Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Endocrine System

One of the oldest forms of Intercellular Communication is chemical signaling, mediated by chemical messengers via the body's fluid environments. Such fluid (humoral) communication has several distinct features: slow dissemination of chemical substances, diffuse effects, and relatively low reliability, as these substances act in minute quantities and are rapidly degraded.

Over the course of evolutionary development, humoral regulation mechanisms were complemented by a more sophisticated nervous regulation, transmitting information via bioelectric potentials directed precisely at the target.

The Emergence of neural regulation went hand in hand with the refinement of humoral regulatory mechanisms, marked by The Development of specialized endocrine Organs, or Glands of Internal Secretion. The chemical, BIOLOGICALLY ACTIVE SUBSTANCES secreted by Endocrine glands are called Hormones (from the Greek hormao meaning to excite or set in motion). Endocrine glands lack excretory ducts and release their synthesized hormones directly into the body's circulating fluids (interstitial fluid and Blood). A distinction is made between hormonal and paracrine (mediated) modes of hormone action.

In hormonal—or strictly endocrine—action, hormones are transported by the bloodstream to a target organ, acting at a distance from their site of synthesis. In paracrine action, hormones released from the site of synthesis enter the extracellular space and act locally on surrounding target Cells.

It is now well established that the PRODUCTION OF BIOLOGICALLY active substances is a function not only of major endocrine organs, but also of individual (endocrine) cells scattered across various Tissues and belonging to the so-called Cytology/cytology/65.html">Diffuse Endocrine System.

Endocrine glands and the hormones they secrete operate in close coordination with The Nervous system, forming a unified integrative neurohumoral regulatory mechanism. The interdependence and mutual conditioning of neural and humoral processes are integrated within the Hypothalamus. Neurosecretory Cells of the hypothalamic nuclei serve as the structural substrate uniting the nervous and endocrine systems, functioning as a site where neural stimuli are transformed into humoral signals and vice versa.