Blood follows a single, looping path through your heart, entering oxygen-poor and leaving oxygen-rich. The full journey passes through four chambers, four valves, and two separate circuits in a sequence that repeats roughly 100,000 times a day. At a resting heart rate of about 75 beats per minute, each complete cycle takes just 0.8 seconds.
The Complete Path in Order
Blood returning from your body has already delivered its oxygen to your tissues. This dark, oxygen-poor blood funnels into two large veins: the superior vena cava (draining your head, arms, and upper body) and the inferior vena cava (draining everything below). Both empty into the right atrium, the heart’s upper-right chamber. Oxygen saturation at this point averages around 79%, compared to about 97% in oxygen-rich blood leaving the lungs.
From the right atrium, blood passes through the tricuspid valve into the right ventricle. When the right ventricle fills and contracts, it pushes blood through the pulmonary valve into the pulmonary artery. This is where the naming gets counterintuitive: the pulmonary arteries are the only arteries in your body that carry oxygen-poor blood. They deliver it to the lungs, where it picks up oxygen and releases carbon dioxide.
Freshly oxygenated blood returns to the heart through the pulmonary veins, again breaking the usual rule. These are the only veins that carry oxygen-rich blood. They empty into the left atrium, the upper-left chamber. Blood then passes through the mitral valve into the left ventricle, the heart’s most muscular chamber. When the left ventricle contracts, it forces blood through the aortic valve into the aorta, the body’s largest artery, which distributes it to every organ and tissue.
So the full sequence is: vena cava → right atrium → tricuspid valve → right ventricle → pulmonary valve → pulmonary artery → lungs → pulmonary veins → left atrium → mitral valve → left ventricle → aortic valve → aorta → body.
Two Circuits, One Continuous Loop
Your heart is really two pumps sitting side by side. The right side handles the pulmonary circuit, a short, low-pressure loop that sends blood to the lungs and back. The left side handles the systemic circuit, which pushes blood to every other part of your body. These two circuits are connected in series: blood must complete one before entering the other, and it passes through the heart twice during each full loop around your body.
The pressure difference between the two sides reflects the workload. The right ventricle generates a peak pressure of about 25 mmHg to push blood through the nearby lungs. The left ventricle produces roughly 130 mmHg, over five times more, because it needs enough force to reach your brain, your toes, and everything in between. That’s why the left ventricle’s muscular wall is significantly thicker than the right’s.
How the Valves Control Flow
Your heart’s four valves are one-way gates that open and close based on pressure differences. They don’t have muscles of their own. Instead, they respond passively to the blood pushing against them.
The two valves between the atria and ventricles (tricuspid on the right, mitral on the left) are called atrioventricular valves. They open when the atria contract and pressure above the valve exceeds pressure below it, letting blood drop into the ventricles. Once the ventricles start contracting and their internal pressure rises above atrial pressure, these valves snap shut. That closure prevents blood from flowing backward into the atria and produces the first sound of your heartbeat.
The two outflow valves (pulmonary and aortic) work on the same principle but in the opposite direction. They stay closed while the ventricles are filling. Once ventricular pressure exceeds the pressure in the pulmonary artery or aorta, these valves open and blood is ejected. When the ventricles relax and pressure drops, blood briefly starts to flow backward, catching the valve leaflets and pushing them shut. That closure produces the second heart sound.
There are brief moments during each cycle when all four valves are closed simultaneously. During these phases the ventricles are either beginning to contract or beginning to relax, but no blood is moving in or out. The volume of blood inside the ventricle stays constant, building or releasing pressure before the next valve opens.
What Happens in the Lungs
The pulmonary artery branches into smaller and smaller vessels until blood reaches the capillaries wrapped around tiny air sacs in the lungs. Here, gas exchange happens almost instantly. Carbon dioxide moves out of the blood and into the air sacs to be exhaled. Oxygen moves in the opposite direction, binding to red blood cells. Blood that entered the lungs at roughly 79% oxygen saturation leaves at about 97%. This oxygen-rich blood collects into the four pulmonary veins and flows back to the heart’s left atrium, ready to be pumped out to the body.
How the Heart Feeds Itself
The heart muscle doesn’t absorb oxygen from the blood passing through its chambers. Instead, it has its own dedicated blood supply. Just above the aortic valve, two small coronary arteries branch off from the aorta and wrap around the heart’s surface, delivering oxygen-rich blood directly to the heart muscle. Most of this flow occurs when the heart is relaxing between beats, because contraction squeezes the coronary vessels and temporarily restricts flow. Since the heart spends about two-thirds of each cycle in this relaxed state, there’s enough time for adequate blood delivery under normal conditions.
Cardiac Output at Rest
At rest, your heart pumps between 5 and 6 liters of blood per minute. That’s roughly your entire blood volume circulating once every minute. Each contraction of the left ventricle ejects a portion of blood called the stroke volume, and multiplying that by your heart rate gives you the total output. During exercise, cardiac output can increase dramatically as both heart rate and stroke volume rise to meet your muscles’ demand for oxygen.
How Fetal Blood Flow Differs
Before birth, the pathway through the heart looks quite different. A fetus gets oxygen from the placenta, not the lungs, so there’s no reason to pump large volumes of blood through lung tissue that isn’t yet breathing. The fetal heart uses three temporary shortcuts, called shunts, to reroute blood away from the lungs and liver.
The foramen ovale is a small opening in the wall between the right and left atria, allowing oxygen-rich blood from the placenta to pass directly to the left side of the heart without going through the lungs. The ductus arteriosus connects the pulmonary artery to the aorta, diverting even more blood away from the lungs. A third shunt, the ductus venosus, bypasses the liver by channeling blood from the umbilical vein directly into the inferior vena cava. All three shunts normally close within hours to days after birth, once the baby takes its first breaths and the lungs take over gas exchange. At that point, the heart transitions to the two-circuit pathway it will use for the rest of life.