Humans, like other life forms, can be viewed as thermodynamically open
systems that continuously consume energy to maintain stability in the internal
milieu in the face of ongoing environmental stress. In contrast to simple unicellular
life forms such as bacteria, higher life forms must maintain stability not
only in individual cells but also for the organism as a whole. To this end, a collection
of physiologic systems evolved to process foodstuffs; to acquire oxygen and dispose of gaseous waste; to eliminate excess fluid and soluble toxins; and
to perform other tasks. These systems—labeled respiratory, circulatory, digestive,
neurological, and so on—share several features.