<span>The ventricles are the part of the heart that are responsible for the pumping action to move blood throughout the body. Therefore, the muscles in the ventricles are much larger and thicker to do their job properly. The atrium only has to pump blood through an open valve (the tricuspid valve) into the right ventricle. There is much less resistance to this flow of blood so much less muscle is needed. </span>
Organ system is the most complex in organization
Answer small and large intestines
I just found this by searching for "mushroom diagram".
