The circulatory system is powered by the heart, a muscle that pumps blood through a network of blood vessels to deliver oxygen and nutrients to every cell in the body while carrying away waste products like carbon dioxide. An adult heart beats roughly 100,000 times a day.
The respiratory system works closely with the circulatory system to supply the body with oxygen. When you inhale, air travels through the trachea into the lungs, where tiny air sacs called alveoli allow oxygen to pass into the bloodstream while carbon dioxide passes out to be exhaled.
These two systems depend on each other constantly: the respiratory system brings in the oxygen that the circulatory system then transports throughout the body, which is why physical activities that raise your heart rate, like running or swimming, also make you breathe faster and deeper.