What is the valve that allows blood to flow from the left atrium into the left ventricle and prevents the backflow of blood?

Updated by: Michael A. Chen, MD, PhD, Associate Professor of Medicine, Division of Cardiology, Harborview Medical Center, University of Washington Medical School, Seattle, WA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

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Learning Outcomes

• Describe the structure of the heart

The heart muscle is asymmetrical due to the distance blood must travel in the pulmonary and systemic circuits. Since the right side of the heart sends blood to the pulmonary circuit, it is smaller than the left side which must send blood out to the whole body in the systemic circuit, as shown in Figure 1.

Figure 1. The heart is primarily made of a thick muscle layer, called the myocardium, surrounded by membranes. One-way valves separate the four chambers.

In humans, the heart is about the size of a clenched fist, and it is divided into four chambers: two atria and two ventricles. There is one atrium and one ventricle on the right side and one atrium and one ventricle on the left side. The atria are the chambers that receive blood, and the ventricles are the chambers that pump blood. The right atrium receives deoxygenated blood from the superior vena cava, which drains blood from the jugular vein that comes from the brain and from the veins that come from the arms, as well as from the inferior vena cava which drains blood from the veins that come from the lower organs and the legs.

In addition, the right atrium receives blood from the coronary sinus which drains deoxygenated blood from the heart itself. This deoxygenated blood then passes to the right ventricle through the atrioventricular valve or the tricuspid valve, a flap of connective tissue that opens in only one direction to prevent the backflow of blood. The valve separating the chambers on the left side of the heart valve is called the bicuspid or mitral valve. After it is filled, the right ventricle pumps the blood through the pulmonary arteries, by-passing the semilunar valve (or pulmonic valve) to the lungs for re-oxygenation.

After blood passes through the pulmonary arteries, the right semilunar valves close preventing the blood from flowing backwards into the right ventricle. The left atrium then receives the oxygen-rich blood from the lungs via the pulmonary veins. This blood passes through the bicuspid valve or mitral valve (the atrioventricular valve on the left side of the heart) to the left ventricle where the blood is pumped out through aorta, the major artery of the body, taking oxygenated blood to the organs and muscles of the body. Once blood is pumped out of the left ventricle and into the aorta, the aortic semilunar valve (or aortic valve) closes preventing blood from flowing backward into the left ventricle. This pattern of pumping is referred to as double circulation and is found in all mammals.

Which of the following statements about the heart is false?

1. The mitral valve separates the left ventricle from the left atrium.
2. Blood travels through the bicuspid valve to the left atrium.
3. Both the aortic and the pulmonary valves are semilunar valves.
4. The mitral valve is an atrioventricular valve.

The heart is composed of three layers; the epicardium, the myocardium, and the endocardium, illustrated in Figure 1. The inner wall of the heart has a lining called the endocardium. The myocardium consists of the heart muscle cells that make up the middle layer and the bulk of the heart wall. The outer layer of cells is called the epicardium, of which the second layer is a membranous layered structure called the pericardium that surrounds and protects the heart; it allows enough room for vigorous pumping but also keeps the heart in place to reduce friction between the heart and other structures.

Figure 2. Blood vessels of the coronary system, including the coronary arteries and veins, keep the heart musculature oxygenated.

The heart has its own blood vessels that supply the heart muscle with blood (Figure 2). The coronary arteries branch from the aorta and surround the outer surface of the heart like a crown. They diverge into capillaries where the heart muscle is supplied with oxygen before converging again into the coronary veins to take the deoxygenated blood back to the right atrium where the blood will be re-oxygenated through the pulmonary circuit. The heart muscle will die without a steady supply of blood. Atherosclerosis is the blockage of an artery by the buildup of fatty plaques. Because of the size (narrow) of the coronary arteries and their function in serving the heart itself, atherosclerosis can be deadly in these arteries. The slowdown of blood flow and subsequent oxygen deprivation that results from atherosclerosis causes severe pain, known as angina, and complete blockage of the arteries will cause myocardial infarction: the death of cardiac muscle tissue, commonly known as a heart attack.

