Endocrine System

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The endocrine system consists of cells, tissues, and organs that secrete hormones as a primary or secondary function.

• The endocrine gland is the major player in this system.
• The primary function of the endocrine gland is to secrete hormones directly into the surrounding fluid.
• The surrounding fluid (interstitial fluid) and the blood vessels then transport the hormones throughout the body.
• The endocrine system includes the pituitary, thyroid, parathyroid, adrenal, and pineal glands.
• The ductless endocrine glands are not to be confused with the body’s exocrine system, whose glands release their secretions through ducts.
  • Examples of exocrine glands include the sebaceous and sweat glands of the skin.
• The endocrine system uses one method of communication called chemical signaling.
• The endocrine organs secrete chemicals—called hormones—into the fluid outside of the tissue cells (extracellular fluid). Hormones are then transported primarily via the bloodstream throughout the body, where they bind to receptors on target cells, creating a particular response.
• Endocrine signaling is typically less specific than neural (nerve) signaling.
• Generally, the nervous system involves quick responses to rapid changes in the external environment, and the endocrine system is usually slower acting.
• Role: taking care of the internal environment of the body, maintaining equilibrium (homeostasis), and in controlling reproduction.
• Although considered “slower”, the nervous system can cause rapid endocrine responses to keep up with sudden changes in both the external and internal environments, when necessary.

Types of Chemical Signaling

• There are four different types of chemical signaling occurring in multicellular organisms: endocrine signaling, autocrine signaling, paracrine signaling, and direct signaling.
  • In endocrine signaling, hormones secreted into the extracellular fluid spreads into the blood or lymphatic system, and can, therefore, travel great distances throughout the body.
  • In contrast, autocrine signaling occurs within the same cell.
  • Paracrine signaling occurs amongst neighboring cells.
    • Although paracrines may enter the bloodstream, their concentration is generally too low to elicit a response from distant tissues.
    • Example: histamine
  • Direct signaling occurs between neighboring cells across gap junctions. Gap junctions are channels that connect neighboring cells, that allow small molecules to move between the neighboring cells.

Endocrine Glands, their Hormones, and Effects

1. Betts, J.G., Young, K.A., Wise, J.A., Johson, E., Poe, B., Kruse, D. H., Korol, O., Johnson, J.E., Womble, M. & DeSaix, P. (2013). Anatomy and Physiology. OpenStax. http://cnx.org/content/col11496/latest/[↩]