When you purchase through links on our site, we may earn an affiliate commission. Here’s how it works. The circulatory system consists of three independent systems that work together ...

but ripped out the brain through the nose and discarded it. In the 2nd century, the Greek physician and philosopher Galen of Pergamon came up with a believable model for the circulatory system.

Let’s follow the blood on a simple loop through the circulatory system to see how it works: Atherosclerosis can gradually make the arteries narrower, affecting the amount of blood that can flow ...