Notes
Introduction to Endocrinology
Sections
Hormones are secreted by specialized cells (usually epithelial cells).
They are transported via the blood and bind with receptors on other tissues, where they initiate physiological responses
Hormones are responsible for long-term regulation of: metabolism, growth, development, reproduction, and the internal environment (i.e., temperature)
In contrast, neural mechanisms trigger rapid, short term responses
Key endocrine structures and examples of their products
Synthesizes anti-diuretic hormone (ADH, aka, vasopressin), corticotropin-releasing hormone (CRH), and gonadotropin-releasing hormone (GnRH), among others.
Pituitary gland
Posterior lobe of the pituitary gland does not synthesize hormones, it release antidiuretic hormone, which, as we've indicated, is synthesized by the hypothalamus.
Anterior lobe synthesizes adrenocorticotropic hormone (ACTH, aka, adrenocorticotropic hormone), growth hormone (GH), thyroid-stimulating hormone (TSH), and the gonadotropins (follicle-stimulating hormone and luteinizing hormone).
Produces the thyroid hormones, T3 and T4, as well as calcitonin.
Clinical correlation: enlarged thyroid gland is called goiter.
Parathyroid glands
Secrete parathyroid hormone (PTH)
Heart
Specialized cells within it secrete atrial natriuretic hormone (aka, atriopeptin); this hormone is secreted in response to atrial stretching to increase sodium and water excretion in the kidneys.
Kidneys
Secrete erythropoietin and renin; erythropoietin stimulates red blood cell production, and renin aids in blood pressure regulation.
Cortex: cortisol
Medulla: catecholamines (norepinephrine and epinephrine)
Specialized cells throughout the gastrointestinal tract secrete hormones that aid in digestion and metabolism; gastrin and secretion are two examples.
Pancreatic islets secrete insulin and glucagon, which are hormones that regulate blood glucose levels.
Gonads
Produce large quantities of the reproductive hormones; the testes are a major source of the androgens, particularly testosterone, in males, and the ovaries are a major source of progestins and estrogens.
Be aware our that diagram is simplified for introductory purposes, and does not include all endocrine tissues nor the complex interactions between endocrine tissues
For example, fat tissues also synthesize and secrete hormones, and, although the testes produce relatively large quantities of androgens, the adrenal cortex and ovaries also contribute to androgen levels
Hormone Classes
Peptides and proteins
Peptides and proteins vary in size, are derived from amino acids, and are synthesized throughout the body
Examples: insulin (which comes from the pancreatic islets), growth hormone (from the anterior pituitary gland), and parathyroid hormone.
Steroid hormones
Steroid hormones are derived from cholesterol, which is present in food and produced endogenously; they are primarily produced in the adrenal cortex and gonads.
Examples: cortisol and the reproductive hormones.
Amines
Amines are derived from tyrosine in the adrenal medulla and thyroid gland.
Examples: catecholamines (epinephrine, aka, adrenaline, and, norepinephrine, aka, noradrenaline), and T3 and T4.
Regulatory mechanisms
Regulatory mechanisms mediate the secretion of hormones and receptor response.
Hormonal feedback mechanisms are physiological processes that are influenced by their own products:
In positive feedback, the hormonal pathway is reinforced by its own products to ensure additional secretion. This type of feedback is relatively rare.
For example, past a certain threshold, circulating estrogen triggers events that ultimately increase estrogen release from the ovaries
In negative feedback loops, physiological products inhibit the hormonal pathway to halt further secretion
For example, testosterone limits its own secretion via its effects on the hypothalamus and pituitary gland
Receptor Regulation:
Hormones can also influence their effects at target tissues by altering the number or affinity of receptors to regulate receptor response:
In up-regulation, the presence of a hormone increases receptor response.
In down-regulation, receptor response decreases.
Full-Length Text
- Here we will learn some key facets of endocrine physiology.
- To begin, start a table, and denote that hormones are:
- Secreted by specialized cells (usually epithelial cells),
- Transported via the blood,
- Bind with receptors on other tissues,
- Initiate physiological responses.
- Generally, hormones are responsible for long-term regulation of: metabolism, growth, development, reproduction, and the internal environment (i.e., temperature).
- For contrast, consider that neural mechanisms trigger rapid, short term responses.
With these general principles in mind, let's draw some key endocrine structures and list a few of their products.
- First, outline a human form.
- Then, draw the hypothalamus and pituitary gland;
- Write that the hypothalamus synthesizes anti-diuretic hormone (ADH, aka, vasopressin), corticotropin-releasing hormone (CRH), and gonadotropin-releasing hormone (GnRH), among others.
