The West is blinking in disbelief â Vladimir Putin just invaded Ukraine. German diplomats, French Eurocrats and American pundits are all stunned.
The  circulatory system is the body's transport system. It is made up of a group of organs that transport blood throughout the body. The heart pumps the blood and the arteries and veins transport it. Oxygen-rich blood leaves the left side of the heart and enters the biggest artery, called the aorta.
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 metabolismâ66% in the form of carbon dioxide (CO2) to the lungs and the remaining 33% to the kidneys as breakdown products of protein metabolism.