The heart is the central organ of the cardiovascular system, also called the circulatory system. This system includes the heart itself plus a vast network of blood vessels (arteries, veins, and capillaries) that carry blood to every tissue in your body. At rest, the heart pumps roughly 5 to 6 liters of blood per minute, delivering oxygen and nutrients while carrying away waste products like carbon dioxide.
What the Cardiovascular System Does
The cardiovascular system is essentially a closed loop of plumbing with the heart as its pump. Blood leaves the heart through arteries, which are thick, muscular vessels built to handle high pressure. It reaches your tissues through capillaries, tiny vessels with walls thin enough for oxygen and nutrients to pass through into surrounding cells. Then veins carry the now oxygen-depleted blood back to the heart to start the cycle over.
This system runs two circuits simultaneously. The pulmonary circuit sends oxygen-poor blood from the right side of the heart to the lungs, where it picks up fresh oxygen and drops off carbon dioxide (which you exhale). The systemic circuit sends that freshly oxygenated blood from the left side of the heart out to the rest of your body. Both circuits operate with every single heartbeat.
How the Heart Is Built
The heart has four 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 atrium and right ventricle handle the lung-bound circuit. The left atrium and left ventricle handle the body-bound circuit. The left ventricle is the strongest chamber because it generates enough pressure to push blood through your entire body.
Four valves act as one-way doors between these chambers and the major arteries. The tricuspid valve sits between the right atrium and right ventricle. The mitral valve sits between the left atrium and left ventricle. The pulmonary valve guards the exit from the right ventricle to the lungs, and the aortic valve guards the exit from the left ventricle to the aorta, the body’s largest artery. These valves open and close with each heartbeat to keep blood flowing in the right direction.
The heart wall itself has three layers: a thin inner lining, a thick muscular middle layer that does the actual contracting, and a protective outer layer.
The Heart’s Built-In Electrical System
Unlike most muscles in your body, the heart doesn’t wait for a signal from your brain to contract. It has its own internal pacemaker, a small cluster of specialized cells in the right atrium called the SA node. This node fires an electrical impulse 60 to 100 times per minute under normal resting conditions, setting the pace for each heartbeat. Well-trained athletes can have resting rates closer to 40 beats per minute because their hearts pump more efficiently.
Each impulse travels from the SA node to a relay point called the AV node, which briefly slows the signal. That tiny delay gives the atria time to finish squeezing blood into the ventricles before the ventricles contract. From the AV node, the signal races down specialized fibers that branch into both ventricles, triggering them to contract almost simultaneously.
What Happens During a Single Heartbeat
Each heartbeat has two main phases. During the contraction phase (systole), the ventricles squeeze and push blood out into the arteries. First, pressure builds inside the ventricles while all valves are momentarily closed. Once ventricular pressure exceeds the pressure in the arteries, the exit valves pop open and blood surges out.
During the relaxation phase (diastole), the ventricles release and refill. As pressure drops, the exit valves snap shut (preventing backflow), and the inlet valves open to let blood pour in from the atria. Most of the filling happens quickly in the first moment of relaxation, with a slower trickle following. Then the atria give a final squeeze to top off the ventricles, and the cycle begins again. At the end of each contraction, about 60 milliliters of blood remains in each ventricle before refilling.
How the Nervous System Controls Heart Rate
While the heart generates its own rhythm, your nervous system adjusts that rhythm to match what your body needs. Two branches of the autonomic nervous system (the part that runs without conscious effort) act like a gas pedal and a brake.
The sympathetic branch is the gas pedal. During exercise, stress, or any situation requiring more blood flow, it releases signaling chemicals that speed up the heart rate, strengthen each contraction, and narrow blood vessels to raise blood pressure. This is part of the “fight or flight” response.
The parasympathetic branch, working mainly through the vagus nerve, is the brake. It dominates during rest and recovery, slowing the heart rate and promoting relaxation. The balance between these two systems is constantly shifting throughout your day, which is why your heart rate rises when you climb stairs and drops when you sit quietly.
How the Heart Works With the Lungs
The cardiovascular and respiratory systems are deeply intertwined. Blood that returns to the heart from the body is low in oxygen and loaded with carbon dioxide. The right ventricle pumps this blood through the pulmonary artery into the lungs, where it 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 from the air you inhale into the blood. The newly oxygenated blood then travels through the pulmonary veins back to the left side of the heart, ready to be pumped out to the body again.
This continuous exchange is why heart and lung health are so closely linked. If either organ struggles, the other feels the strain almost immediately.