Blood

I always hate learning about the circulatory system and the respiratory system because it always stresses me out. It's like when you think about your breathing suddenly you can't breath correctly anymore until you forget about thinking about your breathing! See!

Yeah, so anyways. It's like there are all of these things that can go wrong and on top of that your heart is working super hard and all the blood is going into whatever vein and then from that into the little branch-y off things until the oxygen gets close enough to the tissues to absorb the oxygen and then it has to come back again to get pumped by the heart and then go to the lungs to get re-oxygenated. And this is all happening in the bu DUM of your pulse. Right now. And now. IT'S SO STRESSFUL. It feels like thinking about it is going to somehow inhibit your heart's ability to function properly...