Blood flows through your heart in a single, looping path: it enters the right side, travels to the lungs to pick up oxygen, returns to the left side, and gets pumped out to the rest of your body. The entire journey through all four chambers takes about one heartbeat, and your heart pushes roughly 70 milliliters of blood with each squeeze, adding up to about 5 liters per minute at rest.
Understanding this flow step by step makes it easier to grasp how your heart keeps every organ supplied with oxygen and why problems at any point in the loop can cause serious symptoms.
The Complete Path, Step by Step
Blood returning from your body has already delivered its oxygen to your tissues. This oxygen-poor blood drains into two large veins: the superior vena cava (carrying blood from your head and arms) and the inferior vena cava (carrying blood from your torso and legs). Both empty into the right atrium, the upper chamber on the right side of your heart.
From the right atrium, blood flows down through the tricuspid valve into the right ventricle. The right ventricle then contracts and pushes that blood through the pulmonary valve into the pulmonary artery, which leads to the lungs. This is the only artery in your body that carries oxygen-poor blood.
Inside the lungs, blood releases carbon dioxide and loads up on fresh oxygen. It then travels back to the heart through four pulmonary veins, entering the left atrium. From the left atrium, blood passes through the mitral valve into the left ventricle, the heart’s most muscular chamber. The left ventricle contracts forcefully, sending oxygen-rich blood through the aortic valve into the aorta, your body’s largest artery. From the aorta, blood branches into smaller and smaller arteries until it reaches the capillaries in your tissues, delivers its oxygen, and begins the return trip through veins back to the right atrium.
Two Loops Working Together
Your circulatory system is really two circuits connected by the heart. The pulmonary circuit is the short loop: right ventricle to lungs and back to the left atrium. Its only job is gas exchange, swapping carbon dioxide for oxygen. The systemic circuit is the long loop: left ventricle to the entire body and back to the right atrium. Every artery in your body branches either directly or indirectly from the aorta, and every vein eventually funnels back into the superior or inferior vena cava.
The right side of the heart handles only the pulmonary loop, so it doesn’t need to generate as much pressure. The left side powers the systemic loop, pushing blood all the way to your feet and back. That’s why the left ventricle’s muscular wall is noticeably thicker than the right’s.
What the Four Valves Do
Four one-way valves keep blood moving forward and prevent it from leaking backward. They open and close passively in response to pressure changes, not from any muscle of their own.
- Tricuspid valve: sits between the right atrium and right ventricle. Its three flaps open to let blood fill the right ventricle, then snap shut when the ventricle contracts so blood can’t surge back into the atrium.
- Pulmonary valve: guards the exit from the right ventricle into the pulmonary artery. It opens during contraction and closes afterward to prevent blood from sliding back into the ventricle.
- Mitral valve: connects the left atrium to the left ventricle. It has only two flaps (unlike the other valves, which have three) and prevents backflow into the atrium when the left ventricle squeezes.
- Aortic valve: sits between the left ventricle and the aorta. It opens to let blood into the aorta and closes to keep that blood from falling back into the ventricle between beats.
When a valve doesn’t close tightly (a condition called regurgitation) or doesn’t open fully (stenosis), the heart has to work harder to move the same amount of blood. This is what doctors are listening for with a stethoscope: abnormal sounds called murmurs that indicate turbulent flow around a faulty valve.
How the Heart Times Each Squeeze
Your heart doesn’t contract all at once. A small cluster of pacemaker cells in the right atrium, called the SA node, fires an electrical signal that spreads across both atria, causing them to contract first and push blood down into the ventricles. The signal then reaches a second relay point, the AV node, located between the atria and ventricles. Here it pauses briefly, giving the ventricles an extra moment to finish filling. After that short delay, the signal races along the walls of both ventricles, triggering a powerful contraction that ejects blood into the pulmonary artery and aorta simultaneously.
This two-step timing is critical. The atria squeeze first (atrial systole), topping off the ventricles. Then the ventricles squeeze (ventricular systole), pushing blood out of the heart. The period when the ventricles relax and refill is called diastole. During diastole, the elastic recoil of your arteries helps push blood back toward the heart, keeping flow continuous even between beats.
What Drives Blood Forward: Pressure
Blood always moves from areas of higher pressure to areas of lower pressure. When a ventricle contracts, the pressure inside it rises above the pressure in the artery beyond it, forcing the outflow valve open. When the ventricle relaxes, the pressure drops below the artery’s pressure, and the valve slams shut. Meanwhile, the pressure inside the ventricle also drops below the atrium’s pressure, which opens the inflow valve and lets blood pour in for the next cycle.
Your arteries store some of that pumping energy in their elastic walls, stretching slightly with each heartbeat and then springing back. This is what you feel as your pulse. It also smooths out the flow so blood doesn’t stop and start with every beat but instead moves in a more continuous stream through your capillaries.
Where Oxygen Levels Change
The right side of the heart handles only oxygen-poor blood. Everything entering the right atrium has already given up most of its oxygen to body tissues. This blood stays oxygen-poor through the right ventricle and into the pulmonary arteries.
The switch happens in the lungs. Tiny air sacs called alveoli sit right next to the thinnest capillaries in your body. Carbon dioxide passes from the blood into the air sacs (you exhale it), and oxygen passes from the air sacs into the blood. By the time blood leaves the lungs through the pulmonary veins, it’s fully oxygenated.
From that point on, the left side of the heart handles oxygen-rich blood. The left atrium, left ventricle, and aorta all carry freshly oxygenated blood that’s ready to fuel your organs, muscles, and brain. Once it delivers that oxygen at the capillary level, it becomes oxygen-poor again and the cycle repeats.
How Much Blood Moves Per Beat
A healthy adult heart ejects about 70 mL of blood per beat, roughly the volume of a small espresso cup. At an average resting heart rate of about 70 beats per minute, that works out to nearly 5 liters per minute flowing through both circuits. During intense exercise, your heart can increase that output four to five times by beating faster and squeezing more forcefully with each contraction.
Both ventricles pump the same volume per beat. If one side consistently pumped more than the other, blood would pool on one side of the circuit, which is essentially what happens in certain types of heart failure. The matched output of the right and left ventricles keeps the two loops balanced and blood circulating smoothly.