Blood flows through the heart in a single, continuous loop: in through the right side, out to the lungs to pick up oxygen, back in through the left side, and out to the rest of the body. The entire circuit takes about one minute at rest, with the heart pumping 5 to 6 liters of blood per minute. Understanding this pathway helps make sense of everything from heart murmurs to why certain conditions affect one side of the heart more than the other.
The Right Side: Receiving and Sending Blood to the Lungs
Blood that has already delivered its oxygen to your muscles, organs, and tissues returns to the heart through two large veins. The superior vena cava collects blood from your head, arms, and upper body. The inferior vena cava collects blood from your lower body and legs. Both empty into the right atrium, the heart’s upper-right chamber.
From the right atrium, blood passes through a one-way valve into the right ventricle, the lower-right chamber. When the right ventricle contracts, it pushes blood through another valve and into the pulmonary artery, which carries it to the lungs. This is the only artery in the body that carries oxygen-poor blood. The right ventricle doesn’t need to generate much pressure for this job because the lungs are just inches away, which is why the muscle wall on the right side of the heart is noticeably thinner than the left.
The Lungs: Where Blood Picks Up Oxygen
Once blood reaches the lungs, it flows through a web of tiny capillaries wrapped around air sacs called alveoli. Carbon dioxide, a waste product your cells produced, moves out of the blood and into the air sacs so you can exhale it. At the same time, fresh oxygen from the air you just inhaled crosses into the blood. This gas exchange happens almost instantly.
The now oxygen-rich blood leaves the lungs through the pulmonary veins, four in total, two from each lung. These are the only veins in the body that carry oxygen-rich blood. They deliver it to the left atrium, the heart’s upper-left chamber, completing the pulmonary circuit.
The Left Side: Pumping Blood to the Body
From the left atrium, blood passes through a valve into the left ventricle, the heart’s largest and most powerful chamber. The left ventricle has to push blood through the aorta, the body’s largest artery, and from there to every organ and tissue from your brain to your toes. That requires significant force, which is why the muscular wall of the left ventricle is roughly three times thicker than the right ventricle’s wall.
When the left ventricle contracts, it sends about 70 milliliters of blood per beat into the aorta. At a resting heart rate of 60 to 100 beats per minute, that adds up to roughly 5 to 6 liters every minute. During intense exercise, cardiac output can increase four or five times that amount as your heart beats faster and ejects more blood with each contraction.
Valves Keep Blood Moving in One Direction
Four valves act as one-way gates to prevent blood from flowing backward. On the right side, the tricuspid valve sits between the right atrium and right ventricle, and the pulmonary valve guards the exit into the pulmonary artery. On the left side, the mitral valve separates the left atrium and left ventricle, and the aortic valve controls flow into the aorta.
Each valve opens when blood pressure on one side exceeds pressure on the other, then snaps shut once the blood has passed. The “lub-dub” sound of a heartbeat is literally the sound of these valves closing. The first sound (“lub”) comes from the tricuspid and mitral valves shutting as the ventricles begin to contract. The second sound (“dub”) comes from the pulmonary and aortic valves shutting after the ventricles finish pushing blood out. A heart murmur is an extra or unusual sound, often caused by blood leaking backward through a valve that doesn’t close completely.
How the Heart Feeds Itself
The heart can’t absorb oxygen from the blood passing through its chambers. It needs its own dedicated supply, delivered by the coronary arteries. These small vessels branch off the aorta just above the aortic valve and spread across the surface of the heart before diving into the muscle tissue.
Most of the heart’s own blood supply arrives between beats, during the relaxation phase called diastole. This happens because the contracting heart muscle squeezes its own blood vessels during each beat, temporarily reducing flow. When the muscle relaxes, the vessels open back up and blood rushes in. This is one reason a very fast heart rate can become a problem: shorter rest periods between beats mean less time for the heart to receive its own blood supply.
The Full Cycle in Sequence
Putting it all together, here’s the path a single red blood cell takes:
- Superior or inferior vena cava brings oxygen-poor blood into the right atrium
- Tricuspid valve opens to let blood into the right ventricle
- Pulmonary valve opens as the right ventricle contracts, sending blood into the pulmonary artery
- Lungs exchange carbon dioxide for fresh oxygen
- Pulmonary veins carry oxygen-rich blood back to the left atrium
- Mitral valve opens to let blood into the left ventricle
- Aortic valve opens as the left ventricle contracts, sending blood into the aorta
- Aorta distributes blood to the rest of the body
The right and left sides of the heart beat simultaneously. While the right ventricle is sending blood to the lungs, the left ventricle is sending blood to the body. This coordinated squeeze happens with every single heartbeat, roughly 100,000 times a day.
How Fetal Blood Flow Differs
Before birth, this pathway works differently because a fetus doesn’t breathe. Oxygen comes from the placenta through the umbilical cord, not from the lungs. Since the lungs aren’t functional yet, the fetal heart uses two built-in shortcuts to reroute blood around them. The foramen ovale is a small opening between the right and left atria that lets blood pass directly from one side to the other. The ductus arteriosus is a short vessel connecting the pulmonary artery to the aorta, diverting blood that would otherwise go to the lungs.
Both of these shortcuts normally close within the first hours or days after birth, once the baby starts breathing and the lungs take over gas exchange. When they don’t close on their own, the result is a congenital heart defect that may need monitoring or treatment depending on how much it affects blood flow.