Closed circulatory system

We were talking about the cardiovascular system in Bio the other day. As humans, we have a closed circulatory system:

Closed circulatory systems have the blood enclosed at all times within vessels of different size and wall thickness. In this type of system, blood is pumped by a heart through vessels, and does not normally fill body cavities.

I started doodling something a bit like this in my notes, and decided to use it as Photoshop practice.
Note my novice use of dodge/burn and clone. I'm a newbie to photoshop, so they were pretty cool to fool around with.