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Your heart is a strong, muscular organ situated slightly to the left of your chest. It pumps blood to all parts of the body through a network of blood vessels by continuously expanding and contracting. On average, your heart will beat 100,000 times and pump about 2,000 gallons of blood each day.

The heart is divided into a right and left side, separated by a septum. Each side has an atrium (which receives blood as it enters) and a ventricle (from which blood is pumped out). The heart has a total of four chambers: right atrium, right ventricle, left atrium and left ventricle.

The right side of the heart collects oxygen-depleted blood and pumps it to the lungs, through the pulmonary arteries, so that the lungs can refresh the blood with a fresh supply of oxygen.

The left side of the heart receives oxygen-rich blood from the lungs, then pumps blood out to the rest of the body's tissues, through the aorta.

Valves maintain direction of blood flow

As the heart pumps blood, a series of valves open and close tightly. These valves ensure that blood flows in only one direction, preventing backflow.

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

Each heart valve, except for the mitral valve, has three flaps (leaflets) that open and close like gates on a fence. The mitral valve has two valve leaflets.

The circulatory system

While the heart and lungs are the largest organs of the circulatory system, the blood vessels are the longest. This extended network of stretchy tubes circulates blood throughout the body. Laid end-to-end, your body's blood vessels would extend about 60,000 miles. That's more than 21 road trips between New York and Los Angeles!

Arteries (along with smaller arterioles and microscopic capillaries) convey oxygen- and nutrient-rich blood to the body's tissues. In turn, veins bring nutrient-depleted blood back to the heart. Along the way, blood is routed through the kidneys and liver, as well, filtering waste products from the blood.

Electrical impulses keep the beat

The heart's four chambers pump in an organized manner with the help of electrical impulses that originate in the sinoatrial node (also called the "SA node"). Situated on the wall of the right atrium, this small cluster of specialized cells is the heart's natural pacemaker, initiating electrical impulses at a normal rate.

The impulse spreads through the walls of the right and left atria, causing them to contract, forcing blood into the ventricles. The impulse then reaches the atrioventricular (AV) node, which acts as an electrical bridge for impulses to travel from the atria to the ventricles. From there, a pathway of fibers (the HIS-Purkinje network) carries the impulse into the ventricles, which contract and force blood out of the heart.

Heart anatomy: By the numbers

1. Superior vena cava: Receives blood from the upper body; delivers blood into the right atrium.

2. Inferior vena cava: Receives blood from the lower extremities, pelvis and abdomen, and delivers blood into the right atrium.

3. Right atrium: Receives blood returning to the heart from the superior and inferior vena cava; transmits blood to the right ventricle, which pumps blood to the lungs for oxygenation.

4. Tricuspid valve: Allows blood to pass from the right atrium to the right ventricle; prevents blood from flowing back into the right atrium as the heart pumps (systole).

5. Right ventricle: Receives blood from the right atrium; pumps blood into the pulmonary artery.

6. Pulmonary valve: Allows blood to pass into the pulmonary arteries; prevents blood from flowing back into the right ventricle.

7. Pulmonary arteries: Carry oxygen-depleted blood from the heart to the lungs.

8. Pulmonary veins: Deliver oxygen-rich blood from the lungs to the left atrium of the heart.

9. Left atrium: Receives blood returning to the heart from the pulmonary veins.

10. Mitral valve: Allows blood to flow into the left ventricle; prevents blood from flowing back into the left atrium.

11. Left ventricle: Receives oxygen-rich blood from the left atrium and pumps blood into the aorta.

12. Aortic valve: Allows blood to pass from the left ventricle to the aorta; prevents backflow of blood into the left ventricle.

13. Aorta: Distributes blood throughout the body from the heart.