Henry Family Blog

The Endocrine System is a very important to your body tissues.  It alters your metabolism, it regulates growth and development, and it influences your reproductive process.  Two types of glands are the exocrine glands and the endocrine glands.  (Exo=outside, Endo=inside)  The Exocrine glands secrete products into ducts which carry the secretions to a target site.  Examples of exocrine glands are sweat, oil, mucus and digestive glands.  Endocrine glands secrete hormones into secretory cells which is then carried away by the blood.  Examples of endocrine glands are pituitary, thyroid, adrenal and pineal glands.

Hormones regulate extracellular fluid, metabolism, biological clock, contraction of cardiac and smooth muscle, glandular secretion, some immune functions, growth, development and reproduction.  There are two classifications of hormones; circulating and local.  Circulating hormones are endocrine hormones and they travel through your blood.  They have distinct target cells waiting for them.  Within the classification of local hormones are Paracrine cells and Autocrine cells.  Paracrine hormones travel in extracellular space and act on neighboring cells.  Autocrine hormones also travel in exracellular space but they act on the same cell that created it.  Neither one of these local hormones travel in the blood.

There are two chemical classes of hormones.  The lipid-soluble class and the water-soluble class.  The lipid-solubles bind to transport proteins and are carried in the blood.  They are direct gene activated and bind to and activate receptors within the cell.  Examples are steroids, thyroid hormones and nitric acid.  Water-solubles circulate freely and no transporter is required.  They are secondary messenger activated and activate plasma membrane receptors.  They set off a cascade of events inside the cell.  Meaning, the first messenger hormone binds to the cell membrane receptor.  Then, the second messenger is released inside the cell where hormone-stimulated responses take place.  Examples are amines, peptides, proteins and glycoproteins. 

To summarize, hormones are released into the blood to regulate functions of cells, tissues and organs.  But why are they being released in the blood in the first place?  Because they are needed to stimulate the production-secretion process.  The stimuli can be humoral, neural or hormonal.  The Humoral stimulus increases glucose in the blood causing the release of insulin by the pancreas.  The Neural stimulus uses a preganglionic sympathetic nerve fiber that stimulates the adrenal medulla to secrete catecholamines.  Finally, the Hormonal stimulus is when the pituitary gland secretes TSH that in-turn causes the thyroid gland to release T3 and T4. 

You must understand that the levels of hormones in blood vary from minute to minute.  There is a constant change in the amount of stimuli and they are regulated by feedback-loop mechanics.

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