Your heart is a muscular pump about the size of your fist, divided into four chambers that work in a precise sequence to push blood through two separate loops in your body. At rest, it completes this entire cycle roughly 75 times per minute, with each cycle lasting about 0.8 seconds. Understanding what each part does, and the order blood flows through them, makes the whole system click into place.
The Four Chambers and What They Do
Picture the heart split into a right side and a left side, each with an upper chamber (atrium) and a lower chamber (ventricle). The right side handles used, oxygen-poor blood. The left side handles freshly oxygenated blood. Blood always moves from atrium to ventricle, never the other direction.
The right atrium collects oxygen-poor blood returning from your body through two large veins called the superior and inferior vena cava. That blood drops into the right ventricle, which pumps it out to your lungs. After picking up oxygen in the lungs, the blood returns to the left atrium, then fills the left ventricle, the most muscular chamber. The left ventricle generates enough pressure to send oxygen-rich blood out through the aorta and to every tissue in your body.
The pressure difference between the two sides is dramatic. The right ventricle only needs to push blood the short distance to your lungs, generating about 15 to 30 mmHg of pressure. The left ventricle, which has to reach your brain, your toes, and everything in between, generates 90 to 140 mmHg. That’s why the left side of the heart is noticeably thicker.
The Four Valves Keep Blood Moving Forward
Four one-way valves prevent blood from flowing backward. Two sit between the atria and ventricles, and two guard the exits from the ventricles:
- Tricuspid valve: between the right atrium and right ventricle
- Pulmonary valve: between the right ventricle and the pulmonary artery (leading to the lungs)
- Mitral valve: between the left atrium and left ventricle
- Aortic valve: between the left ventricle and the aorta (leading to the body)
When a ventricle contracts, the valve behind it snaps shut so blood can’t wash back into the atrium, while the valve in front opens to let blood out. When the ventricle relaxes, the exit valve closes and the entry valve opens, allowing the chamber to refill. The familiar “lub-dub” sound of a heartbeat is these valves closing in sequence.
Two Circulation Loops
Your cardiovascular system runs two distinct circuits simultaneously, and the heart sits at the center of both.
The pulmonary circuit is the short loop. The right ventricle pumps oxygen-poor blood through the pulmonary artery to the lungs. Inside the lungs, tiny blood vessels called capillaries sit right next to air sacs. Carbon dioxide leaves the blood, oxygen enters it, and the now oxygen-rich blood travels back to the left atrium through the pulmonary veins. The oxygen saturation of blood on the right side of the heart averages around 78 to 80 percent. By the time it returns from the lungs, that number jumps to roughly 97 percent.
The systemic circuit is the long loop. The left ventricle pumps oxygen-rich blood through the aorta, which branches into progressively smaller arteries, eventually reaching capillaries threaded through every organ and tissue. At the capillary level, oxygen and nutrients pass into cells while carbon dioxide and waste products pass into the blood. The now oxygen-depleted blood collects into small veins, then larger veins, and finally drains into the superior and inferior vena cava, arriving back at the right atrium to start the cycle over.
The Heart’s Electrical System
Your heart doesn’t wait for instructions from your brain to beat. It has its own built-in electrical system that fires automatically. The sequence starts at a small cluster of cells in the right atrium called the SA node, often referred to as the heart’s natural pacemaker.
The SA node sends an electrical signal that spreads across both atria, causing them to contract and push blood down into the ventricles. The signal then reaches a second relay point, the AV node, which sits between the atria and ventricles. The AV node deliberately pauses the signal for a fraction of a second. This brief delay is critical: it gives the atria time to finish emptying before the ventricles fire. From the AV node, the signal travels down a pathway called the bundle of His and fans out through a network of fibers called Purkinje fibers, which trigger the ventricles to contract in a coordinated squeeze from bottom to top. This pushes blood upward and out through the pulmonary artery and aorta.
The Cardiac Cycle: Squeeze and Fill
Each heartbeat has two phases. During systole, the ventricles contract and eject blood. During diastole, they relax and refill. At a resting heart rate of 75 beats per minute, systole takes about a third of each cycle (roughly 270 milliseconds) while diastole takes the remaining two-thirds (roughly 530 milliseconds). Your heart actually spends more time relaxing than squeezing.
Diastole is more complex than it sounds. It includes a brief moment where the ventricles relax but no blood is yet flowing in, then a rapid filling phase as blood rushes from the atria, a slower filling phase, and finally the atrial contraction that tops off the ventricles just before the next squeeze. Systole begins with a quick pressure buildup while all valves are closed, then the exit valves pop open and blood is ejected.
When your heart rate increases during exercise, diastole shortens more than systole. This is why extremely high heart rates can become a problem: the heart doesn’t get enough time to fill, and each beat pumps less blood.
The Heart’s Own Blood Supply
Despite being full of blood at all times, the heart muscle can’t absorb oxygen from the blood passing through its chambers. It needs its own dedicated supply, delivered by the coronary arteries that wrap around the outside of the heart.
Two main coronary arteries branch off the aorta right at its base. The left main coronary artery feeds the left side of the heart, splitting into the left anterior descending artery (supplying the front of the heart and the wall between the ventricles) and the left circumflex artery (wrapping around to the outer side and back). The right coronary artery supplies the right side of the heart, including the SA and AV nodes that control your heart rhythm.
This is why coronary artery blockages are so dangerous. If a coronary artery narrows or gets blocked, the section of heart muscle it feeds is starved of oxygen. That’s a heart attack. The specific artery involved determines which part of the heart is affected and how serious the damage is. A blockage in the right coronary artery, for instance, can disrupt the electrical nodes and cause dangerous rhythm problems.
The Heart Wall
The heart wall itself has three layers. The inner lining, called the endocardium, provides a smooth surface for blood to flow across. The thick middle layer, the myocardium, is the muscle that does the actual pumping. The outer layer, the epicardium, acts as a protective covering. The entire heart sits inside a thin, fluid-filled sac called the pericardium, which reduces friction as the heart beats inside your chest.
The myocardium of the left ventricle is the thickest section of heart muscle in the body, reflecting the high pressure it must generate with every beat. In conditions like long-standing high blood pressure, this muscle can thicken further as it works harder, which over time can stiffen the wall and make filling more difficult.