How the Heart Works

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.

The heart is made of three layers of tissue.

• Endocardium, the thin inner lining of the heart chambers that also forms the surface of the valves.
• Myocardium, the thick middle layer of muscle that allows your heart chambers to contract and relax to pump blood to your body.
• Pericardium, the sac that surrounds your heart. Made of thin layers of tissue, it holds the heart in place and protects it. A small amount of fluid between the layers helps reduce friction between the beating heart and surrounding tissues.

Some conditions can affect the heart's tissue. Examples include:

• Cardiomyopathy, in which the heart muscle becomes enlarged, thick, or rigid. As cardiomyopathy worsens, the heart becomes weaker and is less able to pump blood through the body and maintain a normal electrical rhythm.
• Heart inflammation, which is inflammation in one or more of the layers of tissue in the heart, including the pericardium, myocardium, or endocardium. This can lead to serious complications, including heart failure, cardiogenic shock, or irregular heart rhythm.

How does the heart form in the womb?

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 tricuspid valve separates the right atrium and right ventricle.
• The mitral valve separates the left atrium and left ventricle.
• The pulmonary valve separates the right ventricle and the pulmonary artery.
• The aortic valve separates the left ventricle and aorta.

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.

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.

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.

• The left coronary artery delivers blood to the left side of your heart, including your left atrium and ventricle and the septum between the ventricles.
• The circumflex artery branches off from the left coronary artery to supply blood to part of the left ventricle.
• The left anterior descending artery also branches from the left coronary artery and provides blood to parts of both the right and left ventricles.
• The right coronary artery provides blood to the right atrium and parts of both ventricles.
• The marginal arteries branch from the right coronary artery and provide blood to the surface of the right atrium.
• The posterior descending artery also branches from the right coronary artery and provides blood to the bottom of both ventricles.

Some conditions can affect normal blood flow through these heart arteries. Examples include:

• Angina
• Heart attack
• Ischemic heart disease

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:

• The anterior cardiac veins.
• The great cardiac vein.
• The middle cardiac vein.
• The small cardiac vein.

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.

• After your atria pump blood into the ventricles, the valves between the atria and ventricles close to prevent backflow. The “lub” is the sound of these valves closing.
• After your ventricles contract to pump blood away from the heart, the aortic and pulmonary valves close and make the “DUB” sound.

What is my pulse, and how do I measure it?

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.

• The signal begins in a group of cells, called pacemaker cells, located in the sinoatrial (SA) node in the right atrium.
• The electrical signal travels through the atria, causing them to pump blood into the ventricles.
• The electrical signal then moves down to a group of pacemaker cells called the atrioventricular (AV) node, located between the atria and the ventricles. Here the signal slows down slightly, allowing the ventricles time to finish filling with blood.
• The AV node fires another signal that travels along the walls of your ventricles, causing them to contract and pump blood out of your heart.
• The ventricles relax, and the heartbeat process starts all over again in the SA node.

Some conditions affect the heart's electrical system. Examples include:

• Arrhythmia, or an irregular heart rhythm. Atrial fibrillation is one of the most common types of arrhythmia.
• Conduction disorders, in which electrical signals either do not generate properly, or they do not travel properly through the heart, or both.

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.

• Systolic pressure is the pressure when the ventricles pump blood out of the heart. The pressure on your arteries is highest during this time.
• Diastolic pressure is the pressure between beats, when the heart is filling with blood. The pressure on your arteries is lowest during this time.

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.

• The parasympathetic system tells your heart to beat slower during rest.
• The sympathetic system tells your heart to beat faster. This is the “fight-or-flight response.” When activated, it releases a chemical signal called norepinephrine that causes the heart to beat faster. Norepinephrine also signals the muscle in your heart to beat harder.

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:

• The renin-angiotensin-aldosterone system, which can also cause the muscle cells in the heart to grow larger so they can pump harder.
• Vasopressin, released from the pituitary gland.

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.