Blood follows a one-way loop through the heart, passing through four chambers and four valves in a fixed sequence that takes less than a second. The path splits into two connected circuits: one sends oxygen-poor blood to the lungs, and the other pumps oxygen-rich blood out to the rest of the body. Here’s exactly how it works, from start to finish.
The Starting Point: Blood Returns to the Heart
Blood that has already delivered its oxygen to your organs and tissues needs to return to the heart for a refill. It gets there through two large veins called the superior and inferior vena cava. The superior vena cava collects blood from your head, neck, arms, and upper chest. The inferior vena cava handles everything below, draining the lower extremities, abdomen, kidneys, and liver. Both veins empty into the right atrium, the heart’s upper-right chamber. This is step one.
Right Side: Sending Blood to the Lungs
Once blood pools in the right atrium, it passes through the tricuspid valve into the right ventricle, the lower-right chamber. The tricuspid valve has three flaps that open to let blood through, then snap shut to prevent it from flowing backward. Think of every valve in the heart as a one-way door.
The right ventricle then contracts and pushes blood through the pulmonary valve into the pulmonary artery. This is the only artery in the body that carries oxygen-poor blood. The pulmonary artery quickly splits into right and left branches, sending blood to both lungs. The right side of the heart operates at relatively low pressure, peaking around 15 to 30 mmHg, because the lungs are close by and don’t require much force to reach.
Inside the lungs, the pulmonary arteries branch into smaller and smaller vessels until they become a dense web of tiny capillaries wrapped around the air sacs where you breathe in oxygen. Carbon dioxide passes out of the blood and into the air sacs to be exhaled, while fresh oxygen passes in. This gas exchange is the entire reason blood makes this trip.
Left Side: Pumping Blood to the Body
Now oxygen-rich, the blood flows through the pulmonary veins (typically four of them, two from each lung) into the left atrium, the upper-left chamber. These are the only veins in the body that carry oxygen-rich blood, which is a detail that trips people up since veins are usually associated with deoxygenated blood.
From the left atrium, blood passes through the mitral valve into the left ventricle. The mitral valve has two flaps instead of three, which is why it’s sometimes called the bicuspid valve. The left ventricle is the largest, most muscular chamber in the heart, and for good reason: it has to generate enough pressure to push blood through the aorta and out to every organ in your body, from your brain down to your toes. Peak pressure in the left ventricle ranges from 90 to 140 mmHg, roughly four to five times higher than the right side.
When the left ventricle contracts, blood is forced through the aortic valve into the aorta, the body’s largest artery. From the aorta, blood branches into progressively smaller arteries and eventually capillaries, where it drops off oxygen and picks up carbon dioxide and other waste. Then it drains into veins, funnels back to the vena cava, and the whole cycle starts again.
The Complete Sequence at a Glance
- Superior and inferior vena cava deliver oxygen-poor blood to the right atrium
- Tricuspid valve opens into the right ventricle
- Pulmonary valve opens into the pulmonary artery, which carries blood to the lungs
- Pulmonary veins return oxygen-rich blood to the left atrium
- Mitral valve opens into the left ventricle
- Aortic valve opens into the aorta, which delivers blood to the entire body
What Makes the Heart Pump: The Electrical Signal
The heart doesn’t just squeeze randomly. Each beat is triggered by an electrical impulse that follows its own precise route. The signal starts in the sinoatrial node, a small cluster of cells in the upper wall of the right atrium that acts as the heart’s natural pacemaker. This node fires automatically, setting your heart rate.
The electrical signal spreads across both atria, causing them to contract and push blood down into the ventricles. The signal then reaches the atrioventricular node, located near the center of the heart between the atria and ventricles. This node briefly delays the signal, giving the ventricles time to fill completely before they contract. From there, the impulse travels down a pathway called the His bundle, which splits into right and left branches running along the wall between the ventricles. These branches fan out into a network of Purkinje fibers that spread the signal across the ventricle walls, triggering a powerful, coordinated squeeze that ejects blood into the pulmonary artery and aorta simultaneously.
How a Single Heartbeat Fills and Ejects Blood
Each heartbeat has two main phases. During diastole, the heart muscle relaxes. The ventricles fill with blood in stages: first a rapid rush as the valves between the atria and ventricles open, then a pause where pressures equalize and flow nearly stops, then a final push from the atria contracting to top off the ventricles. During systole, the ventricles contract. For a brief moment at the start, all four valves are closed and pressure builds rapidly inside the ventricles without any blood moving. Once ventricular pressure exceeds the pressure in the arteries beyond, the pulmonary and aortic valves pop open and blood is ejected.
The average heart pushes out about 70 milliliters of blood per beat, roughly the volume of a shot glass and a half. At a resting heart rate of 70 beats per minute, that adds up to nearly 5 liters per minute, enough to circulate your entire blood volume in about one minute. This volume per beat is called stroke volume, and the percentage of blood the ventricle actually ejects compared to how much it held when full is the ejection fraction, a number doctors use to gauge how well the heart is pumping.
Why the Two Sides Work Differently
The right and left sides of the heart beat at the same time, but they serve very different circuits. The right side handles pulmonary circulation, a short, low-pressure loop to the lungs and back. The pulmonary artery travels only about 5 centimeters before splitting into its two main branches, and the entire capillary network in the lungs is designed for easy, low-resistance blood flow to maximize gas exchange.
The left side handles systemic circulation, a high-pressure circuit that must reach every tissue in the body. That’s why the left ventricle’s muscular wall is significantly thicker than the right ventricle’s. Despite the pressure difference, both sides pump the same volume of blood per beat. If they didn’t, blood would pool on one side of the circuit, which is essentially what happens in certain types of heart failure.