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In this article:
Key Points
- The heart is a four-chambered pump that sends blood to two places: the lungs to get more oxygen, and the body that uses the oxygen for making energy.
- The nervous system controls the pumping of the heart, speeding it up during stress or exercise and slowing it down during rest.
- Ischemia is a condition during which the heart does not get enough oxygen-rich blood.
If you've just learned that you have heart disease, you may be feeling overwhelmed, confused, or upset. Emotions aside, you may also find the disease difficult to understand.
Heart disease is complex. And while you and your doctor may have talked about your own case of angina, and about heart disease in general, you probably still have questions.
A good way to find answers is to learn about your body's cardiovascular system (the heart and the network of arteries and veins that carry blood throughout the body). This section of LifeHeart.com explains the basics of a healthy heart and how it functions.
The hardworking heart
The heart is a major organ of your body. It lies inside the chest between your lungs and behind your breastbone, also called the sternum. It rests on the diaphragm, a muscle that separates the chest and abdominal cavities.1 Basically, the heart is a four-chambered muscle that works like two pumps. The right pump sends blood out to the lungs to get more oxygen. Blood that is oxygen-poor is often described as being dark blue in color. The left pump sends oxygen-rich blood out to the body so that the body has enough nourishment to work effectively. Blood that is bright red in color is loaded with oxygen.
The heart is made up of three layers of tissue: The smooth inner lining is the endocardium, the middle muscle layer is the myocardium, and the fluid-filled outer sac is the pericardium. The myocardium squeezes (called a contraction) to help blood pass through the chambers of the heart and out to the lungs and the rest of the body. It then relaxes to allow more blood to enter. This squeezing and relaxation is the heartbeat, also known as the heart rate (which sounds like "lub-DUB, lub-DUB, lub-DUB"). You also can "feel" your heartbeat as your pulse by pressing on an artery in your wrist, forearm, or neck.
A resting, healthy adult has a heartbeat of about 60 to 75 beats per minute, or an average of about one beat per second. Over a lifetime, this adds up to nearly 2.5 billion beats—proof that the heart is a very hardworking muscle (Learn more about pulse in Blood Pressure).
The heart's four chambers at work
Each of the heart’s four chambers plays a role in helping blood pass into and back out of the heart. The smaller two chambers, near the top of the heart, are called the right and left atria (or atrium, if you're talking about them separately). The larger two chambers, on the bottom, are the right and left ventricles. Each of the chambers is working during every one-second heartbeat in a process called contraction and relaxation. Four one-way valves keep blood moving in the right direction.1 Here's how it's done:
- The right atrium is relaxed when it receives dark blue blood from the rest of the body. During contraction, it pumps the blood through the tricuspid valve into the right ventricle. When it relaxes again, more blood is allowed to enter.
- Next, the right ventricle contracts and pumps dark blue blood through the pulmonary valve into the lungs where the blood picks up oxygen. When the right ventricle relaxes again, more blue blood is received from the right atrium.
- At the same time, the left atrium and left ventricle are working, too. The oxygen-rich, bright red blood leaving the lungs returns to the heart and enters the left atrium. When the left atrium squeezes it sends blood into the left ventricle through the bicuspid valve, then it relaxes and receives more oxygen-rich blood from the lungs.
- Finally, the left ventricle—the workhorse of the heart—must contract hard enough to send blood with enough force through the aortic valve to reach the entire body. Then the left ventricle rests long enough to fill up with blood again from the left atrium.
The first sound you hear of your heartbeat ("lub") is the closing of the tricuspid and bicuspid valves when the ventricles begin to contract. The second sound ("DUB") is the closing of the pulmonary and aortic valves after blood is pumped from the ventricles when the contraction is finished. Remarkably, all of this happens in about one second.
The heart's electrical system
The heart's four chambers work in a process called contraction and relaxation. The electrical system of the heart (the conduction system) tells the heart when to contract, or beat. The timing of the electrical signal must be perfect so that all four chambers can do their part within a fraction of every second.
The conduction system is influenced by the body's nervous system via electrical signals that travel from the brain to the heart. The nervous system can change the speed of your heartbeat in a matter of seconds. If you are feeling stressed, for example, your heartbeat may speed up. Your heartbeat will also increase when your body needs more blood, such as when you exercise.2 If you are relaxed, your heartbeat may slow down.
Your doctor measures the health of your conduction system with a device called an electrocardiogram, more commonly known as the EKG or ECG. If your EKG test results are not normal, it may be a sign that your heart may not be working properly. In addition, an EKG may detect an abnormal heartbeat called an arrhythmia that may mean the heart is not working effectively.
The heart is also a furnace
The body is capable of only a certain amount of physical effort before the muscles become exhausted. But the heart must keep beating continuously with rest periods of only a fraction of a second, every second. To do that, the heart must have a constant supply of fuel to keep going.
Where does this fuel come from? All cells in the body contain structures called mitochondria. They are the cells' powerhouses. Mitochondria use oxygen and special proteins called enzymes to change nutrients into energy. This energy is used to create a fuel called adenosine triphosphate, or ATP. ATP remains available in each cell until the cell receives a signal that more fuel is needed.
The heart itself has thousands of these tiny powerhouses in every cell to produce the ATP it needs. In fact, about one-quarter to one-third of the entire contents of each heart muscle cell are mitochondria.3 This is how the heart acts as a furnace, a comparison often made by Carl S. Apstein, M.D., an expert in cardiac metabolism at the Boston University School of Medicine.4
The amount of oxygen the heart uses is very important. The less oxygen the heart receives, the less work it can perform. Oxygen supply can be low if there are too few red blood cells in the blood (anemia). It can also be low if not enough oxygen-rich blood reaches the heart (ischemia) because of atherosclerosis or coronary artery disease. Learn more in Coronary Artery Disease.
Sources
1. The New Encyclopedia Britannica, volume 5 Micropedia Ready Reference, 2003, pp 782-783.
2. The World Book Encyclopedia, volume 9, 2004, pp 132-147.
3. Opie, L.H. "Mechanisms of Cardiac Contraction and Relaxation." Heart Disease: A Textbook of Cardiovascular Medicine, 6th ed., Ed. Eugene Braunwald. Philadelphia: W.B. Saunders, 2001. 443.
4. Apstein, Carl S., M.D. Used with permission
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