Chapter 16: The Endocrine System

Chapter 16: The Endocrine System. Dr. Porta-Miller BIO2102: Anatomy & Physiology II Marieb Human Anatomy & Physiology, 12ed.

Learning Objectives (LOs) Part 1:. Compare and contrast the endocrine and nervous systems in terms of their speed, duration, and methods of communication within the body. What are some of the main processes controlled by the endocrine system? Define hormone and know the main classes of hormones. How does hormone type influence mechanisms of cell entry and blood transport? Describe how blood levels of hormones are controlled by negative feedback mechanisms. Be able to explain the 3 triggers of hormonal release (humoral, neural, hormonal) and provide examples of each. What factors determine if a target cell is activated? What changes do hormones typically produce in a target cell? Describe up-regulation versus down-regulation of hormone receptors. Discuss how hormones circulate in the blood, the variation in duration of hormone effect, and how hormonal signals end. Explain how hormones can have antagonistic, synergistic, or permissive interactions and provide one example of each. Discuss the posterior pituitary-hypothalamic interactions and which hormones are released from the posterior pituitary (including their regulation of release and target organ effects)..

The Endocrine System is One of the Body’s Two Major Control Systems.

Reproduction Growth and development Maintenance of electrolyte, water, and nutrient balance of blood Regulation of cellular metabolism and energy balance Mobilization of body defenses.

Overview of Endocrine Organs & the Hormones They Release.

Hormones: long-distance chemical messengers transported in the bloodstream that elicit target cell effects after a period of a few seconds to several days Higher the concentration of hormone, the stronger the “message” A Hormone’s chemical structure determines its solubility in water, which in turn determines: How it will be transported in the blood (which is mostly water!) How long it will take to be degraded Which receptors it will act upon.

An internal or external stimulus triggers hormone secretion As levels of hormone rise, it causes target organ or tissue effects These increased hormone-induced effects produce changes that remove the initial stimulus and inhibit further hormone release As a result, blood levels of hormones vary within a narrow, desirable range.

Triggers for Hormone Release: Humoral Stimuli. “Humor” refers to moisture or bodily fluids Some endocrine glands secrete their hormones in direct response to changing blood levels of critical ions and nutrients Ex. Ca2+ in blood Declining (low) blood Ca2+ concentration stimulates parathyroid glands to secrete parathyroid hormone (PTH) PTH causes Ca2+ concentrations to rise, and then the stimulus is removed.

Nerve fibers can stimulate hormone release E.g., Stress response the sympathetic nervous system stimulates the adrenal medulla to release epinephrine and norepinephrine.

Many endocrine glands release their hormones in response to hormones that are produced by other endocrine organs E.g., Hypothalamic hormones regulate release of most anterior pituitary hormones Anterior pituitary hormones stimulate other target endocrine organs to secrete their hormones This is the Hypothalamic–pituitary–target endocrine organ feedback loop (core concept!!) Hormones from final target organs inhibit release of anterior pituitary hormones, and thus their own release.

Water-soluble hormones (all amino acid (protein)–based hormones except thyroid hormone) are the majority of hormones Act on plasma membrane receptors Cannot cross plasma membrane to enter cell Most are coupled via G proteins to second messengers that mediate the target cell’s response Lipid-soluble hormones (steroid and thyroid hormones) are released by reproductive organs and adrenal cortical glands Act on intracellular receptors that directly activate genes Can diffuse across plasma membrane to enter cell.

Hormones circulate to virtually all tissues, but only cells with receptors for that hormone are affected Tissue cells with receptors for a specific hormone are target cells of that hormone Hormones alter target cell activity, increasing or decreasing the rates of normal cellular processes.

Target cells must have specific receptors for a hormone to bind E.g., adrenocorticotropic hormone (ACTH) receptors are found only on certain cells of the adrenal cortex; but thyroxine receptors are found on nearly all cells of the body (thyroxine stimulates cellular metabolism) Degree of target cell activation (its response) depends on three factors: 1. Blood levels of hormone 2. Relative number of target cell receptors 3. Affinity (strength) of binding between hormone and receptor.

