Body has 100 trillion cells which functions in a coordinated way. This coordination requires normal internal Environment. This internal environment is affected by response of each system to each other. This coordination is studied in physiology.
The term Milieu interior is dependent upon pH of the blood and response of various systems to each other in changed condition.
With the evolution unicellular organism developed into multicellular one. During evolution, unicellular organism was surrounded by sea water. Sea water was carrying oxygen & nutrients directly into the cell and waste products were thrown out. This system slowly developed into closed circulatory system with heart as a pump and for control of various systems there was development of CNS & Endocrine system.
We study this coordination and factors affecting this in Physiology.