what are the major physiological functions of the circulatory system?
<answer> To deliver oxygen and substrate to cells. <context> The circulatory system includes the heart, blood vessels, and regulatory controls (such as the autonomic nervous system, catecholamines, and hormones). Its main function is to deliver oxygen and substrate (glucose) to cells. The circulatory system also transports byproducts of cellular metabolism—66% in the form of carbon dioxide (CO2) to the lungs and the remaining 33% to the kidneys as breakdown products of protein metabolism.