Explore the heart's four-chambered structure, layered walls, valves, and built-in electrical system that coordinates each heartbeat. Learn how disruptions to this precise sequence of electrical and mechanical events produce arrhythmias.
If you have recently received a diagnosis of a cardiac arrhythmia — or if someone you love has — it is completely natural to feel unsettled by unfamiliar words and ideas. One of the most reassuring things you can do at this stage is simply learn how your heart is built and how it works. Not at a doctor's level of detail, but enough to follow the conversation, make sense of what you are being told, and feel less at the mercy of terminology you have never encountered before. That is exactly what this course is for.
This overview is your map before you explore the territory. It introduces the major ideas you will study — each of which gets its own focused lesson — and shows you how they fit together as one coherent system.
An arrhythmia is an abnormality in the heart's rhythm — the timing, pattern, or coordination of its beating. To understand what goes wrong in an arrhythmia, you first need a clear picture of what "right" looks like. The heart is not just a lump of muscle that squeezes and relaxes. It is a precisely engineered structure: four chambers, four valves, a dedicated electrical system, its own blood supply, and layers of protective tissue — all working together in a sequence that must be coordinated down to fractions of a second. When you understand the normal system, the nature of disruptions to that system becomes genuinely comprehensible rather than mysterious.
Your heart sits in the middle of your chest, tucked between your two lungs, in a region doctors call the mediastinum. It tilts slightly to the left, which is why you feel your heartbeat on the left side of your chest. It is roughly the size of your own closed fist, and in a healthy adult it weighs somewhere between half a pound and three-quarters of a pound.
Knowing where the heart sits helps you understand why doctors listen to your chest in certain places, why an enlarged heart appears in a particular way on an X-ray, and why the position of the heart matters when interpreting the electrical recordings you will learn about later.
At its most essential, the heart is a pump — actually, two pumps working side by side. Its job, running continuously every minute of your life, is to push blood around two circuits simultaneously: one to your lungs (to pick up oxygen and release carbon dioxide), and one to the rest of your body (to deliver that oxygen and collect waste products). Every cell in your body depends on this circulation to stay alive.
Understanding the heart as a pump helps you appreciate why rhythm matters so much. A pump that beats too fast, too slowly, or in a disorganized way cannot do its job efficiently — and that inefficiency has real consequences for how you feel and, in some cases, for your health.
The wall of the heart is made up of three distinct layers, each with its own role:
Each layer has clinical significance. The myocardium, for instance, can be damaged by a heart attack, and that damage can create the very conditions — areas of scarring, irregular electrical behavior — that give rise to certain arrhythmias.
The heart has four hollow chambers — two on the right side and two on the left. Each side has an upper chamber called an atrium (plural: atria) and a lower chamber called a ventricle.
The right side of the heart collects blood returning from the body (which has given up its oxygen) and sends it to the lungs. The left side receives the freshly oxygenated blood coming back from the lungs and pumps it out to the body. A thick muscular wall called the septum separates the right side from the left, keeping the two circuits completely distinct.
The atria and ventricles also have different roles: the atria act primarily as receiving chambers and give the ventricles a final top-up of blood just before each beat; the ventricles are the main pumping chambers. The left ventricle, in particular, must pump blood all the way around the body — and so its walls are noticeably thicker and more muscular than the others.
Between each chamber and the vessel or chamber it connects to, there is a valve. The heart has four valves in total, and their job is elegantly simple: they open to let blood move forward, and they close to prevent it from flowing backward.
Two valves sit between the atria and the ventricles (called atrioventricular valves), and two sit at the exits of the ventricles (called semilunar valves). When valves work normally, blood flows in one direction only — a critical feature for an efficient pump.
Valve problems can affect both the heart's mechanical function and, indirectly, its electrical behavior — making them part of the broader picture for understanding arrhythmias.
This is perhaps the most important piece of the puzzle for understanding arrhythmias. Your heart does not rely on signals from your brain to beat — it generates its own electrical impulses from within, using a dedicated network of specialized cells.
This electrical system starts at a natural pacemaker in the upper right chamber called the sinoatrial (SA) node, which fires a small electrical signal 60 to 100 times per minute at rest. That signal travels through the atria (causing them to contract), reaches a relay station called the atrioventricular (AV) node, pauses briefly, then continues down a pathway called the Bundle of His, splits into bundle branches, and fans out through a network called the Purkinje fibers — triggering the ventricles to contract in a coordinated wave.
This precisely sequenced electrical journey, from the SA node to the last ventricular muscle fiber, is what produces a normal, coordinated heartbeat. An arrhythmia, at its core, is a disruption somewhere along this journey — the signal starting in the wrong place, traveling down the wrong path, moving too quickly, too slowly, or looping back on itself.
The heart's electrical signal travels a precise, ordered route — from the SA node at the top, through the AV node, down the Bundle of His, and out to the ventricles via the Purkinje fibers. Arrhythmias arise when this pathway is disrupted.
A single heartbeat involves a carefully timed sequence of electrical and mechanical events. The atria contract first, topping up the ventricles with blood. Then the ventricles contract powerfully, sending blood out to the lungs and body. Then everything relaxes, the chambers refill, and the cycle repeats.
Doctors refer to the contraction phase as systole and the relaxation phase as diastole — you may hear these words when your blood pressure is discussed (the two numbers in a blood pressure reading reflect the pressure in your arteries during each of these phases).
This cycle happens 60 to 100 times per minute at rest in a healthy adult. Over an average lifetime, the heart completes this cycle more than two billion times without stopping.
An electrocardiogram, almost always called an ECG (or EKG), is a recording of the electrical activity in your heart, captured via sensors placed on your skin. It is the primary tool doctors use to detect and diagnose arrhythmias. The squiggly line on an ECG tracing corresponds directly to the electrical events described above: a small bump called the P wave represents the atria depolarizing (preparing to contract), a tall spike called the QRS complex represents the ventricles depolarizing, and a gentle hump called the T wave represents the ventricles resetting. Understanding what normal looks like on an ECG helps you understand what abnormal looks like — and why your doctor paid attention to a particular feature of your own tracing.
Arrhythmias arise when something disrupts the normal electrical sequence. That disruption can happen in several ways:
All of these disruptions are easier to understand once you know what the normal system looks like. Each lesson in this course builds one piece of that foundation — so that by the time you finish, you will have a genuinely clear picture of how your heart works, and what it means when that system is challenged.
You do not need to absorb everything at once. Take it one concept at a time, and know that everything you learn here is real, accurate, and useful — both for conversations with your cardiologist and for your own peace of mind.