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. Image description available.
Figure 6.1 Endocrine System. Endocrine glands and cells are located throughout the body and play an important role in maintaining equilibrium (homeostasis). From Betts, et al., 2013. Licensed under CC BY 4.0. [Image description.]

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—taking care of the internal environment of the body, maintaining equilibrium (homeostasis), and controlling reproduction (see Table 6.1). So how does the fight-or-flight response, mentioned earlier, happen so quickly if hormones are usually slower acting? It is because the two systems are connected. It is the fast action of the nervous system in response to the danger in the environment that stimulates the adrenal glands to secrete their hormones, epinephrine and norepinephrine. As a result, the nervous system can cause rapid endocrine responses to keep up with sudden changes in both the external and internal environments when necessary.

Table 6.1: Endocrine and Nervous Systems

Characteristic Endocrine System Nervous System
Signaling mechanism(s) Chemical Chemical/electrical
Primary chemical signal Hormones Neurotransmitters
Distance traveled Long or short Always short
Response time Fast or slow Always fast
Environment targeted Internal Internal and external

From Betts, et al., 2013. Licensed under CC BY 4.0.

Table 6.2: Endocrine Glands and Their Major Hormones

Endocrine Gland Associated Hormones Chemical Class Effect
Pituitary (anterior) Growth hormone (GH) Protein Promotes growth of body tissues
Pituitary (anterior) Prolactin (PRL) Peptide Promotes milk production
Pituitary (anterior) Thyroid-stimulating hormone (TSH) Glycoprotein Stimulates thyroid hormone release
Pituitary (anterior) Adrenocorticotropic hormone (ACTH) Peptide Stimulates hormone release by adrenal cortex
Pituitary (anterior) Follicle-stimulating hormone (FSH) Glycoprotein Stimulates gamete production
Pituitary (anterior) Luteinizing hormone (LH) Glycoprotein Stimulates androgen production by gonads
Pituitary (posterior) Antidiuretic hormone (ADH) Peptide Stimulates water reabsorption by kidneys
Pituitary (posterior) Oxytocin Peptide Stimulates uterine contractions during childbirth
Thyroid Thyroxine (T4), triiodothyronine (T3) Amine Stimulate basal metabolic rate
Thyroid Calcitonin Peptide Reduces blood Ca2+ levels
Parathyroid Parathyroid hormone (PTH) Peptide Increases blood Ca2+ levels
Adrenal (cortex) Aldosterone Steroid Increases blood Na+ levels
Adrenal (cortex) Cortisol, corticosterone, cortisone Steroid Increases blood glucose levels
Adrenal (medulla) Epinephrine, norepinephrine Amine Stimulates fight-or-flight response
Pineal Melatonin Amine Regulates sleep cycles
Pancreas Insulin Protein Reduces blood glucose levels
Pancreas Glucagon Protein Increases blood glucose levels
Testes Testosterone Steroid Stimulates development of male secondary sex characteristics and sperm production
Ovaries Estrogens and progesterone Steroid Stimulates development of female secondary sex characteristics and prepares the body for childbirth

From Betts, et al., 2013. Licensed under CC BY 4.0.


Hormone Receptors

The message a hormone sends is received by a hormone receptor, a protein located either inside the cell or within the cell membrane. The receptor will process the message by initiating other signaling events or cellular mechanisms that result in the target cell’s response. Hormone receptors recognize molecules with specific shapes and side groups and respond only to those hormones that are recognized. The same type of receptor may be located on cells in different body tissues and trigger somewhat different responses. Thus, the response triggered by a hormone depends not only on the hormone but also on the target cell.

Image Descriptions

Figure 6.1 image description: This diagram shows the endocrine glands and cells that are located throughout the body. The endocrine system organs include the pineal gland and pituitary gland in the brain. The pituitary is located on the anterior side of the thalamus, while the pineal gland is located on the posterior side of the thalamus. The thyroid gland is a butterfly-shaped gland that wraps around the trachea within the neck. Four small, disc-shaped parathyroid glands are embedded into the posterior side of the thyroid. The adrenal glands are located on top of the kidneys. The pancreas is located at the center of the abdomen. In females, the two ovaries are connected to the uterus by two long, curved tubes in the pelvic region. In males, the two testes are located in the scrotum below the penis. [Return to Figure 6.1].


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Medical Terminology: An Interactive Approach Copyright © 2022 by LOUIS: The Louisiana Library Network is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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