In the peripheral border areas, where blood capillaries and parenchymal cells are in direct contact, the most vital processes of our organism take place. Here, the exchange of substances occurs between the cell and the surrounding media – this is the essence of life. The individual cell within the tissue is not passively nourished; rather, it actively participates in its nourishment by extracting the necessary portion from the tissue fluid that surrounds it. In this sense, nutrition is both quantitatively and qualitatively the result of cellular activity, although the cell is naturally also dependent on the quantity and quality of the available nutrients – that is, the supply – but is by no means compelled to absorb everything that flows to it, regardless of what or how much it is. Just like the individual cell of an alga extracts from the nutrient fluid.