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Your heart is in the center of your chest, near your lungs. It has four hollow heart chambers surrounded by muscle and other heart tissue. The chambers are separated by heart valves, which make sure that the blood keeps flowing in the right direction. Read more about heart valves in Blood Flow.
The two upper chambers of your heart are called atria, and the two lower chambers are called ventricles. Blood flows from the body and lungs to the atria and from the atria to the ventricles. The ventricles pump blood out of the heart to the lungs and other parts of the body. An internal wall of tissue divides the right and left sides of your heart. This wall is called the septum.
Chambers of the heart. Your heart has four chambers. Two upper chambers, called the left and right atria, and two lower chambers, called the left and right ventricles, contract in a steady rhythm known as your heartbeat. During a normal heartbeat, blood from your tissues and lungs flow into your atria, then into your ventricles. Walls inside your heart, called the interatrial septum and intraventricular septum, help keep the blood on both sides from mixing. Medical Animation Copyright © 2019 Nucleus Medical Media, All rights reserved.
The heart is made of three layers of tissue.
Heart muscle. In order to pump blood more efficiently, your heart muscle, called myocardium, is arranged in a unique pattern. Three layers of myocardium wrap around the lower part of your heart. They twist and tighten in different directions to push blood through your heart. Medical Animation Copyright © 2019 Nucleus Medical Media, All rights reserved.
Some conditions can affect the heart's tissue. Examples include:
How does the heart form in the womb?
The heart begins forming very early in pregnancy and is the first organ to function while a baby is growing in the womb. The heart begins as two tubes of cells. The tubes fuse into a single tube that has an immature atrium and ventricle, which begin beating in the third week of pregnancy. The tubes loop to create a heart that looks more like the mature heart. The septum begins to form and separate the atria and ventricles into four chambers. Cells from different parts of the unborn baby, or embryo, move to the heart to form the heart valves. The heart is nearly fully formed by the ninth week of pregnancy. Problems at any point in this process can cause the heart to develop abnormally and lead to congenital heart defects.
Arteries and veins link your heart to the rest of the circulatory system. Veins bring blood to your heart. Arteries take blood away from your heart. Your heart valves help control the direction the blood flows.
Heart valves control the flow of blood so that it moves in the right direction. The valves prevent blood from flowing backward.
The heart has four valves.
The valves open and shut in time with the pumping action of your heart's atria and ventricles. The opening and closing involves a set of flaps called cusps or leaflets. The cusps open to allow blood to flow out of a chamber and close to allow the chamber to refill with blood. Heart valve diseases can cause backflow or slow the flow of blood through the heart.
Valves of the heart. Two valves sit like doors between your atria and ventricles to prevent blood from flowing backward into your atria. The tricuspid valve opens into your right ventricle, and the mitral valve opens into your left ventricle. Strong thin tissues called chordae tendineae hold your valves in place during the forceful contractions of your ventricles. Blood leaving the ventricles passes through another set of valves, the pulmonary valve, between your right ventricle and pulmonary trunk, and the aortic valve, connecting your left ventricle and aorta. Medical Animation Copyright © 2019 Nucleus Medical Media, All rights reserved.
Oxygen-poor blood from the body enters your heart through two large veins called the superior and inferior vena cava. The blood enters the heart's right atrium and is pumped to your right ventricle, which in turn pumps the blood to your lungs.
The pulmonary artery then carries the oxygen-poor blood from your heart to the lungs. Your lungs add oxygen to your blood. The oxygen-rich blood returns to your heart through the pulmonary veins. Visit our How the Lungs Work Health Topic to learn more about what happens to the blood in the lungs.
The oxygen-rich blood from the lungs then enters the left atrium and is pumped to the left ventricle. The left ventricle generates the high pressure needed to pump the blood to your whole body through your blood vessels.
When blood leaves the heart to go to the rest of the body, it travels through a large artery called the aorta. A balloon-like bulge, called an aortic aneurysm, can sometimes occur in the aorta.
Circulation and the heart. Your heart is divided into left and right halves, which work together like a dual pump. On the right side of your heart, oxygen-poor blood from your body’s tissues flows through large veins, called the superior and inferior vena cava, into your right atrium. Next, the blood moves into your right ventricle, which contracts and sends blood out of your heart to your lungs, to gather oxygen and get rid of carbon dioxide. On the left side of your heart, oxygen-rich blood from your lungs flows through your pulmonary veins into your left atrium. The blood then moves into your left ventricle, which contracts and sends blood out of your heart through the aorta to feed your cells and tissues. Medical Animation Copyright © 2019 Nucleus Medical Media, All rights reserved.
Like other muscles in the body, your heart needs blood to get oxygen and nutrients. Your coronary arteries supply blood to your heart. These arteries branch off from the aorta so that oxygen-rich blood is delivered to your heart as well as the rest of your body.
Arteries supplying oxygen to the body. The coronary arteries branch off the aorta and supply the heart muscle with oxygen and nutrients. At the top of your aorta, arteries branch off to carry blood to your head and arms. Arteries branching from the middle and lower parts of your aorta supply blood to the rest of your body. Medical Animation Copyright © 2019 Nucleus Medical Media, All rights reserved.