- Notice that many of the hypothalamic hormones include the word "releasing;" an indication of the hormone's action.
- Make a notation that "releasing" means it stimulates the "release" of another hormone from another endocrine source.
- "Inhibiting" means that the hormone blocks the release of some other hormone.
- A key target of hypothalamic hormones is the pituitary gland, which is structurally and functionally divisible into anterior and posterior lobes;
- Indicate that, though the posterior lobe of the pituitary gland does not synthesize hormones, it does release antidiuretic hormone, which, as we've indicated, is synthesized by the hypothalamus.
- We'll learn the details of the hypothalamic-pituitary system, elsewhere.
- Write that the anterior lobe synthesizes adrenocorticotropic hormone (ACTH, aka, adrenocorticotropic hormone), growth hormone (GH), thyroid-stimulating hormone (TSH), and the gonadotropins (follicle-stimulating hormone and luteinizing hormone).
- As you may have guessed, the names of these hormones tells us their effects on target organs; for example, thyroid-stimulating hormone stimulates the thyroid gland.
- Next, indicate that the thyroid gland, which is the butterfly or bow-tie shaped gland in the neck, produces the thyroid hormones, T3 and T4, as well as calcitonin.
- Show that the four small parathyroid glands, which are on the posterior aspect of the thyroid gland secrete parathyroid hormone (PTH); as we'll learn elsewhere, this hormone regulates calcium levels.
- Draw the heart, and indicate that specialized cells within it secrete atrial natriuretic hormone (aka, atriopeptin); this hormone is secreted in response to atrial distension to increase sodium and water excretion in the kidneys.
- Next, indicate that the kidneys secrete erythropoietin and renin; erythropoietin stimulates red blood cell production, and renin aids in blood pressure regulation.
- Draw the adrenal gland, which rests on the superior surface of the kidney; this mass of endocrine tissue comprises an outer cortex and an inner medulla;
- Write that cortisol is produced in the cortex; as we'll learn elsewhere, cortisol and other glucocorticoids regulate blood glucose levels.
- Epinephrine (aka, adrenaline) and norepinephrine (aka, noradrenalin) are released from the adrenal medulla; these catecholamines operate in coordination with the sympathetic division of the autonomic nervous system as part of the "fight or flight" response to stress.
- Across from the kidneys, draw the stomach and indicate that specialized cells throughout the gastrointestinal tract secrete hormones that aid in digestion and metabolism; gastrin and secretion are two examples.
- Then, show the nearby pancreas, and indicate that pancreatic islets secrete insulin and glucagon, which are hormones that regulate blood glucose levels.
- Lastly, show that the gonads produce large quantities of the reproductive hormones; the testes are a major source of the androgens, particularly testosterone, in males, and the ovaries are a major source of progestins and estrogens.
- Be aware our that diagram is simplified for introductory purposes, and does not include all endocrine tissues nor the complex interactions between endocrine tissues.
- For example, fat tissues also synthesize and secrete hormones, and, although the testes produce relatively large quantities of androgens, the adrenal cortex and ovaries also contribute to androgen levels.
Let's take a moment now and organize the numerous hormones into three key classes.
- Peptides and proteins, which vary in size; these are derived from amino acids, and are synthesized throughout the body.
- Denote the following examples: insulin (which comes from the pancreatic islets), growth hormone (from the anterior pituitary gland), and parathyroid hormone.
- Steroid hormones are derived from cholesterol, which is present in food and produced endogenously.
- Steroid hormones are primarily produced in the adrenal cortex and gonads – where we showed that cortisol and the reproductive hormones are produced.
- Amines, which are derived from tyrosine in the adrenal medulla and thyroid gland.
- We saw that the adrenal medulla produces the catecholamines, including epinephrine (aka, adrenaline) and norepinephrine (aka, noradrenaline), and, thyroid gland produces such hormones as T3 and T4.
Finally, let's look at regulatory mechanisms that control the secretion of hormones and receptor response.
- Write that hormonal feedback mechanisms are physiological processes that are influenced by their own products:
- In positive feedback, the hormonal pathway is reinforced by its own products to ensure additional secretion. This type of feedback is relatively rare.
- For example, past a certain threshold, circulating estrogen triggers events that ultimately increase estrogen release from the ovaries.
- In negative feedback loops, physiological products inhibit the hormonal pathway to halt further secretion.
- For example, testosterone limits its own secretion via its effects on the hypothalamus and pituitary gland.
- Hormones can also influence their effects at target tissues by altering the number or affinity of receptors to regulate receptor response.
- In up-regulation, the presence of a hormone increases receptor response;
- In down-regulation, receptor response decreases.