Part 5- Discourse

So i was really thinking about part 5 of the Discourse and I don't really know why Descartes is so obsessed with the blood flowing through the body. He seems to be very interested with how our bodies are so complex unlike other things. The most important part of our body is our heart considering it gives us warmth and blood flow. How would he know so much about the body in the 1600's? I just find this very interesting espeically after taking biology classes and learning how everything works and him being on target without all of the equiptment that we have today.