What is atrial fibrillation, and why has Apple decided that it’s worth screening for it? The first question is much easier to answer, so let’s get that out of the way.

Your heart has four chambers, two atria and two ventricles. The atria are smaller chambers at the top of the heart, and their contraction fills the larger ventricles with blood. The ventricles then provide the powerful push that sends the blood either to the lungs to pick up oxygen or out to the body once it is oxygenated.

Got no rhythm

The proper coordination of the beating of all these parts requires a carefully synchronized spread of electrical signals through the four chambers. Given the complexity involved in getting this to work, it shouldn’t be surprising that it sometimes goes wrong. The fault for problems can be anything from a temporary physical change to a permanent problem with your heart’s development that started back when you were an embryo. The consequences can range from irrelevant to fatal.