A hypothesis is a general statement that has little or no data backing. Once a hypothesis is tested and can be backed by data from different independent researchers, it becomes a theory.
Hence, the scenario described a theory rather than a hypothesis.
By delivering oxygen to the cells. The circulatory system transports the blood around the body while the respiratory is responsible for creating the oxygen and transporting it to the blood.
No because they still have a lot to them a nucleus membrane bound organelles etc and they are just as important