Imagine this scenario: You're out on an easy recovery run in the summer, and you're wearing a heart-rate monitor. You're on an out-and-back course and you're planning to run for 40 minutes. It's pretty hot and humid, just a bit more intense than the weather has been in previous days. You turn around at the 20-minute mark, check your heart rate and it's at 150 beats per minute (bpm).
That's a bit high for an easy recovery run, but you know you're running easy, so you assume the heart-rate monitor is just off a bit. You run just as easy on the way back, and right before you end the run you look down and see that you're now at 162 bpm. Something must be wrong because you know you didn't run any faster on the return trip, yet your heart-rate monitor shows a 12 bpm increase in the last 20 minutes of the run.
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Is your heart-rate monitor broken? Probably not.
What you experienced on that run is a phenomenon called cardiac drift. When you engage in aerobic exercise, your body has to get oxygen-rich blood to working muscles so that the mitochondria in your muscles' cells can produce energy to contract the working muscles.
Blood is made of cells and plasma, and the plasma is over 90 percent water. When you exercise and start to sweat, you loose some of that water from the blood plasma traveling through your body in order to get oxygen to the working muscles.