circulation (of blood)
Circulation is the endless loop your blood travels: the heart squeezes, pushing blood out through wide pipes called arteries, on through a fine mesh of tiny vessels that reach every corner of you, and back again through veins to be pumped once more. Picture a single drop setting off from the heart, dropping oxygen and food at your toes and fingertips, picking up waste, and returning to the start — over and over, a full circuit in well under a minute: about 20 to 30 seconds at rest, and faster when you exercise, all day, all your life.
It matters because this loop is how every cell gets fed and kept clean. Your lungs load the blood with oxygen, your gut hands over fuel, and the moving stream carries both to muscles, brain, and skin while sweeping away the rubbish. Stop the loop and the deliveries stop with it — which is exactly why a blocked vessel or a stalled heart is so deadly so fast.
Here's the surprise: for over a thousand years, learned doctors taught the opposite. Following Galen, they believed the liver constantly brewed fresh blood that the body then used up like fuel, one-way, never returning. In 1628 William Harvey did the arithmetic — the heart pushes out far more blood in an hour than the body could possibly make — and proved it must be the same blood, going round and round in a closed circle.
heart → arteries → capillaries (the body's cells) → veins → back to the heart → lungs → and round again
One full circuit of the closed loop.
William Harvey laid out the proof in 1628 in a slim Latin book, De Motu Cordis ("On the Motion of the Heart"). He couldn't yet see how blood crossed from arteries to veins; the missing link — the capillaries, too fine for the naked eye — was spotted under a microscope by Marcello Malpighi a few years after Harvey's death.