Every time your heart beats it pushes blood through your blood vessels. This force of blood rushing through your blood vessels is your blood pressure. It's what moves "fresh" blood to your tissues and organs and what carries away the 'old' blood. As your circulatory system listens and reacts to messages from your body, different systems act to keep your blood pressure under control.
Your heart, a small muscle about the size of your fist, beats about 100,000 times a day, pumping 40,000 gallons of blood along the 60,000 miles of blood vessels that make up your circulatory system. With each beat, blood rushes out of the heart into the aorta, an artery the width of a garden hose. Branching from the aorta, smaller and smaller arteries carry the blood to the rest of your body. The smallest of these branch arteries, arterioles, carry blood to every organ and tissue in your body. Within these tissues, blood moves into the microscopic capillary vessels. There it unloads oxygen and nutrients into the tissues and carries away the carbon dioxide and wastes. Mirroring the arteries, a completely separate system of blood vessels, the veins, channels the old blood back to your heart.
While it moves those gallons of blood, your circulatory system monitors what your body is doing and adapts your blood flow to your different actions. Your network of nerves, like telephone lines, transmit messages between the circulation system and the rest of your body. When you walk up a flight of stairs, your nerves tell the tiny muscles surrounding your arteries to narrow, forcing blood quickly to your legs. When you are done eating dinner, the nerves tell the muscles to open your arterioles, letting more blood to your stomach. This responsiveness is called microcirculation.
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