart has four special spaces inside called chambers. Some animals might have only two or three chambers.
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In humans, the four chambers are two atria (say: AY-tree-uh) and two ventricles (say: VEN-trih-kuls). If we talk about just one, it's an atrium.
There's a right atrium and a right ventricle. These chambers receive blood that comes back to the heart from the body. They then pump this blood to the lungs. In the lungs, the blood picks up oxygen (the air we breathe) and drops off carbon dioxide (a waste gas).
Blood that has picked up oxygen from the lungs then goes to the left atrium and left ventricle. These chambers are very strong! The left atrium and ventricle send the oxygen-rich blood out to the rest of the body. The left ventricle works much harder than the right ventricle because it has to pump blood to your entire body.
Blood Vessels and Valves
Blood travels through your body in special tubes called blood vessels. These are mainly arteries and veins.
- Veins carry blood to the heart.
- Arteries carry blood away from the heart.
The main artery leaving the right ventricle is the pulmonary artery. (Pulmonary means related to the lungs.) This artery takes blood to the lungs. The main artery leaving the left ventricle is the aorta (say: ay-OR-tuh). The aorta sends oxygen-rich blood to your whole body.
The veins that bring blood into the right atrium are the superior vena cava (say: VEE-nuh KAH-vuh) and inferior vena cava. These bring blood from your body to the right side of your heart. The veins that bring blood into the left atrium are the pulmonary veins. These bring oxygen-rich blood from your lungs to the left side of your heart.
When blood moves from the atria to the ventricles, it passes through heart valves. Valves are like one-way doors. They make sure blood only flows in one direction and doesn't go backward.
The four main valves of the heart are:
- Valves between atria and ventricles:
- Tricuspid valve (say: try-KUS-pid): Blood goes from the right atrium to the right ventricle.
- Mitral valve (say: MY-truhl): Blood goes from the left atrium to the left ventricle.
- Valves from ventricles to arteries:
- Pulmonic valve (say: pul-MON-ik): Blood goes out of the right ventricle to the lungs through the pulmonary artery.
- Aortic valve (say: ay-OR-tik): Blood goes out of the left ventricle to the body through the aorta.
Layers of the Heart
The heart has three important layers:
- The outer layer is the pericardium (say: peh-rih-KAR-dee-um). This is a tough sac that protects the heart.
- The middle layer is the myocardium. This is the strong heart muscle itself.
- The inner layer is the endocardium (say: en-doh-KAR-dee-um). This is a thin, smooth lining inside the heart's chambers.
How the Heart Beats: The Cardiac Cycle
A heart beat happens when the heart muscle contracts. This means the heart squeezes in, making its chambers smaller. This squeezing pushes blood out of the heart and into the blood vessels. After the heart contracts, the muscle relaxes, or stops squeezing. The chambers get bigger again, and blood coming back to the heart fills them up.
When the heart muscle contracts (pushes in), it's called systole (say: SIS-toh-lee). When the heart muscle relaxes (stops pushing in), it's called diastole (say: dy-AS-toh-lee).
Both atria contract together (atrial systole). Then, both ventricles contract together (ventricular systole). But the atria contract before the ventricles. Even though the atria and ventricles contract at different times, all four chambers relax at the same time. This is called cardiac diastole.
The order of events for one heartbeat is: 1. Atrial systole (atria squeeze) 2. Ventricular systole (ventricles squeeze) 3. Cardiac diastole (all chambers relax)
When this whole process happens one time, it's called a cardiac cycle.
The Heart's Natural Pacemaker
The heart muscle cells contract because tiny amounts of electricity flow through the heart. This electricity makes the cells get smaller.
The electricity starts in a special group of cells called the sino-atrial node (or SA Node). The SA Node is located in the right atrium. These cells create an electrical impulse that acts like the heart's natural pacemaker. It sets the speed and timing for all the heart muscle cells to contract. This first contraction is the atrial systole.
The electrical impulse then travels to another group of cells called the atrio-ventricular node (or AV Node). The AV Node makes the electrical impulse slow down a little. This slowdown is important because it makes sure the ventricles contract after the atria. This way, all the blood can leave the atria and fill the ventricles before the ventricles squeeze.
After the AV Node, the electrical impulse travels through a special conduction system in the ventricles. Conduction means electricity traveling through something. This system brings the electrical impulse to all parts of the ventricles.
- First, it goes through the bundle of His (named after Dr. Wilhelm His, Jr., who discovered it). A "bundle" means a group of wires or strings together.
- The bundle of His then splits into two bundle branches: the left bundle branch (for the left ventricle) and the right bundle branch (for the right ventricle).
- Finally, at the end of the bundle branches, the electrical impulse goes into the ventricular muscle through tiny fibers called Purkinje Fibers (say: pur-KIN-jee). This is what makes the ventricles contract, causing ventricular systole.
So, the electrical path in your heart goes like this: Sino-Atrial Node → Atria (systole) → Atrio-Ventricular Node → Bundle of His → Bundle branches → Purkinje Fibers → Ventricles (systole)
Understanding an ECG
An ECG stands for ElectroCardioGram. You might also see it written as EKG, which comes from the German word ElektroKardioGramm. An ECG is a test that shows what the electricity in your heart is doing.
To do an ECG, electrodes (small sticky pads) are placed on a person's skin. These electrodes pick up the electrical signals traveling through the heart. A special machine then records these signals on paper, creating the ECG report.
Doctors look at the ECG to learn about a person's heart health. It can show problems like heart attacks or issues with the heart's rhythm (how the electricity flows through the heart's conduction system).
The ECG shows different waves that match the heart's actions:
- The P-wave shows when the atria contract (atrial systole).
- The QRS or QRS-complex shows when the ventricles contract (ventricular systole). It's called a "complex" because it has three different waves: the Q-wave, R-wave, and S-wave.
- The T-wave shows when the ventricles relax (ventricular diastole). The atria also relax at this time, but their electrical signal is too small to be seen separately on the ECG.
There are also important time intervals on an ECG:
- The PR-Interval is the time between the start of atrial systole (P-wave) and the start of ventricular systole (QRS).
- The QT-Interval is the time from when the QRS starts to when the T-wave ends.
- The ST-segment is the flat line between the end of the QRS and the start of the T-wave.
Images for kids
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Human heart during an autopsy
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Computer-generated animation of a beating human heart
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Layers of the heart wall, including visceral and parietal pericardium
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Autonomic innervation of the heart
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Development of the human heart during the first eight weeks (top) and the formation of the heart chambers (bottom). In this figure, the blue and red colors represent blood inflow and outflow (not venous and arterial blood). Initially, all venous blood flows from the tail/atria to the ventricles/head, a very different pattern from that of an adult.
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The cardiac cycle as correlated to the ECG
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The x-axis reflects time with a recording of the heart sounds. The y-axis represents pressure.
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Transmission of a cardiac action potential through the heart's conduction system
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3D echocardiogram showing the mitral valve (right), tricuspid and mitral valves (top left) and aortic valve (top right). The closure of the heart valves causes the heart sounds.
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Cardiac cycle shown against ECG
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Heart and its blood vessels, by Leonardo da Vinci, 15th century
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Animated heart
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The tube-like heart (green) of the mosquito Anopheles gambiae extends horizontally across the body, interlinked with the diamond-shaped wing muscles (also green) and surrounded by pericardial cells (red). Blue depicts cell nuclei.
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Basic arthropod body structure – heart shown in red
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The human heart viewed from the front and from behind
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Frontal section of the human heart
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Heart illustration with circulatory system
See also
In Spanish: Corazón para niños
