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. Author manuscript; available in PMC: 2015 Feb 20.
Published in final edited form as: Annu Rev Biochem. 2008;77:777–798. doi: 10.1146/annurev.biochem.77.070606.101605

Figure 1.

Figure 1

Interacting physiological systems are shown schematically as two interacting systems. These systems are affected by inputs from the environment and by inputs from other physiological systems. System outputs can originate within the system independently of other inputs or can be dependent on the inputs from the environment and/or other systems. Organisms can be divided into physiological systems in a number of different ways; for example, human physiology can be described using 11 systems: skin, respiratory system, circulatory system, central nervous system, endocrine system, reproductive system, lymphoid system, musculoskeletal system, urinary system, digestive system, and special sense organs (33, 34). The increased mortality of the organism as it ages is due to the decline over time of various systems outputs. These can be intrinsic where the system outputs become inappropriate during aging because of alterations within the system. Dysfunction in system outputs can also occur for undamaged systems when the inputs are inappropriate.