The heart is a muscular pump that moves blood through your entire body, delivering oxygen and nutrients to every tissue and carrying away waste products like carbon dioxide. At rest, it pumps about 5 to 6 liters of blood per minute, beating 60 to 100 times each minute in most adults. During intense exercise, that output can surge to more than 35 liters per minute. Every function of the heart supports one goal: keeping blood flowing in the right direction, at the right pressure, at the right time.
How Blood Moves Through the Four Chambers
The heart has four hollow chambers arranged in two pairs. The two upper chambers, called atria, receive incoming blood. The two lower chambers, called ventricles, pump blood out. The right side handles blood that needs oxygen, and the left side handles blood that already has it.
Here’s the sequence: blood returning from your body enters the right atrium, then passes down into the right ventricle, which pumps it to your lungs. In the lungs, the blood picks up fresh oxygen and releases carbon dioxide. That oxygen-rich blood flows back to the heart’s left atrium, drops into the left ventricle, and gets pumped out to the rest of your body through the aorta, the largest artery. Then the cycle repeats.
Four one-way valves keep blood moving forward and prevent it from leaking backward. Each valve opens to let blood through, then snaps shut. The tricuspid valve sits between the right atrium and right ventricle. The pulmonary valve guards the exit from the right ventricle to the lungs. The mitral valve connects the left atrium to the left ventricle. And the aortic valve controls the final exit into the aorta. The “lub-dub” sound of your heartbeat is these valves closing in sequence.
Two Circuits Working Together
Your cardiovascular system runs two separate loops of circulation at the same time. The pulmonary circuit is the shorter loop: the right ventricle sends oxygen-poor blood to the lungs, where it picks up oxygen and drops off carbon dioxide, then returns it to the left side of the heart. The systemic circuit is the longer loop: the left ventricle pushes oxygen-rich blood out through arteries that branch into smaller and smaller vessels, eventually reaching tiny capillaries in every organ and tissue. There, oxygen and nutrients pass into cells, and waste products pass back into the blood. That now oxygen-depleted blood collects into veins that merge into larger veins, ultimately draining into the right atrium through two major vessels, completing the circle.
The left ventricle does significantly more work than the right because it has to generate enough pressure to push blood all the way to your toes and back. That’s why the left ventricle has thicker, more muscular walls. A healthy systolic blood pressure (the peak pressure when the left ventricle contracts) is typically around 120 mm Hg. If arteries stiffen over time, the left ventricle has to squeeze harder to push the same volume of blood, which raises systolic pressure.
The Heart’s Built-In Electrical System
Your heart doesn’t need your brain to tell it when to beat. It generates its own electrical signals through a built-in conduction system. The process starts at a small cluster of cells in the right atrium called the SA node, often referred to as the heart’s natural pacemaker. This node fires an electrical impulse that spreads across both atria, causing them to contract and push blood into the ventricles.
The signal then reaches a second relay point, the AV node, which sits between the atria and ventricles. Here, the impulse pauses for a fraction of a second. That tiny delay is critical: it gives the ventricles time to fill with blood before they contract. After the pause, the signal travels down a pathway that splits into two branches, one for each ventricle, and fans out through a network of specialized fibers that trigger both ventricles to contract almost simultaneously. This coordinated squeeze is what ejects blood into the lungs and body with each beat.
How Efficiently the Heart Pumps
Not all the blood in the ventricle gets pushed out with each beat. A healthy heart ejects about 50% to 70% of the blood in its left ventricle during each contraction. This percentage is called the ejection fraction, and it’s one of the most important measures of heart function. A mildly reduced ejection fraction falls between 41% and 49%, while 40% or below is considered significantly reduced, a sign the heart isn’t pumping forcefully enough to meet the body’s needs.
Three main factors determine how much oxygen the heart itself needs to do its job: heart rate, how forcefully the muscle contracts, and the tension in the walls of the ventricles. When you exercise, your heart rate and contraction strength both increase, so the heart’s own oxygen demand rises sharply. A fit heart handles this efficiently. People who are more physically active tend to have lower resting heart rates, sometimes as low as 40 beats per minute in trained athletes, because each beat is stronger and moves more blood.
How the Heart Feeds Itself
The heart pumps blood for the entire body, but it also needs its own dedicated supply. The coronary arteries, which wrap around the outside of the heart, branch off from the aorta and deliver oxygen-rich blood directly into the heart muscle. This is a separate system from the blood flowing through the chambers inside. If a coronary artery becomes narrowed or blocked, the heart muscle downstream gets starved of oxygen. That’s the basic mechanism behind a heart attack.
Because the heart never stops working, it has an extraordinarily high demand for oxygen compared to other organs. Even at rest, the heart extracts a large proportion of the oxygen from the blood its coronary arteries deliver, leaving little reserve. This is why partial blockages that might go unnoticed in other tissues can become dangerous in the heart, especially during physical stress when the muscle needs even more fuel.
The Heart as a Hormone-Producing Organ
Beyond pumping, the heart plays a lesser-known role: it produces hormones that help regulate blood pressure and fluid balance. When blood volume or pressure gets too high, cells in the atria release a hormone called atrial natriuretic peptide (ANP) into the bloodstream. ANP signals the kidneys to excrete more salt and water, which reduces blood volume. It also causes blood vessels to relax and widen, which lowers pressure directly. This feedback loop means the heart doesn’t just respond to blood pressure; it actively helps control it.
This endocrine function was first confirmed in 1981, when researchers found that extracts from heart tissue could trigger significant changes in kidney output. It reframed the heart from a purely mechanical pump to an organ that communicates chemically with the rest of the body, fine-tuning the very conditions it operates under.