Up-regulation: target cells add receptors in response to persistently low hormone levels Allows target cell to become more sensitive to less available hormone Down-regulation: target cells remove receptors in response to persistently high hormone levels Desensitizes the target cells so they respond less vigorously to hormonal stimulation, preventing them from overreacting to persistently high hormone levels.

Steroids and thyroid hormone are bound to plasma proteins Most other hormones can circulate freely without carriers (why?!) Concentration of circulating hormone reflects: 1. Rate of the hormone’s release 2. Speed at which it is inactivated and removed from body To end hormonal signals, they must be broken down and removed from blood Some hormones are rapidly degraded by enzymes in their target tissues Most hormones are removed from blood by the liver or kidneys Half-life: time required for level of hormone in blood to decrease by half.

How long does it take for a hormone to have an effect? It varies!.

Permissiveness: when one hormone cannot exert its effects without another hormone being present E.g., reproductive hormones regulate development of reproductive system, but thyroid hormone is also necessary for normal timely development of reproductive structures Synergism: occurs when more than one hormone produces the same effects at the target cell and their combined effects are amplified E.g., glucagon and epinephrine both cause liver to release glucose into the blood, when acting together the amount of glucose released is ~150% of what is released if each hormone acts alone Antagonism: occurs when one hormone opposes the action of another hormone (how does this occur?!) E.g., insulin (lowers blood glucose) is antagonized by glucagon (raises blood glucose).

Hypothalamus is connected to pituitary gland via infundibulum Pituitary has 2 major lobes: Posterior pituitary Mainly neural tissue Stores: Oxytocin & ADH Anterior pituitary Mainly glandular tissue Considered the “master endocrine gland”  many hormones it produces regulate activity of other endocrine glands.

Posterior Pituitary-Hypothalamic Relationships. This figure summarizes the two ways in which the hypothalamus controls the release of hormones from the pituitary gland. The figure illustrates the mechanisms in posterior pituitary. For long description in Notes pane, press F6..

The Posterior Pituitary and Hypothalamic (neuro)Hormones.

Learning Objectives (LOs) Part 2:. Discuss the anterior pituitary-hypothalamic interactions and which hormones are released from the anterior pituitary (including their regulation of release and target organ effects). Understand and describe how the posterior and anterior pituitary differ – anatomically and functionally. Identify the location and functions of the major endocrine glands discussed and list the hormones secreted by each major endocrine gland, their target tissues, and their primary effects on the body. Describe an example of negative feedback regulation involving the hypothalamus, anterior pituitary, and a peripheral endocrine gland (e.g., the regulation of thyroid hormones by TRH, TSH, and thyroid hormone)..

Anterior Pituitary-Hypothalamic Relationships. This figure summarizes the two ways in which the hypothalamus controls the release of hormones from the pituitary gland. The figure illustrates the mechanisms in anterior pituitary. For long description in Notes pane, press F6..

Growth hormone (G H) Functions: Anabolic (tissue building) hormone Metabolic effects: mobilizes fat, breaks down glycogen (“anti-insulin effects of GH”) Regulation of secretion Hypothalamus releases GHRH (triggered by low G H, low blood glucose, stress or exercise)  stimulating the anterior pituitary to secrete GH When there are high GH levels, this signals the hypothalamus to release GHIH (negative feedback!).

Thyroid-stimulating hormone (TSH) Function: stimulates normal development and secretory activity of thyroid gland Secretion triggered by thyrotropin-releasing hormone (T R H) from hypothalamus Secretion of T R H and T S H inhibited by rising blood levels of thyroid hormones (negative feedback).

Adrenocorticotropic hormone (ACTH) Function: stimulates adrenal cortex to release corticosteroids (mostly glucocorticoids) Hypothalamus secretes corticotropin-releasing hormone (CRH) (triggered by stress and hypoglycemia) Secretion of CRH and ACTH is inhibited by rising blood levels of glucocorticoids (negative feedback!).

Luteinizing hormone (LH) Functions: targets gonads to promote production of gonadal hormones (estrogen, progesterone, & testosterone), triggers ovulation Follicle-stimulating hormone (FSH) Functions: targets gonads to stimulate production of gametes (egg/sperm), promotes ovarian follicle maturation Prolactin (PRL) Function: promotes lactation.

