The Heart's Electrical System

The heart is a pump, but a pump needs something to tell it when to squeeze, and in what order. Your heart comes with its own built in electrical system, and it's the reason your heartbeat is so beautifully timed.

The heart is a four chambered pump with one way valves. But knowing the parts doesn't explain the most remarkable thing about it: the timing. The chambers don't all squeeze at once. They contract in a precise order, top first, then bottom, over and over, around 100,000 times a day, with no instruction from your brain. What makes that happen is electricity, generated by the heart itself. Your heart has its own wiring. A tiny built in spark generator sets the beat, and a set of pathways carry that spark through the muscle in exactly the right sequence. Follow that spark on its journey and the whole rhythm makes sense. The spark: the natural pacemaker It all begins in a small patch of special cells in the upper right chamber, called the sinoatrial node , or SA node. This is the heart's natural pacemaker. All on its own, it fires off an electrical signal, normally somewhere between 60 and 100 times a minute. Nobody tells it to. It simply does, for your whole life. Every single heartbeat starts here. Step one: the top chambers squeeze The signal spreads out across the two upper chambers, the atria, and makes them contract. That gentle squeeze pushes blood down into the two lower chambers, getting them ready to do the powerful work. So the first thing the spark does is fill the main pumps. The pause: a clever built in delay Next, the signal reaches a kind of relay station between the upper and lower chambers, called the atrioventricular node , or AV node. And here something clever happens: the AV node deliberately holds the signal back for a fraction of a second. That tiny pause matters enormously. It gives the upper chambers just enough time to finish emptying their blood into the lower chambers before those lower chambers fire. Without the delay, the chambers would squeeze at the same time and the pumping would be far less efficient. Step two: the main pumps fire After the pause, the signal races down a pathway through the middle of the heart, the bundle of His . This pathway splits into branches and spreads through a web of fibres into the walls of the two lower chambers, the ventricles. This makes them contract hard, driving blood out to the lungs and the body. Then everything relaxes, the SA node fires again, and the whole sequence repeats. One full cycle is one heartbeat. Seeing the electricity: the ECG Because all this is electrical, it can be measured from the outside. When a doctor or nurse attaches small stickers to your chest and prints out a tracing, that's an ECG , an electrocardiogram, and it's literally a picture of this electrical journey. The tracing has a repeating pattern of bumps, and each one has a name. One small bump shows the upper chambers being triggered (the P wave). A tall sharp spike shows the powerful lower chambers firing (the QRS complex). A final gentle bump shows the heart resetting itself before the next beat (the T wave). A doctor reading an ECG is following the exact same journey you just followed, beat by beat. So the heart isn't just a pump. It's a pump with its own electrician, firing a spark in perfect sequence to keep blood moving in the right direction, at the right time. You never have to give it a moment's thought. What a doctor checks when assessing the heart's rhythm The electrical system cannot be seen, but its effects can be felt, heard and recorded. Feeling. Feeling the pulse at the wrist tells a doctor two things: the rate, how fast the heart is beating, and the rhythm, how regular each beat is. Listening. Listening with a stethoscope lets a doctor hear whether the beats are evenly spaced or irregular, which can hint at a problem with the wiring rather than the pump itself. Special tests. An ECG is the main tool for this system. Small stickers on the chest record the electrical signal directly as it travels through the SA node, the AV node, and finally the ventricles. This produces the tracing you read about above. What doctors learn. An ECG tells a doctor where in the electrical pathway a problem might be. It can show whether the natural pacemaker is firing correctly. It can also show whether the signal is delayed or blocked on its way to the lower chambers, or whether the rhythm has become disorganised. When this system is affected When the heart's electrical system is not working smoothly, people may notice a racing heartbeat or a fluttering, pounding sensation in the chest. Other signs include dizziness, fainting, and an irregular pulse. Doctors call an irregular heart rhythm an arrhythmia, which can range from harmless to serious depending on the type. These symptoms are explored further in the lesson "Palpitations: When Your Heart Feels Different".

The heart generates its own electricity, starting at the SA node, pausing briefly at the AV node, and recorded by doctors as the repeating tracing of an ECG.