Why do we have a circulatory system whereas unicellular organisms simply obtain nutrients directly through their cell membranes?

Unicellular organisms, such as bacterium and some Protista, are composed of only one cell. Therefore, the transportation of substances across the single cell membrane of unicellular organisms results in the near-immediate delivery of the substances to their final destination. However, multicellular organisms are composed of many cells. A much more complex system is needed in order to transport nutrients, oxygen, carbon dioxide, wastes, and other substances from one cell, tissue, or organ to another within a multicellular organism. The vessels, veins, and arteries of the circulatory system allow for the transport of these substances to occur throughout a multicellular organism. Hence, the circulatory system enables gas and nutrient levels to be maintained throughout a multicellular organism. This is one way in which all parts of a multicellular organism are able to maintain hemostasis.