Two major regulatory systems make important contributions to homeostasis: the nervous system and the endocrine system. In order to maintain relatively constant conditions in the internal environment of the body, each of these systems influences the activity of all the other organ systems. The nervous system coordinates fast, precise responses, such as muscle contraction. Electrical impulses generated by this system are very rapid and of short duration (milliseconds). The endocrine system regulates metabolic activity within the cells of organs and tissues. In contrast to the nervous system, this system coordinates activities that require longer duration (hours, days) rather than speed. Examples of such activities include growth; long-term regulation of blood pressure; and coordination of menstrual cycles in females. The endocrine system carries out its effects through the production of hormones, chemical messengers that exert a regulatory effect on the cells of the body. Secreted from endocrine glands, which are ductless structures, hormones are released directly into the blood. They are then transported by the circulation to the tissues upon which they exert their effects. Because they travel in the blood, the serum concentrations of hormones are very low (10-11 to 10-9 M); therefore, these molecules must be very potent.

Generally, a single hormone does not affect all of the body's cells. The tissues that respond to a hormone are referred to as the target tissues. The cells of these tissues possess specific receptors to which the hormone binds. This receptor binding then elicits a series of events that influences cellular activities.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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