Some conditions can affect normal blood flow through these heart arteries. Examples include:
The coronary veins return oxygen-low blood from the heart's muscles back to the right atrium so it can be pumped to the lungs. They include:
Your heartbeat is the contraction of your heart to pump blood to your lungs and the rest of your body. Your heart's electrical system determines how fast your heart beats.
The contraction of the atria and ventricles makes a heartbeat. When your heart beats, it makes a “lub-DUB” sound. You may have heard this if you listened with a stethoscope or with your ear on someone's chest.
Your heartbeat. Your heart beats an average of 60 to 100 beats per minute. In that one minute, your heart pumps about five quarts of blood through your arteries, delivering a steady stream of oxygen and nutrients all over your body. Medical Animation Copyright © 2019 Nucleus Medical Media, All rights reserved.
What is my pulse, and how do I measure it?
Your pulse is the rate your heart beats. It is also called your heart rate. To find your pulse, gently place your index and middle fingers on the artery located on the inner wrist of either arm, below your thumb. You should feel a pulsing or tapping against your fingers.
Watch the second hand or set the timer on your stopwatch or phone, and count the number of beats you feel in 30 seconds. Double that number to find out your heart rate or pulse for one minute.
Electrical signals cause muscles to contract. Your heart has a special electrical system called the cardiac conduction system. This system controls the rate and rhythm of the heartbeat.
With each heartbeat, an electrical signal travels from the top of the heart to the bottom. As the signal travels, it causes the heart to contract and pump blood. The heartbeat process includes the following steps.
Your heart’s conduction system. When special cells called pacemaker cells generate electrical signals inside your heart, the heart muscle cells, called myocytes, contract as a group. Medical Animation Copyright © 2019 Nucleus Medical Media, All rights reserved.
Some conditions affect the heart's electrical system. Examples include:
How fast and hard your heart beats is controlled by signals from your body’s nervous system, as well as by hormones from your endocrine system. These signals and hormones allow you to adapt to changes in the amount of oxygen and nutrients your body needs. For example, when you exercise, your muscles need more oxygen, so your heart beats faster. When you sleep, your heart beats slower.
Your blood pressure is the force of the blood pushing against the walls of your arteries as the heart pumps blood. It is made up of two numbers: systolic and diastolic.
For most adults, healthy blood pressure is usually less than 120 over 80, which is written as your systolic pressure number over your diastolic pressure number.
High blood pressure is what happens when blood flows through blood vessels at higher-than-normal pressures.
Your heart rate is controlled by the autonomic nervous system, also called the involuntary nervous system because it happens without your thinking about it. There are two opposing effects of the autonomic nervous system on your heart.
In a healthy person, the heart rate reflects a balance between these two systems.
A number of hormones from the endocrine system affect your heart and blood vessels.
Low levels of the hormone epinephrine, also called adrenaline, cause blood vessels to relax and widen. High levels of this same hormone, along with the hormone norepinephrine, cause the blood vessels to narrow and the heart rate to rise, increasing blood pressure.
Hormones also control how much water and salt your kidneys remove from your blood to excrete as urine. When your blood volume is low, such as when you are losing blood, certain hormones prevent water loss to help maintain your blood volume and blood pressure. The hormones also cause the blood vessels to narrow to maintain blood pressure. These hormones include:
Some hormones cause the kidneys to remove more water and salt from the blood. The decreased blood volume and salt cause your blood vessels to relax and lower your blood pressure. Atrial natriuretic peptide is a hormone made and released by heart cells when the pressure inside the atria is elevated.
The thyroid gland releases thyroid hormones that increase the heart rate. Problems with your thyroid gland can lead to heart problems such as an irregular heartbeat. Too much thyroid hormone can cause the heart to beat faster. Too little thyroid hormone can slow your heart rate.
The NHLBI is part of the U.S. Department of Health and Human Services’ National Institutes of Health (NIH)—the Nation’s biomedical research agency that makes important scientific discoveries to improve health and save lives. We are committed to advancing science and translating discoveries into clinical practice to promote the prevention and treatment of heart, lung, blood, and sleep disorders, including heart conditions. Learn about current and future NHLBI efforts to improve health through research and scientific discovery.
Learn about the following ways the NHLBI continues to translate current research and science into improved health for people who have heart conditions. Research on this topic is part of the NHLBI's broader commitment to advancing heart and vascular disease scientific discovery.
Learn about some of the pioneering research contributions we have made over the years that have improved clinical care.
In support of our mission, we are committed to advancing heart research, in part, through the following ways.
Learn about exciting research areas the NHLBI is exploring about the heart.
We lead or sponsor many studies on the heart. See if you or someone you know is eligible to participate in our clinical trials.
To learn more about clinical trials at the NIH Clinical Center or to talk to someone about a study that might fit your needs, call the Office of Patient Recruitment 800-411-1222.
Learn more about participating in a clinical trial.
View all trials from ClinicalTrials.gov.
Visit Children and Clinical Studies to hear experts, parents, and children talk about their experiences with clinical research.
After reading our How the Heart Works Health Topic, you may be interested in additional information found in the following resources.
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