We don’t think of the human body as a piece of electronics, but a surprising amount of our bodies work on electricity. The heart is certainly one of these. When you think about it, it is pretty amazing. A pump the size of your fist that has an expected service life of nearly 100 years.
All that electrical activity is something you can monitor and–if you know what to look for–irregular patterns can tell you if everything is OK in there. [Ohoilett] is a graduate student in the biomedical field and he shares some simple circuits for reading electrocardiogram (ECG) data. You can see a video fo the results, below.
The design centers around an instrumentation amplifier which is ideal because it has a high input impedance and very high immunity to common mode noise. The simplest circuit is just an instrumentation amp and little else. The disadvantage is that it requires a bipolar power supply. [Ohoilett] also shows how you can add a few more parts to make the power supply simpler. In either case, an oscilloscope provides the output display (if you insist, there is a video on the page that shows the ECG connected to an Arduino).
If you want to know more about what to do with the data, you might enjoy this post. If you decide to get serious, you might check out the very robust open hardware project for ECG and other bioelectrical measurements.