We looked at the internal environment in module 1 and learnt that it is important to maintain conditions at relatively constant levels. Recall that the internal environment is everything inside the body but does not include the cells. The internal environment is the space the cells live in. Any change in these values will result in a not-so-ideal internal environment, which will cause cells to function improperly. The average human is made up of 60% water or 42 liters (for a 70kg person). This water is found in 3 different compartments in the body - the intracellular compartments (everything inside the cells), the interstitial compartment (everything outside the cells but not in the plasma) and the plasma. Both the plasma and the interstitial compartment can be combined into the extracellular compartment (everything outside the cells including plasma). Most of the water (67%) is found inside the cells (the intracellular compartment).