In a unicellular organism, the interior  region of  the organism  is no longer in contact with the external environment.  Thus simple processes  of  diffusion, phagocytosis, exocytosis and contact with sea water helped in exchanging  the necessary molecules  and nutrients. But a further increase in size and complexity of the organism lead to concentration of the pool of cytoplasm. The fluid in-between the cells was known as interstitialfluids.

These fluids, due to constant exchange of molecules between the various cells lead to changes in the internal environment which influenced the functioning of the cell. Physiology, we learnt earlier,  is concerned with such basic activities as reproduction, growth, metabolism, excitation and contraction as they are carried out within the fine structure  -  the cells, tissues, organs and organ systems of the body. The various body systems of our body are enumerated next.