What happens to your heart rate when you excercise?

Depending on how light or heavy exercise is will depend on the change that occurs.

Typically during exercise heart rate will increase. This is in order to increase blood flow to the working muscles to allow for increased respiration - in order for the muscles to work efficiently. Therefore the more strenuous the exercise, the more your heart rate will increase.

Blood pressure will also increase during exercise and again depends on intensity levels. During exercise such as running/cycling/swimming systolic pressure will increase progressively whereas diastolic pressure will increase only slightly.

During weight lifting exercises, both systolic and diastolic pressures will rise.

Obviously these effects will vary from person to person, depending on age, gender, exercise level, exercise intensity etc. etc.