Answer:
1.) The heart is unlike many man made pumps because it's more like multiple pumps. The heart pumps blood throughout the whole body and to most organs like the lungs. Man made pumps could never go as fast and filter out the blood to our small circulatory and organ systems.
2.) We need a circulatory system because it carries blood throughout our bodies. Without it we wouldn't get oxygen to our lungs or heart therefore they'd stop working.
3.) Some structures that ensure unidirectional flow are Heart Valves which ensure that unidirectional blood flow reaches through the cardiac cycle. Some examples of these valves are, atrioventricular valves, tricuspid valves, and the mitral valves.
Hope this helps. Was a week late but it showed up on my recommended. :)
