Aspartame is the most controversial food additive in history, and its approval for use in food was the most contested in FDA history. In the end, the artificial sweetener was approved, not on scientific grounds, but rather because of strong political and financial pressure.
The arterioles then divide further into capillaries. Capillaries are the smallest blood vessels and their walls are only one cell thick which allows diffusion between the blood and cells to occur. Nutrients and oxygen from the blood diffuses through the capillary walls into the tissues that need it. This is aided by the relatively high blood pressure that also squeezes the oxygen and nutrients through the capillary wall and into the cells of the tissue.
Other animals (including many vertebrates) oxygenate via other organs like the skin or gills. Oxygen dissolves in the liquid lining of the alveoli and then diffuses down a concentration gradient through the walls of the alveoli and capillaries into the plasma and into the red blood cells.xygen enters the tissues from the bloodstream because our bodies  need oxygen and food to release energy. The cells in the tissues  use the oxygen for various functions.