We accept the fact that our body is made up of billions of cells. Each cell carries out routine chemical reactions to sustain itself and perform specific chemical reactions that help the cell to execute its unique function. The rate of these chemical reactions is under the influence of regulatory systems that involve other cells. For example, during stressful situations, the blood glucose would be raised to meet the demand of the effectors (especially skeletal muscle) for fight and flight responses. This is orchestrated by activation of the sympathetic nervous system which leads to increase in stress hormone levels in the blood (epinephrine by adrenal medulla and cortisol by adrenal cortex). The stress hormones work on effector cells (especially liver cells) to increase production of glucose for use by skeletal muscles for fight and flight. It is clear that the fight and flight responses from this example requires intensive intercellular communication that involves the nervous system, the endocrine system, the musculoskeletal system, etc.