The primary purpose of circulation is to deliver oxygen to tissues for cellular respiration and ATP production within mitochondria.

“So the the reason you breathe is to bring oxygen into the body most people know this and then once oxygen is in your lungs it goes into your blood and then it goes to the heart and then the heart kind of boom distributes this you know across a whole organism. Uh and then when oxygen gets you know to your big toe or to your muscle or to your neuron and you know the your hippocampus or some brain region um what happens is the oxygen enters the cell and then once it's inside the cell it looks for mitochondria looks I mean it's attracted by a concentration gradient. So that's the mitochondria is where oxygen is consumed.”