Th main reason is becuse the whole cardiovascular system becomes alot more efficent. all aspects become adapted due to a higher fitness.
Inb a long reponse answer you should go into delpth about the different adaptations the body makes.
Resting heart reate decreases because the body is able to use the oxygen it is supplies with alot more efficently. the muscles of the heart become alot stronger and are able to pump out more blood with each beat.
Stroke volume increases- the amount of blood pumped out of the heart with each beat. This is due to the left ventricle being able to fill more completely. therefore more blood cane be pumped out and less beats per minute so therefore a lower resting heart reat.
Haemoglobin levels increase, which allows the body's oxygen carrying ability to increase. this is due to an increase in blood plasma. this allows more oxygen to be transported so less being needed to be pumped out therefore a lower resting heart rate. these all realate to a lower resting heart rate and im sure there are more, if you elaborate on all these ideas then you should come up with a deatiled response, hope i have been of some help
