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The woman who has heard the best breakup line (and by best I mean worst!By Regina Bailey The circulatory system serves to move blood to a site or sites where it can be oxygenated, and where wastes can be disposed.As the bulb is squeezed, it forces the water along to the bucket.
As oxygen and other chemicals diffuse out of the blood cells and into the fluid surrounding the cells of the body's tissues, waste produces diffuse into the blood cells to be carried away.
Their cells are close enough to their environment for oxygen, other gases, nutrients, and waste products to simply diffuse out of and into their cells.
In animals with multiple layers of cells, especially land animals, this will not work, as their cells are too far from the external environment for simple osmosis and diffusion to function quickly enough in exchanging cellular wastes and needed material with the environment.
Needless to say, this is a very inefficient system.
Insects can get by with this type system because they have numerous openings in their bodies (spiracles) that allow the "blood" to come into contact with air.
The closed circulatory system of some mollusks and all higher invertebrates and the vertebrates is a much more efficient system.