Heart Rate Monitor
When your heart pumps blood into the rest of your body, the blood that comes in is
different in one key way from the blood that was there before: it contains more oxygen.
When oxygen binds to hemoglobin in your red blood cells, it changes the color of your
blood – it makes it more red. This being the case, it is also true that the fresh blood will
transmit less red light than the bluer blood that it replaced.
The heart rate monitor uses this property of your blood to measure your pulse. The
device consists of a little clip that goes over a finger or perhaps an ear lobe – or even a
toe. On one side of the clip is a light; on the other is a light sensor. The sensor
measures the light that is transmitted through your finger. There is always a fair amount
of light transmitted through you – a lot of the tissues that make up your body are less
than totally opaque. But when the new, red blood arrives in your finger from your heart,
more of the light will be absorbed in your finger, and less will be transmitted. This will
give a dip in the light transmission.
By looking at the time between successive dips, you can determine your pulse. In fact,
that is what the unit was designed to do. But it can do more: since the computer will
measure the light intensity as a function of time, you can look at how it varies.
An electrocardiogram is a set of measures of voltages across a person’s body while they
are at rest. Muscles in the body work by moving ions across the cell membranes. When
any muscle in the body contracts, it produces an electric field. After the contraction, as
the ions move back, there will also be a complementary field. (These two phases are
called “depolarization” and “repolarization.”) You can measure the electric field produced
by any muscle in your body, but the muscle that generally gives the strongest signal is –
no surprise – your heart. It turns out that nerve cells produce electric fields too, but