Thyroid gland Largest pure endocrine gland Contains Colloid-filled follicles Follicular cells produce thyroglobulin, a glycoprotein used to make thyroid hormone Colloid is thick fluid storing the thyroglobulin proteins with attached iodine, from which thyroid hormone is produced and then secreted by the follicular cells Parafollicular cells amongst the follicles produce different hormone, calcitonin (lowers calcium blood levels).

TH is the body’s major metabolic hormone, it affects nearly ALL cells Produced in 2 forms T4 (tetraiodothyronine or thyroxine) – secreted by thyroid follicles/follicular cells T3 (triiodothyronine) – formed at target tissues by converting T4 to T3 Unlike other amines, TH is lipid soluble (like steroids) Must travel in bloodstream bound to carriers Can enter target cells to bind to intracellular receptors within the nucleus Triggers transcription of various genes for protein synthesis.

4 parathyroid glands are usually found embedded in posterior aspect of thyroid gland Contain parathyroid cells that secrete the protein parathyroid hormone (P T H) PTH is most important hormone in Calcium homeostasis Falling blood Ca2+ levels stimulate PTH secretions Rising blood Ca2+ levels inhibit PTH secretion Target organs are the bones, kidneys, and small intestine….

Stimulate osteoclasts to digest bone matrix and release Ca2+ to blood Enhances reabsorption of Ca2+ by kidneys Promotes activation of vitamin D by kidneys, which leads to increased absorption of Ca2+ by intestinal mucosa.

Pyramid-shaped organs found above the kidneys Each adrenal gland is structurally & functionally 2 different endocrine glands: Adrenal cortex: 3 outer layers of glandular tissue that synthesize and secrete several different steroid hormones Adrenal medulla: inner region of nervous tissue that is part of sympathetic nervous system; secretes 2 amine hormones Adrenal hormones help us cope with stressful situations and regulate electrolyte balance.

Produces corticosteroid hormones 3 layers (zones) of the cortex Zona glomerulosa produces mineralocorticoids Hormones (like aldosterone) that help to regulate blood electrolyte concentrations (primarily Na+ and K+) Zona fasciculata produces glucocorticoids (cortisol!) Help us resist stressors Influences energy metabolism of most body cells Provokes gluconeogenesis (forming glucose from fats/proteins) Enhances vasoconstriction by SNS during stress/low BP Zona reticularis produces gonadocorticoids Mostly weak androgens (male sex hormones) that get converted in tissue cells to more potent male hormones (testosterone), some converted to estrogens Exact functions still unclear; thought to contribute to female libido, is main source of estrogens in postmenopausal people.

Medullary chromaffin cells Synthesize 2 catecholamines epinephrine (80%) and norepinephrine (20%) Cells are stimulated to secrete catecholamines during the fight-or-flight (stress) response (SNS!) Effects of catecholamines Blood pressure increases via vasoconstriction and increased heart rate Blood diverted from nonessential organs to heart and skeletal muscle Blood glucose levels rise.

Pinealocytes secrete melatonin Blood levels rise and fall in a daily cycle (peak levels at night make us sleepy; lowest levels around noon) Melatonin effects: The suprachiasmatic nucleus of the hypothalamus (“biological clock”) has tons of Melatonin receptors, so exposure to bright light (which suppresses melatonin secretion) can reset the clock timing As a result, changing levels of melatonin may influence rhythmic variations in physiological processes like body temperature, sleep, and appetite.

Mixed gland with both exocrine and endocrine cells Acinar (exocrine) cells produce enzyme-rich fluid for digestion (more in Ch. 23!) Pancreatic islets consist of endocrine cells Alpha cells produce glucagon Beta cells produce insulin Insulin and glucagon regulate blood glucose levels in an antagonistic fashion Glucagon causes blood glucose levels to rise Insulin causes blood glucose levels to fall.

Slides beyond this point are useful study tools .

Anterior Pituitary Hormones: Summary of Regulation and Effects.

Anterior Pituitary Hormones: Summary of Regulation and Effects cont..

Anterior Pituitary Hormones: Summary of Regulation and Effects cont...

Glucocorticoids and the Long-Term Stress Response.

Insulin & Glucagon from the Pancreas Regulate Blood Glucose Levels.