Because if you exercise you sweat and sweat is made of salt water so if you drink lots of plain water theres not alot of salt left in your body
The answer to this question is the drug "Beta-1 Adrenergic" which is also known as ADRB1. It is a certain drug that affects and decreases the heart rate by simply blocking a receptor on cardiac pacemaker cells. This drug also probably binds to Beta-1 Adrenergic.
To compare an see if anything changes.
say a patient is really sick at one point,an you would want to see in reports how fast or slow they got better.