A graded family of pipes
Blood leaves the heart in big, high-pressure vessels and gradually moves into smaller and smaller ones, all the way down to tubes barely wider than a single blood cell, then gathers back into ever-larger vessels on the way home. The order is: artery → arteriole → capillary → small vein → large vein. Each step is built differently for what it must do.
Arteries have thick, elastic, muscular walls because they carry blood at high pressure straight from the heart. The biggest ones stretch with each heartbeat and recoil between beats, smoothing the flow. Veins are thinner-walled and floppier; they operate at low pressure and act as a reservoir, holding most of the body's blood at any moment. Many veins contain one-way valves that keep blood moving toward the heart.
Where the real work happens
Arteries and veins are transport. The actual business of the circulation — handing oxygen, glucose and signals to cells, and taking back carbon dioxide and waste — happens in the capillaries. These are the tiniest vessels, with walls just one cell thick, lined by a delicate layer called the endothelium. That thinness is the point: molecules can slip across in either direction. The delivery of blood to tissue at this level is called perfusion.
Arterioles: the adjustable taps
Just before the capillaries sit the arterioles — small arteries wrapped in a ring of smooth muscle. By tightening or relaxing, they dial flow into each tissue up or down, like a tap. Collectively they are the body's main control point for systemic vascular resistance, the overall opposition to flow. When arterioles narrow across the body, resistance and blood pressure rise; when they widen, both fall.
On the return side, the low-pressure veins do more than carry blood back. Because they hold so much volume and can squeeze it back toward the heart, they are central to venous return — how much blood actually makes it back to fill the heart for the next beat. We will return to this in a later guide.