How your heart works — the apartment in your chest

In clinic I explain the heart the same way every time, because it is the explanation that survives the walk back to the car park: your heart is an apartment. It has four rooms. It has doors between them. It has wiring hidden in the walls. And it has plumbing running along the outside that keeps the whole flat supplied.

Almost every heart test ever ordered is an inspection of one of those four things. Once you can picture the apartment, a report stops being a page of code and becomes a surveyor’s note about a particular part of your home.

The four rooms

Your heart has four rooms, arranged as two small ones upstairs and two large ones downstairs. The upstairs rooms receive blood; the downstairs rooms pump it out. Doctors call the upper rooms the atria and the lower ones the ventricles, and your report will use those words.

But the important thing is not the names — it is that the apartment is really two flats side by side, sharing one wall, and there is no door in that wall.

The right-hand flat takes tired blood back from your body and sends it the short distance to your lungs to be refreshed. The left-hand flat receives that refreshed blood and pushes it out to everywhere else — your brain, your kidneys, your legs. That is a far longer journey, which is why the left lower room is the thick, muscular one. It is doing the heavy lifting, and almost every number on an echo report that people worry about is a measurement of that room.

Two things go wrong with rooms. They can stretch and get bigger — the word on a report is dilated — the way a room looks when a wall has been knocked through. Or their walls can get thicker — hypertrophy — which sounds like strength but is usually the opposite: it is what a wall looks like after years of being pushed against, most often by high blood pressure.

The four doors

Between each upper room and the lower room beneath it there is a door. And where each lower room opens onto its outward journey there is another. Four doors in total. Their names — mitral, aortic, tricuspid, pulmonary — appear on every echo report, and each gets its own line.

These are one-way doors, and that is the whole point of them. Blood should move through the apartment in one direction and never come back. A door swings open as the room behind it squeezes, then falls shut so nothing returns.

So there are exactly two ways a door can misbehave, and every valve finding you will ever read is one of them:

• It does not close properly, so blood leaks backwards. Your report will say regurgitation, or simply leak, graded from trivial to severe.
• It does not open fully, because it has stiffened, so blood is forced through a narrower gap. Your report will say stenosis, graded mild to severe.

The wiring

An apartment needs more than rooms and doors. Something has to tell the rooms when to squeeze, and in what order — the upper rooms a fraction of a second before the lower ones, so that each room is filled before it is asked to pump.

Your heart runs its own electrical supply for this. It does not take current from anywhere else. Deep in the wall of the right upper room sits a small cluster of cells that fires by itself, roughly sixty to a hundred times a minute, every minute of your life — the timer. Doctors call it the sinoatrial node.

From the timer, the signal spreads across both upper rooms and makes them contract. Then it reaches a junction box at the dividing wall — the atrioventricular node — which does something clever: it holds the signal back for a fraction of a second. That deliberate pause is what lets the upper rooms finish emptying before the lower ones begin to squeeze.

From the junction box, the wiring runs down the dividing wall and splits into branches — one to the right lower room, one to the left, and the left one divides again. Each lower room gets its own supply, which is why an electrical fault can affect one side and not the other.

Wiring goes wrong in ways that are easy to picture:

• The timer runs slow or fast — a pulse that is too slow or too quick at rest.
• The junction box delays too long or drops signals altogether — reported as heart block.
• A branch stops conducting — bundle branch block, meaning one lower room now gets its signal late, by a roundabout route.
• The upper rooms stop firing in an orderly way and quiver instead — atrial fibrillation, the commonest of all rhythm problems.

The plumbing

Here is the part that surprises almost everybody, and it is worth stopping on.

Your heart is full of blood. Litres of it pass through those four rooms every minute. And the heart muscle cannot use a single drop of it. The walls of the apartment are not fed by what flows through the rooms, any more than the bricks of a building drink from the water tank inside it.

Instead, the heart wall has its own dedicated supply: three main pipes that branch off the aorta right at the front door and run back along the outside surface of the heart, sending smaller and smaller branches down into the muscle. These are the coronary arteries.

Each pipe feeds a particular territory of wall, the way a riser feeds a particular column of flats. And that gives the whole thing a logic that turns out to be extremely useful: if you know which part of the wall is in trouble, you can usually tell which pipe is narrowed. This is why a doctor can look at an ECG and say which artery is likely to be involved.

When a pipe narrows, the stretch of wall it feeds runs short of supply — not always, but as soon as you ask more of it, by climbing stairs or hurrying for a train. That shortage is what you feel as chest tightness on exertion, and what doctors call angina. When a pipe blocks completely and suddenly, that stretch of wall is cut off entirely, and unless the supply is restored quickly the muscle there dies. That is a heart attack.

The four are connected

The apartment is a single home, so trouble in one system shows up in another. This is not a complication of the metaphor — it is the most clinically useful part of it.

A blocked pipe starves a stretch of wall, and a starved wall stops moving properly. So a test that looks at the rooms can reveal a section that has gone quiet — and that quiet section points straight back at the pipe that feeds it. Damaged wall can also disturb the wiring that runs through it, which is why rhythm problems often follow a heart attack. And a door that has leaked for years forces the room behind it to handle extra volume, so it stretches — a door problem, arriving as a room problem.

This is the honest reason a single normal report cannot promise you a normal heart, and the reason your doctor keeps ordering things that seem, from the outside, to overlap.

Which inspection looks at what

Each test surveys a different part of the apartment. This is the map, not the detail — each report has its own page, which is where the numbers get read.

Notice how much of that table overlaps, and that no row inspects everything. That is the single most useful thing to carry away from this page.

What to do with this

When you next read a report, or sit in a consulting room, try placing what you are told into the apartment. Is this about a room, a door, the wiring, or the plumbing? Is this test even capable of seeing the thing I am worried about?

Those two questions will make you a better-informed patient than any amount of searching for individual numbers — and they are the questions the rest of this site is built to answer, one report at a time.

Common questions

Why did my doctor order so many different tests?

Because no single test can inspect the whole apartment. One test looks at the rooms and doors, another at the wiring, another at the plumbing. A doctor ordering a second test is usually not doubting the first one — they are inspecting a different part of the flat. If two tests seem to disagree, that is often because they were never looking at the same thing.

Which test is the most important one?

The one that answers your particular question. There is no ranking. A test that is exactly right for chest pain on walking may be useless for palpitations, because the first is a question about the plumbing and the second is a question about the wiring.

My report says everything is normal. Does that mean my heart is completely fine?

It means the part of the apartment that test inspects looked normal on the day it was done. A normal echo says the rooms and doors are working; it does not clear the plumbing, because an echo cannot see the pipes at all. This is exactly why a doctor may reassure you about one report and still want another test.

Where is the blockage in my heart, then — in the heart itself?

Almost never inside the rooms. When people say a blockage in the heart, they nearly always mean a narrowing in one of the supply pipes running along the outside of the heart wall. The blood inside the rooms flows on; it is the wall itself that goes short.

Does a bigger heart mean a stronger heart?

No — usually the opposite. In the body, a muscle that grows with training is a good sign. In the heart, a room that has enlarged has usually done so because it has been struggling against something for a long time, and a stretched room pumps less efficiently, not more.