For every of the 24 sub-units there is special chemical signal. Depending on the need for oxygen a cell can give many or less signals. Beside these signals the release of oxygen is also controlled by temperature. Every sub-unit has a specific temperature optimum. The blood does not flow through blood vessels.
Spiders have an open blood circulation system.
Blood vessels do transport the blood to a specific place but thereafter the blood flows freely in the open spaces between the organs. On the back of the abdomen the heart is located. Elastic muscles around this cavity contract, enlarging the tube. Because of the valves in the tube the blood can flow in only one direction. If the tube is filled with fluid the muscles weakens and the blood is pressed out of the tube.