The pool of immediately available ATP is surprisingly small in muscles. There is only enough ATP available at any time for about 1 or 2 seconds of muscle contraction. Phosphocreatine can supply additional ATP, but after no more than 10 seconds of work anaerobic and aerobic metabolism need to take over. It can be instructive to compare the three types of muscle. Skeletal muscle is controlled by motor neurons and can contract rapidly.
Cardiac muscle has more mitochondria than skeletal muscle.
Our next topic is circulation. These proportions are roughly the same in all vertebrates. As you are probably aware, blood has fluid and cellular components. The fluid is called plasma. The cells are of course the red blood cells, white blood cells, and platelets.