Circulation

So it occurs to me that no one ever properly explained the circulatory system to me. I always pictured it as a sort of pipe weaving its way through the body, with blood rushing through it thanks to the heart. Each blood cell would make a complete circuit of the body.

But that can't be right. It must be two series of tubes, one high-pressure and one low-pressure. The blood is forced across tissue by the difference in pressure. The heart pumps blood from the low-pressure system to the high-pressure system to maintain the differential. Each blood cell crosses over at some point and then immediately returns to the heart - so for instance, it might go from the high-pressure system to the low-pressure system somewhere in the leg, and immediately return to the heart.

Now it strikes me that the lungs must be set up to intercept each blood cell either before or after it goes through the heart, because of course one of the main points is to carry oxygen throughout the body.

Anyway, it's a testament to my pretty devastating lack of intelligence that I never learned how circulation works. That's, like, second grade, right?