The heart is an incredibly simple organ, it's a pump powered by wires. Let's break it down into the structure itself, the plumbing, and the electricity. 

Structure

Firstly, we lied. The heart isn't just a pump, it's actually two pumps: one on the left, and one on the right - these are called ventricles. 

The left ventricle does most of the heavy lifting, it pumps oxygen-rich blood all around your body. As such, it is bigger and has more muscle.
The right ventricle pumps oxygen-depleted blood into the lungs so that it can become oxygen-rich again. 

There are two chambers sitting on top of the ventricles. They work as collecting buckets for the pumps - they are called the atria (plural of atrium). 
The left atrium collects the oxygen-rich blood coming from the lungs.
The right atrium collects the oxygen-depleted blood coming from the rest of the body. 

The atria and the ventricles are separated by valves, little flaps that open and close.
The valve separating the left atrium and left ventricle is the mitral valve.
The valve separating the right atrium and the right ventricle is the tricuspid valve. 
They close to allow the atria to fill up, and then open to deliver the blood into the ventricles.  When they close, we hear the first of the two heart sounds ("Ba-dum").

So if they are the "Ba", what is the "Dum"? The second heart sound comes from the closure of another pair of valves - namely the aortic and the pulmonary. They open when the ventricles are squeezing, pumping blood to the body and the lungs. 

The septum is the big wall running through the middle, separating the left and right pumping systems.

Sometimes, the pumps can lose power, or the valves can get a bit leaky like an old bathroom. That's when you might run into problems...

Plumbing

Like any organ, the heart muscle needs a blood supply of its own. There are three main arteries that supply the heart muscle - right coronary artery, left anterior descending artery, and left circumflex artery. 

Fancy names, we get it. Basically, there is one on the right and two on the left (to feed the larger muscle). These three arteries are the first arteries the left ventricle feeds with oxygen-rich blood, because the heart is a greedy thing. But also, the body generally likes to prioritise our organs - can you guess the next stop? That's right, the brain. 

When we get blockages to these vessels, there's a lack of fuel being directed to the heart itself which can damage the muscles...the wires...the valves - it all depends on which vessel, the degree of blockage, and the time it takes for the blockage to be cleared. 

Electricity

The heart has to keep a rhythm - the atria receiving the blood, the ventricles contracting at the right time, the opening and closing of the valves...the hearts movements are controlled by the heart's conduction system - and this dance needs one hell of a choreographer, namely the sinoatrial node. 

The sinoatrial node is often called the 'pacemaker of the heart'. Like the drummer in a band, it sets the tempo and keeps the beat. The sinoatrial node lives in the right atrium, and is responsible for the beginning of the electrochemical cascade. It flicks the switch, and a Mexican wave of voltage moves through the heart, operating the machinery from top to bottom. Then, the system resets, and it happens all over again...and again...and again, around 60 to 100 times a minute, every minute for the whole of our lives. And you think you work hard? The sinoatrial node brings the heart rate up or down depending on the demand from the body. 

The sinoatrial node is the start of the circuit. It connects to small wires that run down through the septum (middle of the heart) before fanning out in all directions to supply the ventricles. As you can imagine, electricity moves faster than the muscle can contract.

To give the ventricles time to fill up with blood, there's a clever delay mechanism at play called the atrioventricular node (it lives in the septum). It acts as a dam in the electrical river, slowing the conduction just long enough to allow the atria to fill the ventricles.

The movement of the electrical signals through the heart give us the tracing known as an ECG (Electrocardiogram). We look for abnormal bumps and spikes in the ECG to point to the location of a possible issue with the heart.