In this lesson, you will take a look inside the heart and identify the chambers and one-way valves that make up this vital organ. You will also follow along as we trace the flow of blood through the heart chambers.
In your body, blood flows within a closed circuit of blood vessels. Blood is able to circulate around your body thanks to a muscular pump known as your heart. As we previously learned, the heart is the ultimate workaholic, and it pushes your body's complete supply of blood through your blood vessels over 1,000 times a day. In this lesson, we will take a look inside the heart and identify the structures that make up this vital organ. You will also follow along as we trace the flow of blood through the heart chambers.
The ventricles pump blood out of the heart.
Your heart has four hollow chambers. The two chambers on top are called the atria. These are the heart chambers that receive blood returning to the heart. Because they are primarily receiving chambers, we do not think of the atria as the true pumps of the heart. Instead, blood flows into the atria under low pressure from the blood vessels and then passively drains through on its way to the lower two chambers, which are called the ventricles. These are the heart chambers that pump blood out of the heart. When your heart contracts, it's fair to think of it as your ventricles contracting, because they are the primary pumping chambers of your heart.
You can imagine that when your ventricles contract, the blood inside is forced out under a lot of pressure, and you may be wondering what prevents blood from flowing backward and clogging up the atria. This backward flow is prevented thanks to four one-way valves. The two atrioventricular valves, or AV valves, separate the atria and the ventricles, which makes it easy to remember their name. These valves are open when the heart is relaxed and passively receiving blood, but when the ventricles contract, the AV valves snap shut, preventing backflow into the atria. Interestingly, it is this snapping shut of the AV valves that produces the familiar 'lub' sound that you hear when you're listening to your heart using a stethoscope.
The characteristic 'dub' sound is caused by the closing of the other two valves. These valves are called the semilunar valves and they separate the ventricles from the blood vessels leaving the heart. When blood is forced out of the ventricles, the semilunar valves open wide, allowing the blood to pass into the arteries. Then, when the ventricles relax, they quickly close with a 'dub,' preventing blood from flowing backward. So, the next time you hear the familiar 'lub-dub, lub-dub' of your heartbeat, you can think about the valves of your heart at work.
The sound of your heartbeat is the valves opening and closing.
Now that we identified the chambers and valves, let's take a closer look at what happens during a heartbeat. First, blood comes into the atria and then passively flows into the ventricles. When the ventricles are almost full, the atria contract in unison and push as much blood as possible into the ventricles. The ventricles are now full and under high pressure. At this time, they contract and push blood out of the heart and into the arteries.
As we mentioned, blood follows a one-way path through the heart. Let's follow this path. First, blood from the body enters the right atrium and passively flows into the right ventricle through the AV valve called the tricuspid valve. This is the valve that separates the right atrium and the right ventricle, and its name comes from the fact that it has three flaps (or cusps) that open and close.
We learned earlier that the AV valves are forced shut when the ventricles contract. What you might be wondering is what stops the valve flaps from being pushed upward into the atria like an umbrella on a windy day. This is prevented by tiny white cords that anchor the cusps to the wall of the ventricle. These white cords are commonly referred to as your heartstrings. If you ever feel in love with someone who tugged on your 'heartstrings,' this is where the term originated. Blood now moves from the right ventricle through the pulmonary semilunar valve, which is the valve that separates the right ventricle and the lungs. In the lungs, the blood picks up oxygen and drops off carbon dioxide before returning to the heart. As we previously learned, this is called the pulmonary circuit.
Diagram of the blood flow path in the heart
Now that blood is freshly oxygenated from its trip to the lungs, it returns to the left atrium of the heart and passively flows through the bicuspid (or mitral valve). This is the valve that separates the left atrium and the left ventricle. The blood continues on its one-way path out of the left ventricle through the aortic semilunar valve, which is the valve that separates the left ventricle and the arteries leading to the body. From there, the blood is carried around the body through the systemic circulation before returning to the heart and starting the process all over again.
Let's review. The heart has four hollow chambers. The smaller chambers on top are called atria, and the more muscular chambers on the bottom are called the ventricles. The atria receive blood returning to the heart; the ventricles pump blood out of the heart.
The chambers are separated by valves that prevent the backflow of blood as it moves through the heart. The valves that separate the atria and the ventricles are conveniently called the atrioventricular valves (or AV valves). The valves that separate the ventricles from the blood vessels leaving the heart are called the semilunar valves.
Deoxygenated blood from the body enters the heart at the right atrium and passively travels past the tricuspid valve to fill the right ventricle. When the ventricle is filled, it contracts, forcing the tricuspid valve to close and forcing the blood through the pulmonary semilunar valve on its way to the lungs.
From the lungs, freshly oxygenated blood flows back into the left atrium and passively passes through the mitral or bicuspid valve to fill the left ventricle. The left ventricle then contracts, pushing the blood through the aortic semilunar valve on its way to the cells of the body.
Following this lesson, you'll be able to describe the structures of the heart and how they work to pump blood through the body.