Endocrine System Overview

Endocrine System Overview

• The endocrine system regulates homeostasis over long periods (hours, days, weeks)
• Hormones are chemicals released into the bloodstream to regulate distant cells
• Endocrinology is the study of hormones and endocrine organs

Major Endocrine Processes

• Reproduction
• Growth and development
• Electrolyte, water, and nutrient balance in the blood
• Cellular metabolism and energy balance
• Immune defenses

Major Endocrine Glands

• Hypothalamus
• Pituitary gland (anterior and posterior)
• Thyroid gland
• Parathyroid glands
• Adrenal glands
• Pineal gland
• Pancreas (endocrine portion)
• Gonads (ovaries and testes)

Hypothalamus-Pituitary Axis

• The hypothalamus is the control center for the endocrine system
• It releases or inhibits hormones that affect the anterior pituitary
• The anterior pituitary releases tropic hormones that act on other endocrine glands
• Peripheral endocrine glands release hormones that affect target cells and body functions
• Target cells have receptors for particular hormones, causing changes in homeostasis

Hypothalamus-Anterior Pituitary Hormones

• 7 hormones are released by hypothalamus to regulate anterior pituitary
• Examples include TRH (thyrotropin-releasing hormone), CRH (corticotropin-releasing hormone), GnRH (gonadotropin-releasing hormone)

Hormones and Their Glands

• Hypothalamus: ADH, oxytocin, releasing/inhibiting hormones (GHRH, GHIH, TRH, CRH, GnRH, PIH)
• Anterior Pituitary: growth hormone, TSH, ACTH, FSH, LH, PRL
• Thyroid Gland: thyroid hormone, calcitonin
• Parathyroid Gland: parathyroid hormone
• Adrenal Cortex: aldosterone, cortisol, DHEA, androgens
• Adrenal Medulla: epinephrine, norepinephrine
• Pineal Gland: melatonin
• Pancreas: insulin, glucagon
• Gonads: estrogen, progesterone, testosterone (more detail in reproductive chapter)

Growth Hormone

• Growth hormone (somatotropin) is an amino acid-based hormone produced by the anterior pituitary.
• Functions: metabolism (throughout life), fat breakdown, increased blood glucose, protein synthesis (muscle mass, cellular growth), bone growth, and increased cell division.
• Regulation: hormonal (GHRH, GHIH from hypothalamus; negative feedback of IGF-I and GH); neural (diurnal rhythms, increase with sleep)

Growth Hormone & IGF's

• Growth hormone actions are mediated by insulin-like growth factors (IGFs), also called somatomedins
• IGF-I is mainly released into blood by the liver, and produced by individual tissues, stimulating soft tissue and bone growth
• IGF-II is primarily involved in fetal development

Growth Hormone Pathway

• Growth hormone (GH) is made and released by the anterior pituitary in response to growth hormone releasing hormone (GHRH) from the hypothalamus
• Target cells include the liver, adipose tissue, and all body cells
• Regulation includes negative feedback from IGFs and GH to the anterior pituitary, and GHIH (somatostatin) from the hypothalamus

Growth Hormone Syndromes

• Hypersecretion: gigantism (whole body enlarged) or acromegaly (enlarged extremities)
• Hyposecretion: dwarfism
• Treatment for dwarfism is effective if diagnosed before epiphyseal plate closure. Other issues include abuse of GH by bodybuilders, athletes, and elderly.

Thyroid Hormone

• Thyroid hormones (T3 and T4) are lipid-soluble, major metabolic hormones
• T4 (tetraiodothyronine) is the stored and secreted form
• T3 (triiodothyronine) is more potent and biologically active
• Produced and released by the thyroid gland; bow-tie shaped organ below larynx
• Function: increased metabolic rate, heat production, growth, CNS development, SNS activity
• Regulation: hormonal (TRH, TSH), neural (body temperature, stress)

Thyroid Hormone Pathway

• Stimulus: TRH (hypothalamus), TSH (anterior pituitary)
• Production & Release: thyroid gland
• Target cells: body cells (metabolism & heat production, tissue growth & development, blood vessel SNS receptors to maintain blood pressure)
• Regulation: negative feedback to hypothalamus and the anterior pituitary from T3 & T4; hormonal inhibition (GHIH, dopamine, glucocorticoids), neural (cold in infants, stress)

Thyroid Hormone Synthesis

• Thyroid hormones are made from amino acid tyrosine and iodine.
• Steps involve: thyroglobulin synthesis, iodide transport into follicle cells, converting iodide to iodine, attaching iodine to tyrosine in the colloid, forming T3 and T4, taking T3- and T4-thyroglobulins back into follicle cells, and cleaving T3 and T4 from thyroglobulins, allowing them to diffuse into the blood

T4 to T3 conversion

• T4 is the major form of thyroid hormone stored and secreted by the thyroid
• T3 is 80% made from T4 by removing iodine from the liver or kidneys; 20% is secreted as T3 by the thyroid gland
• T3 is 10 times more potent and biologically active

