Homeostasis: the process by which the body"s substances and characteristics (such as temperature and glucose level) are maintained at their optimal level. Ingestive behaviour: eating or drinking (intake of food, water, and minerals) A physiological regulatory mechanism is one that maintains the constancy of some internal characteristic of the organism in the face of external variability ex: keeping body temperature constant despite changes in ambient temperature. Contains four essential features: system variable: a variable that is controlled by a regulatory mechanism, for example, temperature in a heating system. Negative feedback: a process whereby the effect produced by an action serves to diminish or terminate that action; a characteristic of regulatory systems. We can achieve water balance by the intake of 2 ingredients: water and sodium chloride. Eating needs to obtain adequate amounts of carbohydrates, fats, amino acids, vitamins, and minerals (other than sodium).