My son who is only 36 months old has a very fast heart beat. his stool content partial blood strain after defication.?

Fast heart beat: The fast heart beat and blood in his stool may be related or two very distinct and separate signs. I would recommend that you take him to his pediatrician and have him evaluated for his bloody stools, and to determine if he has a normal, but fast, heart rhythm (due to dehydration, anemia, stress, etc.) versus a truly abnormal heart rhythm (much less common).

Check his Hemoglobin: If he is passing several stool with blood he can be anemic and his fast heart rate secondary to the anemia. Other thing to do an ECG at his pediatrician office.