The heart is a muscular pump that moves blood through your entire body, delivering oxygen and nutrients to every tissue and carrying waste products away. At rest, it pumps about 5 to 6 liters of blood per minute, adding up to roughly 2,000 gallons over the course of a single day. But pumping is only part of the story. The heart also generates its own electrical signals, responds to your nervous system in real time, and even releases hormones that help regulate blood pressure.
How Blood Moves Through the Heart
The heart has four chambers: two on the right and two on the left. Each side does a different job, and blood flows through them in a specific order, kept on track by four one-way valves that prevent it from flowing backward.
Oxygen-poor blood returning from your body enters the right atrium through two large veins. From there it passes through the tricuspid valve into the right ventricle, which pumps it out through the pulmonary valve and into the pulmonary artery heading toward the lungs. In the lungs, the blood picks up fresh oxygen and releases carbon dioxide. It then travels back to the heart through the pulmonary veins, entering the left atrium. From the left atrium it passes through the mitral valve into the left ventricle, the heart’s most powerful chamber. The left ventricle generates enough pressure to push blood through the aortic valve, into the aorta, and out to the rest of your body.
Each valve has small flaps called cusps that open to let blood through, then snap shut so the chamber can refill. When a valve doesn’t open or close properly, blood can leak backward or get blocked, forcing the heart to work harder.
Two Circulation Loops
Your heart essentially runs two separate circuits at the same time. The right side powers the pulmonary circuit, sending oxygen-depleted blood to the lungs and receiving it back freshly oxygenated. The left side powers the systemic circuit, pushing that oxygen-rich blood through arteries, into tiny capillaries where oxygen and nutrients pass into tissues, then collecting the now oxygen-poor blood through veins to start the cycle again.
The left ventricle has thicker, stronger walls than the right because it needs to generate much higher pressure. Peak pressure in the left ventricle reaches about 120 mmHg during a contraction, compared to only about 25 mmHg on the right side. That difference makes sense: the right ventricle only needs to push blood the short distance to the lungs, while the left ventricle sends blood to your head, your toes, and everywhere in between.
The Cardiac Cycle: Squeeze and Fill
Every heartbeat consists of two main phases. During systole, the ventricles contract and push blood out. During diastole, they relax and refill.
Systole begins with a brief moment called isovolumic contraction, when the ventricles start squeezing but all the valves are closed. Pressure builds rapidly inside the chambers. Once ventricular pressure exceeds the pressure in the arteries, the outflow valves pop open and blood is ejected. Each ventricle holds about 130 milliliters of blood before it contracts and ejects roughly half of that, leaving about 60 milliliters behind.
Diastole is the mirror image. The ventricles relax, pressure drops, the outflow valves close, and for a brief moment all valves are shut again. As soon as ventricular pressure falls below the pressure in the atria, the inflow valves open and blood rushes in. Most of the filling happens quickly in the first part of diastole, with a slower trickle continuing until the next contraction begins. This filling phase is why high heart rates can become a problem: the faster the heart beats, the less time it has to fill between contractions.
The Heart’s Built-In Electrical System
Your heart doesn’t wait for instructions from your brain to beat. It generates its own rhythm through a specialized cluster of cells called the sinoatrial (SA) node, often described as the heart’s natural pacemaker. The SA node fires an electrical impulse that spreads across both atria, causing them to contract and push blood into the ventricles.
The signal then reaches the atrioventricular (AV) node, which sits near the center of the heart. The AV node introduces a deliberate delay of a fraction of a second. That tiny pause is critical: it gives the atria time to finish emptying before the ventricles fire. After the delay, the signal travels down a bundle of specialized fibers through the center of the heart and fans out into a network that reaches the walls of both ventricles, triggering a powerful, coordinated contraction from the bottom up.
When any part of this electrical pathway malfunctions, the result is an arrhythmia, a heartbeat that’s too fast, too slow, or irregular.
How Your Nervous System Adjusts Heart Rate
While the SA node sets the baseline rhythm, your nervous system constantly fine-tunes it based on what your body needs. Two branches of the autonomic nervous system pull the heart rate in opposite directions, like a gas pedal and a brake.
The sympathetic branch acts as the accelerator. When you exercise, feel stressed, or need more blood flow, it releases norepinephrine (and epinephrine from the adrenal glands), which acts on receptors in the SA node to speed up the heart rate and make each contraction stronger. This means more blood gets pumped with every beat and it gets pumped faster.
The parasympathetic branch, working through the vagus nerve, acts as the brake. It releases acetylcholine, which slows the pacemaker cells in the SA node and reduces heart rate. At rest, the parasympathetic system is typically dominant, which is why a calm, relaxed heart beats slower than you might expect. The constant push and pull between these two systems is what allows your heart to smoothly ramp from a resting rate of 60 or 70 beats per minute up to 150 or more during intense exercise, then settle back down when you stop.
The Heart as a Hormone-Releasing Organ
Beyond pumping, the heart plays a role that surprises most people: it functions as an endocrine organ. When the heart’s chambers stretch from increased blood volume or pressure, cells in the atrial walls release hormones called natriuretic peptides. These hormones signal the kidneys to excrete more sodium and water, which reduces blood volume and lowers blood pressure.
This discovery fundamentally changed how scientists understand the heart. Rather than a simple mechanical pump, it’s an active participant in a body-wide network that maintains fluid balance, electrolyte levels, and stable blood pressure. The heart senses when pressure is too high and directly helps correct it, creating a feedback loop between itself and the kidneys that operates continuously without any conscious effort on your part.