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Endochrine System

The Endocrine System consists of endocrine glands (pituitary, thyroid, parathyroids, adrenals, and pineal) and several organs that contain endocrine tissue (hypothalamus, thymus, pancreas, ovaries, testes, kidneys, stomach, liver, small intestine, skin, heart and placenta). Endocrine glands secrete hormones into the blood.


  • Maintenance of the internal environment in the body (maintaining the optimum biochemical environment).
  • Integration and regulation of growth and development.
  • Control, maintenance and instigation of sexual reproduction, including gametogenesis, coitus, fertilisation, foetal growth and development and nourishment of the newborn

Within the endocrine system there are two types of glands, the endocrine and the exocrine. Exocrine glands have ducts, where products that are not hormonally related are directed to membrane surfaces.

Endocrine glands are ductless and their function is to release hormones only into the blood and lymph. Endocrine glands comprise of the pituitary, thyroid, parathyroid, adrenal, pineal and thymus gland As well as the main endocrine organ, there are other organs and tissues which produce hormones, which may include the heart, kidneys, small intestine and stomach which release hormone producing cells from pockets in the wall of the organ.


A hormone is a chemical signal sent from cells in one part of an organism to cells in another part (or parts) of the same individual.  They are often regarded as chemical messengers.  Although typically produced in very small quantities, hormones may cause profound changes in the cells that it is intended for. Their effect may be stimulatory or inhibitory. In some cases, a single hormone may have multiple targets and cause different effects in each target.  Hormones effectively alter cell activity.

Together, these hormone-secreting structures form an endocrine system that helps maintain homeostasis, coordinate behaviour, and regulate growth, development and other physiological activities.

Hormones are the chemical messenger molecules of the endocrine system. They travel by circulating blood and can either work on local target cells called local hormones, or target distant cells called endocrines. There are four classes of hormones: steroids, biogenic amines, proteins and peptides and eicosanoids.
The seven actions of hormones: 

  • They regulate the chemical composition and volume of the extracellular fluid.
  • They regulate contraction of cardiac and smooth muscle.
  • They regulate metabolism.
  • They regulate the immune system.
  • They help maintain homeostasis.
  • They help in growth and development.
  • They help the process of reproduction.

The Chemical Structure of Hormones

Hormones are made up of a variety of amino acids, chains of amino acids known as peptides, and derivatives of lipids and glycoproteins. Hormones can either be water or lipid soluble: thyroid and steroid hormones are lipid soluble and all others are water soluble.  After hormones are secreted into the blood, lipid hormones will bind to proteins which enable them to remain in the circulation longer than water soluble hormones

Actions of Hormones

There are two main mechanisms of hormone regulation; one which mediates the target cells response to the particular hormone known as the second messenger system and one which involves the direct DNA activation by the hormone itself. Second messenger mechanisms are when protein, peptide and various amine hormones all interact with their target cells. This occurs in the cyclic ATP system. 

Hormone - Target Cells

Hormones circulate the blood constantly and come into contact with all cells. The particular hormone will only affect a certain cell however, which are called target cells. A target cell will respond to a hormone depending on the presence of receptors, either within the cell or in the plasma membrane.

Hormones are measured by their half-life, which is the concentration of hormone in the blood stream. This both detects the rate that it is released as well as the speed of its removal from the body. Some hormones will promote target organ responses at once; some will take hours or days before any effect is noticed by the body. The control and release of different hormones is due to a negative feedback system. Hormone secretion is triggered by various internal and external stimuli on the body.

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