It seems like a farfetched notion that a small chemical can enter the bloodstream and cause an action at a very distant location in the body. Yet this scenario occurs everyday. The ability to maintain homeostasis and respond to stimuli is largely due to hormones secreted within your body. Without hormones, you could not grow, maintain a constant temperature, produce offspring, or perform the basic actions that are essential for life.
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The
endocrine system produces hormones that are instrumental in maintaining homeostasis and regulating reproduction and development. A hormone is a chemical messenger produced by a cell that effects specific change in the cellular activity of other cells (target cells). Unlike exocrine glands (which produce substances such as saliva, milk, stomach acid, and digestive enzymes), endocrine glands do not secrete substances into ducts (tubes). Instead, endocrine glands secrete their hormones directly into the surrounding extracellular space. The hormones then diffuse into nearby capillaries and are transported throughout the body in the blood.
The endocrine and nervous systems often work toward the same goal—both influence other cells with chemicals (hormones and neurotransmitters). However, they attain their goals differently. Neurotransmitters act immediately (within milliseconds) on adjacent muscle, gland, or other nervous cells, and their effect is short-lived. In contrast, hormones take longer to produce their intended effect (seconds to days), may affect any cell, nearby or distant, and produce effects that last as long as they remain in the blood (up to several hours).
Hormones can be chemically classified into four groups:
Amino acid-derived hormones are modified amino acids.
Polypeptide and protein hormones are chains of amino acids of less than or more than about 100 amino acids, respectively. Some protein hormones are actually glycoproteins, containing glucose or other carbohydrate groups.
Steroid hormones are lipids that are synthesized from cholesterol. Steroids are characterized by four interlocking carbohydrate rings.
Eicosanoids are lipids that are synthesized from the fatty acid chains of phospholipids found in plasma membrane.