The circulatory system is basically comprised of a pump that pushes stuff (including cells, plasma, and hormones) through a series of pipes. The pipes come in a variety of sizes. The aorta and vena cavas are enormous and have a large volume capability. They each branch off into smaller tubes called veins and arteries. Those branch to the even smaller venules and arterioles. Smaller still are the capillaries. As those tubes branch into different directions and sizes, they gain access to just about every cell in the human body; therefore, the circulatory system has access to all parts of the body.
The circulatory system is like a highway and road system, because modern-day road systems give drivers access to just about everything. On a road, I can drive to a small country farmhouse, but I can also drive to a huge corporate headquarters, like Google. As the road system needs to handle more traffic volume, the road size changes. I live in San Diego, where the 163 and 15 interchange is 16 lanes wide. It's a huge section of road because it needs to carry a lot of traffic. That's like the aorta carrying all 70 ml of blood being pumped out of the left ventricle. My neighborhood is a series of narrow, one-way streets. That's like the capillaries squeezing in between individual cells. The roads are like the blood vessels and the cars are like the substances moving within the blood vessels.
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