Basic Anatomy & Physiology of the Endocrine System
Basic Anatomy & Physiology of the Endocrine System
The endocrine system is composed of endocrine glands distributed throughout the body. These glands are pituitary, thyroid, parathyroid, adrenal, pineal, pancreas, gonads (ovaries and testes), and thymus (Figure 6.1).
The endocrine system consists of cells, tissues, and organs that secrete hormones as a primary or secondary function. The primary function of the endocrine glands is to secrete hormones into the bloodstream. Hormones are chemical messengers that will influence metabolic activities, growth, and development. Some glands have both endocrine and nonendocrine functions. For example, the pancreas contains cells that function in digestion as well as cells that secrete the endocrine hormones like insulin and glucagon, which regulate blood glucose levels. The hypothalamus, thymus, heart, kidneys, stomach, small intestine, liver, skin, female ovaries, and male testes are other organs that contain cells with endocrine function. Moreover, fat (adipose) tissue has long been known to produce hormones, and recent research has revealed that even bone tissue has endocrine functions. 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. As just noted, the pancreas also has an exocrine function: most of its cells secrete pancreatic juice through the pancreatic and accessory ducts to the lumen of the small intestine.
Endocrine Signaling
The endocrine system uses one method of communication called chemical signaling. These chemical signals are sent by the endocrine organs. 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. For example, the hormones released when you are presented with a dangerous or a frightening situation, called the fight-or-flight response, occur through the release of hormones from the adrenal gland—epinephrine and norepinephrine—within seconds. In contrast, it may take up to 48 hours for target cells to respond to certain reproductive hormones.
In addition, endocrine signaling is typically less specific than neural (nerve) signaling. The same hormone may also play a role in a variety of different physiological processes depending on the target cells involved. For example, the hormone oxytocin generates uterine contractions in women who are in labor. This hormone is also important in generating the milk-release reflex during breastfeeding and may be involved in the sexual response and in feelings of emotional attachment in both males and females.
Generally, the nervous system involves quick responses to rapid changes in the external environment, and the endocrine system is usually slower acting—ta