Excretion

The function termed ‘excretion’ includes all the processes of removing the waste products derived from the cellular metabolism. It is a process essential at all levels of living systems organization. It involves transport of molecules across the plasmatic membranes of cells that form the wall of compartmentalized structures present in specialized excretory organs. In this article, the nature of the main molecules involved in nitrogen and carbon excretion is described in the framework of adaptive and evolutionary strategies of organisms. Excretion is discussed also in terms of the water and electrolyte balance of the organism that strongly depends on organism–environment interactions and lifestyle. The basic mechanisms (active and passive transport of solutes, hydrostatic and osmotic forces) that govern the function of excretory organs are described together with a general outline of patterns of membrane transporters in the various functional units of excretory organs. Biological cases of particular interest from the ecological point of view are discussed as case studies of different adaptive strategies related to the environment.