Thyroid Hormone Syndromes

• Hyperthyroidism: excess T3 and T4 (Graves' disease, tumors; symptoms: high metabolism, sweating, tachycardia, bulging eyes)
• Hypothyroidism: low T3 and T4 (lack of iodine, deficiency, or disease; symptoms: low metabolism, cold, sluggish, dry skin, edema, myxedema, cretinism, goiter)

Calcitonin

• Produced by parafollicular cells of the thyroid.
• Reduces blood calcium levels, inhibits bone breakdown, and stimulates calcium storage in bones.
• Regulation: blood calcium (humoral); not crucial in humans

Parathyroid Hormone

• Produced by the parathyroid glands (tiny glands on the thyroid's posterior side)
• Increases blood calcium (amino acid based)
• Function: Increases plasma Ca++, decreases plasma PO4--, stimulates vitamin D.
• Blood (humoral) regulation: related to blood Ca2+ levels
• Requires calcium homeostasis; removal can be fatal

Parathyroid Hormone Pathway

• Stimulus: low blood calcium
• Target organ: bone (increase in Ca2+ release, increase in bone breakdown, Ca2+ and PO4 release via stimulation of osteoclasts) and kidneys (increase in blood calcium retention, decrease in blood phosphate, vitamin D activation by kidneys. and small intestine vitamin D needed for calcium absorption in diet), blood (humoral): High Ca2+ in blood directly inhibits parathyroid hormone production and release.

Parathyroid Hormone Syndromes

• Hyperparathyroidism is usually caused by tumors and results in bone loss and nervous system issues.
• Hypoparathyroidism can be caused by removal (thyroid surgery) or magnesium deficiency and the resulting issue includes muscle spasm, nervousness, and other issues

Adrenal Glands

• Small, capped glands above the kidneys
• Adrenal Cortex (outer region): steroid hormones (mineralocorticoids, glucocorticoids, sex hormones)
• Adrenal Medulla (middle region): catecholamines (epinephrine, norepinephrine)

Adrenal Medulla

• Catecholamines (epinephrine, norepinephrine) produced and released by the adrenal medulla
• Target cells: SNS organ targets
• Function: Enhance sympathetic effects, stress response, and blood pressure regulation.
• Regulation: the sympathetic nervous system

Adrenal Cortex

• Steroid hormones, specifically mineralocorticoids (aldosterone, fluid balance: increases Na+ in blood, decreases K+ in blood), glucocorticoids (cortisol, metabolism: increase blood glucose, increase protein breakdown, increase fat breakdown, immune response block pathways & antibody production), and sex hormones (androgens) produced and released by the adrenal cortex NOTE: Aldosterone deficiency is FATAL; regulated by the renin-angiotensin system and the kidneys

Mineralocorticoids (Aldosterone)

• Primarily aldosterone; regulates fluid balance; increases Na+ retention, decreases K+ excretion
• Deficiency is fatal
• More details in Chapter on Renin-Angiotensin System and kidneys

Gonadocorticoids (Sex Hormones)

• Includes androgens such as DHEA, androstenedione, testosterone, the weak precursors of testosterone in females during puberty, and estrogen
• Adrenal cortex produces DHEA
• Ovaries and testes produce estrogen and testosterone

Glucocorticoids (Cortisol)

• Primarily cortisol; regulates metabolism
• Increase blood glucose levels; sequester glucose to the brain, increase protein breakdown, and increased fat breakdown
• Associated with stress response, suppresses the immune system, blocks inflammatory pathways and antibody production

Cortisol Pathway

• Stimulus: CRH (hypothalamus), ACTH (anterior pituitary)
• Targets: all body cells
• Functions: gluconeogenesis, immune system suppression, breakdown of fats, and proteins
• Regulation: Neural (stress), hormonal (negative feedback from high cortisol to CRH and ACTH)

Cortisol Syndromes

• Hyposecretion: Addison's disease (weight loss, low blood glucose, low blood ions, dehydration, and hypotension)
• Hypersecretion: Cushing's syndrome (“moon face”, increased abdominal/back fat, muscle/bone weakness)

Insulin

• Lowers blood glucose; amino acid based
• Produced and released by the pancreas.
• Functions: glucose uptake in muscle and fat cells, glycogen storage, inhibits glucose production
• Regulation: hormonal (neural, blood (humoral), increased by glucose, epinephrine, thyroxine, glucocorticoids; decreased by somatostatin. PSNS- post-eating)

Glucagon

• Increases blood glucose; amino acid based
• Produced and released by the pancreas
• Functions: glucose synthesis, glucose release, glycogen breakdown
• Regulation: hormonal (low glucose), neural (stimulated by sympathetic nervous system), inhibited by insulin and somatostatin
• Glucagon opposes Insulin

Type I and Type II Diabetes Mellitus

• Type I: lack of insulin secretion from the pancreas; typically treated with injected insulin
• Type II: normal insulin secretion, but target cells don't respond; treated with diet, exercise, sometimes insulin

Explore the complexities of the endocrine system and its role in regulating homeostasis over long periods. This quiz covers the major endocrine glands, key processes, and the hypothalamus-pituitary axis, shedding light on how hormones impact various body functions.