Physiology is the study of function of the living organism, and encompasses both normal and pathological states: the body can be divided into the internal and the external environment. The external environment is made up of the parts of the body that are directly connected to the outside world (i. e. the respiratory system, the gi system, and urogenital tract). The internal environment is basically the extracellular fluid, including the blood and interstitial fluid; put another way, the cells are bathed in the internal environment. There are various transport mechanisms that perform these functions. It can be controlled by both the nervous system (responsible for fast, minute-to-minute changes) and endocrine system (responsible for slower, long-term changes), and is often maintained by a negative feedback mechanism. It is this counteraction of change that gives the mechanism its negative feedback name: about 60% of any person is made up of water; this volume is referred to as total body water (tbw).