How the Endocrine System Works
Endocrine regulation of physiology usually adheres to a slower time course than the neural and cardiovascular systems’ moment-to-moment management. Except in emergencies, endocrine regulation occurs over hours, days, months and occasionally years. However, the large role played by the brain in long-term endocrine regulation, discovered late in the 20th century, blurs the distinction between nervous system and endocrine system.
Endocrine System Components
Many different cells in the body secrete molecules named hormones into blood and into the interstitial fluid surrounding cells. Hormones only cause a reaction in cells that recognize the hormone’s presence.
The name hormone originates from a Greek word that means to ‘set in motion’. If a cell has a detector for the presence of a hormone, then that hormone is capable of setting in motion the cell’s molecular pathways.
In some cases, hormone-secreting cells are clustered into structures that can be observed as separate from the surrounding tissue. Such identifiable clusters of hormone secreting cells are the traditional endocrine glands. Traditional endocrine glands are scattered throughout the human body, and they provided the first understanding how the endocrine system functions. All are distinct, complex structures that secrete their hormones into the blood stream for delivery to target tissues.
A hormone-responding cell may be located at a distant point in the body from where the hormone was secreted. However, the term ‘distant’ is a relative expression. The hormone-responding cell may be near the hormone secreting cell (paracrine). Sometimes the responding cell is even the same cell that secreted the hormone (autocrine). All three of these types of hormone secreting cells are now considered part of the endocrine system.
Pituitary Gland and Brain Hormones
The brain dominates the nervous system. So, the discovery that the brain also participates as an endocrine gland was a surprise. This unexpected arrangement opened a new division of endocrinology named neuroendocrinology. The most obvious neuroendocrine structure in the brain is the hypothalamus.
The hypothalamus is a group of deep brain neuron cell clusters, called nuclei, that secretes hormones in addition to its many strictly neural functions. It works in coordination with a traditional endocrine gland that is located below the brain, the pituitary gland.
The pituitary gland possesses three sections based upon its histology. The anterior portion secretes protein hormones into its circulatory capillaries. Anterior pituitary hormones are carried to other hormone producing glands throughout the body setting in motion hormone synthesis and secretion pathways at those sites.
Terminal ends of neurons whose cell bodies lie in the hypothalamus form the posterior portion of the pituitary gland. Two hormones, oxytocin and vasopressin, are secreted directly into the blood stream by these hypothalamus nerve terminals. The target of oxytocin is reproductive structures and the target of vasopressin is the kidney.
The third portion of the pituitary gland is the intermediate lobe. It contains a collection of colloid structures forming a boundary between the anterior and posterior pituitary. This lobe is small in adult humans and does not secrete hormone during adult life.
Synthesis and release of pituitary hormone is controlled by small peptide hormones secreted by the hypothalamus into capillaries that connect the anterior pituitary to the brain. Hypothalamus hormones are called releasing hormones. Secretion of hypothalamus releasing hormones occurs in response to a wide array of sensory information arriving at the brain from multiple body regions.
Other Endocrine System Tissues
Other example of novel endocrine glands include the hormones controlling hunger and satiation, the heart’s atrial natriuretic peptide secreting cells, and bone osteocyte secretion of the hormone FGF23.
There is also a daunting array of cells and hormones that participate in local paracrine and autocrine endocrinology.
Classes of Hormones
Hormones in blood reach all the body’s cells. However, each hormone influences particular cells and not others. Initiation of a hormone response depends upon the target cell’s set of proteins. Each hormone binds to a unique protein designated its hormone receptor.
Classifiers of hormones traditionally labeled them as hydrophilic or hydrophobic. That system is less than optimal now that paracrine and autocrine hormones are included. It is better to think of the two classes of hormones as 1) those hormones that interact with proteins embedded in cell plasma membrane and 2) those hormones that penetrate the cell membrane to interact with intracellular proteins.
In general, hormone derivatives of amino acids or chains of amino acids, peptides and proteins, find their receptors embedded within cell plasma membranes. The exception is thyroid hormone, a derivative of tyrosine, which enters the cell and binds to an intracellular receptor.
Other hormones that enter cells and bind intracellular receptors include those that are derivatives of cholesterol. Cholesterol derivative hormones are also known as the steroids. Steroid hormones are synthesized in the adrenal glands, the ovary, the testes and the brain.
Autocrine and paracrine hormones are fatty acid derivations. They are the eicosanoids, prostaglandins, leukotrienes, prostacyclin and thromboxane. These hormones also find their receptors embedded in cell plasma membranes.
Hormone Feedback Regulation
Sophisticated feedback loops manage the hypothalamus, pituitary and peripheral organ endocrine activity. Yet, it is important to realize that not all endocrine feedback loops include the same components. Some peripheral endocrine feedback loops lack participation by the pituitary and hypothalamus. For example, feedback regulation of plasma calcium ion includes the parathyroid gland, bone, the intestine and the kidney.
As illustrated, feedback loops indicate a portion of the output [B] of a process [A] returns to the input part of the system and information carried by B alters input, which in turn alters output. In physiology, a predominance of negative feedback loops exists. Rare positive feedback loops include those that regulate contraction of the uterus during childbirth, blood clotting and ovulation of the ova.
As you study the physiology of each anatomic system remember to look for the role of the endocrine system there. Every cell in the body is affected by at least one or more hormone.
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Margaret Thompson Reece PhD, physiologist, former Senior Scientist and Laboratory Director at academic medical centers in California, New York and Massachusetts and CSO at Serometrix LLC is now CEO at Reece Biomedical Consulting LLC.
Dr. Reece is passionate about helping students, online and in person, pursue careers in life sciences. Her books “Physiology: Custom-Designed Chemistry” (2012), “Inside the Closed World of the Brain” (2015) and the workbook (2017) companion to her online course “30-Day Challenge: Craft Your Plan for Learning Physiology” are written for those new to life science. More about her books is available at https://www.amazon.com/author/margaretreece.
Dr. Reece offers a free 30 minute “how-to-get-started” phone conference to students struggling with human anatomy and physiology. Schedule an appointment by email at DrReece@MedicalScienceNavigator.com.