Well because heart rate is controlled by two branches of the autonomic involuntary nervous system. And the heart pumps blood through a complex of blood vessels. The brain gets blood from the circulatory system with the supply of oxygen rich blood and your brain regulates the heart rate and blood pressure. Your heart, brain, and stomach can not be in the same organ system because if they were they wouldn’t work correctly and your heart helps pump blood which the brain gets the rich blood from. And your stomach is it’s own organ system that had small and large intestines.