Blood flows through your heart in a single, continuous loop that passes through four chambers and four valves in a specific order. The entire journey, from oxygen-poor blood entering the right side to oxygen-rich blood leaving the left side, takes less than a second at a resting heart rate. Here’s exactly how it works, step by step.
The Complete Sequence, Start to Finish
Blood moves through your heart in this order:
- Superior and inferior vena cava (two large veins delivering oxygen-poor blood from the body)
- Right atrium (upper right chamber)
- Tricuspid valve
- Right ventricle (lower right chamber)
- Pulmonary valve
- Pulmonary arteries (carry blood to the lungs)
- Lungs (blood picks up oxygen and drops off carbon dioxide)
- Pulmonary veins (four veins carry oxygen-rich blood back)
- Left atrium (upper left chamber)
- Mitral valve
- Left ventricle (lower left chamber)
- Aortic valve
- Aorta (the body’s largest artery, sending blood to all tissues)
From the aorta, blood travels through progressively smaller arteries and capillaries, delivers oxygen to your tissues, collects carbon dioxide, and returns through veins to the vena cava. Then the cycle starts again.
The Right Side: Sending Blood to the Lungs
The right side of your heart handles only oxygen-poor blood. Two large veins, the superior vena cava (draining your head and arms) and the inferior vena cava (draining your torso and legs), empty into the right atrium. When the right atrium contracts, it pushes blood through the tricuspid valve into the right ventricle below.
The right ventricle then contracts and pushes blood through the pulmonary valve into the pulmonary arteries, which carry it to the lungs. This is the only place in your body where arteries carry oxygen-poor blood. The right ventricle doesn’t need to generate much force for this job because the lungs are close by. It produces a peak pressure of about 25 mmHg, roughly one-fifth the pressure generated by the left ventricle.
The Lungs: Where Gas Exchange Happens
Inside your lungs, blood flows through tiny capillaries wrapped around air sacs. Carbon dioxide passes out of the blood and into the air you exhale, while fresh oxygen passes in the opposite direction. This transformation is what changes blood from oxygen-poor (darker red) to oxygen-rich (bright red). The newly oxygenated blood then returns to the heart through four pulmonary veins, two from each lung, entering the left atrium.
The Left Side: Pumping Blood to the Body
Oxygen-rich blood pools in the left atrium and flows through the mitral valve into the left ventricle. The left ventricle is the most muscular chamber in your heart, and for good reason: it has to push blood through the aortic valve and into the aorta with enough force to reach every tissue in your body, from your brain to your toes. It generates a peak pressure of about 130 mmHg, which is why the left ventricle wall is significantly thicker than the right.
Once blood enters the aorta, it branches into smaller and smaller arteries, eventually reaching capillaries where oxygen and nutrients are delivered to cells. The now oxygen-depleted blood collects in veins that merge into larger veins, ultimately feeding back into the vena cava and the right atrium. The loop is complete.
Two Circuits Working Together
Your heart is really two pumps working side by side. The right side powers the pulmonary circuit, a short loop between the heart and lungs. The left side powers the systemic circuit, which supplies every organ and tissue in your body. Both sides beat at the same time and push the same volume of blood per beat. If they didn’t stay perfectly matched, blood would back up on one side.
A helpful way to remember the difference: in the pulmonary circuit, arteries carry oxygen-poor blood and veins carry oxygen-rich blood, the reverse of what most people expect. In the systemic circuit, arteries carry oxygen-rich blood and veins carry oxygen-poor blood, which matches the common assumption.
How the Valves Keep Blood Moving Forward
Your four heart valves are one-way gates that open and close based on pressure differences between chambers. They prevent blood from flowing backward.
When the ventricles start contracting, pressure inside them quickly rises above the pressure in the atria. That pressure difference forces the tricuspid and mitral valves shut, which is what produces the first heart sound (the “lub” in “lub-dub”). As ventricular pressure continues to climb, it exceeds the pressure in the pulmonary artery and aorta, pushing the pulmonary and aortic valves open so blood can be ejected.
When the ventricles relax, the process reverses. Blood in the pulmonary artery and aorta briefly pushes backward, snapping the pulmonary and aortic valves closed. That’s the second heart sound (“dub”). As ventricular pressure drops further, it falls below atrial pressure, and the tricuspid and mitral valves open again, allowing the ventricles to refill passively.
The Electrical System That Drives It All
Blood doesn’t flow through the heart randomly. Each beat is triggered by an electrical signal that follows a precise path to coordinate the contractions.
The signal starts in a cluster of pacemaker cells called the SA node, located in the right atrium. This is your heart’s natural pacemaker, firing 60 to 100 times per minute at rest. The electrical impulse spreads across both atria, causing them to contract and push blood into the ventricles.
The signal then reaches the AV node, a second cluster of cells sitting between the atria and ventricles. Here, it pauses for a fraction of a second. That brief delay is critical: it gives the ventricles time to finish filling with blood before they contract. After the pause, the signal travels rapidly down through the walls of both ventricles, triggering a powerful contraction that pumps blood into the pulmonary artery and aorta simultaneously.
What Happens When Flow Is Disrupted
The entire system depends on valves that open fully and seal tightly. Two common problems can disrupt that. Stenosis is when a valve becomes stiff or narrowed, often from calcium buildup, forcing the heart to work harder to push blood through a smaller opening. Blood flow velocity across a stenotic valve increases, much like water speeds up through a pinched garden hose, and the chamber behind the narrowed valve has to generate more pressure to compensate.
Regurgitation is the opposite problem: a valve doesn’t close completely, allowing blood to leak backward. In mitral valve prolapse, for example, one or both leaflets of the mitral valve sag back into the left atrium during contraction, letting oxygen-rich blood flow the wrong direction. Over time, either problem can force the heart to enlarge and weaken because it’s constantly working harder to move the same volume of blood.