Answer:With this system, blood always flows in only one direction inside the heart. There are also valves at the bottom of the large arteries that carry blood away from the heart: the aorta and the pulmonary artery. These valves keep the blood from flowing backward into the heart once it has been pumped out.
Explanation:
I would say something that makes you use the spinal cord the most (if you catch my drift)...
amount of times you do something in a certain amount of time
