The human heart has four chambers: the right atrium, left atrium, right ventricle, and left ventricle. The two upper chambers (atria) receive blood entering the heart, while the two lower chambers (ventricles) pump blood out. A wall of tissue called the septum divides the heart’s right and left sides, keeping oxygen-rich and oxygen-poor blood completely separate.
The Two Atria: Receiving Chambers
The right atrium and left atrium sit at the top of the heart and act as holding rooms for incoming blood. The right atrium collects oxygen-poor blood returning from the body through two large veins, the superior and inferior vena cava. The left atrium receives oxygen-rich blood coming back from the lungs through the pulmonary veins.
Both atria are relatively thin-walled because they don’t need much force to move blood. Their job is straightforward: fill up, then squeeze blood down into the ventricles below. A healthy left atrium holds roughly 45 milliliters of blood on average, while the right atrium holds about 37.5 milliliters, based on measurements from a study of over 700 healthy adults published in the European Heart Journal.
The Two Ventricles: Pumping Chambers
The right ventricle and left ventricle are the heart’s real workhorses. After receiving blood from the atria above, they generate the pressure needed to push blood out of the heart entirely.
The right ventricle pumps oxygen-poor blood to the lungs through the pulmonary artery. This is a relatively short trip, so the right ventricle doesn’t need to be especially powerful. Its wall is thin, normally between 1 and 5 millimeters.
The left ventricle pumps oxygen-rich blood to every organ and tissue in your body through the aorta, the largest artery. Because it needs to generate enough pressure to reach your head, your toes, and everything in between, the left ventricle is the most muscular chamber. Its wall is roughly 6 to 10 millimeters thick in a healthy heart, several times thicker than the right ventricle’s wall. When fully filled, the left ventricle holds about 94 milliliters of blood on average, then squeezes down to about 34 milliliters after each contraction.
How Blood Flows Through All Four Chambers
Blood moves through the heart in a fixed, one-way loop. Oxygen-poor blood enters the right atrium, drops into the right ventricle, and gets pumped to the lungs. In the lungs, it picks up fresh oxygen and releases carbon dioxide. The now oxygen-rich blood returns to the left atrium, drops into the left ventricle, and gets pumped out to the rest of the body. After delivering oxygen to tissues, the blood cycles back to the right atrium and the process starts again.
This means the right side of the heart handles only oxygen-poor blood, while the left side handles only oxygen-rich blood. The septum between them ensures the two never mix.
The Four Valves That Separate Chambers
Each chamber connects to the next through a one-way valve that prevents blood from flowing backward. There are four valves total:
- Tricuspid valve: sits between the right atrium and right ventricle
- Pulmonary valve: sits between the right ventricle and the pulmonary artery (leading to the lungs)
- Mitral valve: sits between the left atrium and left ventricle
- Aortic valve: sits between the left ventricle and the aorta (leading to the body)
The tricuspid and mitral valves open when the atria contract, letting blood pour into the ventricles. They snap shut when the ventricles contract, so blood can only exit forward through the pulmonary or aortic valves. The “lub-dub” sound of a heartbeat is actually the sound of these valves closing in sequence.
Why the Atria and Ventricles Contract at Different Times
The four chambers don’t all squeeze at once. The atria contract first, pushing blood down into the ventricles. A fraction of a second later, the ventricles contract to push blood out of the heart. This timing matters because the ventricles need a moment to fill completely before they fire.
The heart manages this through its electrical conduction system. A natural pacemaker at the top of the right atrium sends an electrical signal that makes both atria contract. That signal then reaches a relay point near the center of the heart called the AV node, which deliberately delays it by a fraction of a second. This brief pause ensures the atria have finished emptying before the ventricles begin their much more powerful contraction. The coordination happens automatically, every beat, roughly 100,000 times a day.