Comprehensive Human Endocrine System: Glands, Hormones, and Regulation

Endocrine system

Long-term communication system that uses hormones as chemical messengers, regulates growth, metabolism, reproduction, and development, maintains homeostasis, and works alongside the nervous system.

Endocrine system vs nervous system

Endocrine signaling is slower with longer-lasting effects; hormones travel through the bloodstream, while nervous system signaling is rapid and uses neurotransmitters acting at synapses.

Endocrine gland

Ductless gland that secretes hormones directly into blood, including pituitary, thyroid, and adrenal glands.

Endocrine cascade

Series of hormone releases where one gland stimulates another gland, amplifying hormonal effects and allowing precise control.

Hormone classes

Includes steroid hormones, non-steroid hormones, and thyroid hormones (unique category).

Steroid hormones

Lipid-soluble hormones derived from cholesterol that can cross cell membranes and bind intracellular receptors to influence gene activity.

Non-steroid hormones

Water-soluble hormones derived from amino acids or peptides that cannot cross cell membranes and bind to membrane receptors, acting through intracellular signaling systems.

Thyroid hormones

Derived from amino acid tyrosine, lipid-soluble, behave similarly to steroid hormones, and bind intracellular receptors.

Paracrine signaling

Chemical messenger acts locally, affecting nearby cells without entering the bloodstream.

Autocrine signaling

Cell targets itself; messenger binds receptors on the same cell, important in growth and immune regulation.

Hormone action (big idea)

Hormones alter the activity of target cells; only cells with correct receptors respond, and response depends on hormone type and receptor location.

Target cell response to hormones

Depends on receptor presence, number of receptors, and hormone concentration; the same hormone can cause different effects in different tissues.

Up-regulation vs down-regulation of hormone receptors

Up-regulation: target cells increase receptor number when hormone levels are low, increasing cell sensitivity; Down-regulation: target cells decrease receptor number when hormone levels are high, decreasing cell sensitivity.

Hormone-receptor interaction

Hormone binds specifically to its receptor, forming a hormone-receptor complex that triggers a cascade of cellular events, determining type and strength of response.

Two main hormone mechanisms

Direct gene activation and second-messenger systems.

Hormone transport in the bloodstream

Water-soluble hormones circulate freely in plasma; lipid-soluble hormones bind to carrier proteins, increasing hormone stability and creating a hormone reservoir.

Direct gene activation mechanism

A lipid-soluble hormone diffuses through the target cell membrane, binds to an intracellular receptor, forms a hormone-receptor complex that binds directly to DNA, activating specific genes and synthesizing new proteins.

Effects of direct gene activation

Slow onset, long-lasting effects, alters protein production; examples include growth and metabolism changes.

cAMP pathway of hormone action

A water-soluble hormone binds to a membrane receptor, activating a G protein that stimulates adenylyl cyclase to convert ATP into cAMP, activating protein kinases that phosphorylate specific proteins to produce the cellular response.

Signal amplification

One hormone activates many receptors, with each step multiplying the signal; small hormone concentrations cause large cellular responses, increasing efficiency and sensitivity of hormone action.

Termination of cAMP hormone action

The hormone dissociates from the membrane receptor, the G protein inactivates, adenylyl cyclase is no longer activated, cAMP is broken down, protein kinases become inactive, and phosphatases remove phosphate groups from proteins, ending the cellular response.

Hormone interaction types

Includes permissiveness, synergism, and antagonism.

Permissiveness

One hormone allows another hormone to act; target cells require both hormones, e.g., thyroid hormone permits growth hormone effects.

Thyroid hormone permissiveness

Thyroid hormone increases receptor expression, allowing other hormones to produce full effects, especially important for epinephrine and growth hormone.

Synergism

Two hormones work together, with the combined effect greater than individual effects, e.g., glucagon + epinephrine increase blood glucose.

Antagonism

Hormones oppose each other, balancing physiological processes, e.g., insulin vs glucagon.

Control of hormonal secretions (overview)

Hormone levels must be precisely regulated to prevent over- or under-secretion, maintaining homeostasis through endocrine reflexes and feedback mechanisms.

Endocrine reflex

A stimulus triggers hormone release, involving an endocrine cell, hormone, and target cell, producing a physiological response that feeds back to regulate secretion.

Negative feedback (endocrine control)

Rising hormone levels inhibit further hormone secretion, while falling hormone levels stimulate hormone release, maintaining hormone levels within a narrow, stable range.

Positive feedback (endocrine control)

Hormone release stimulates additional hormone release, increasing rather than stabilizing the response; rare and temporary in the endocrine system.

Types of hormone regulation

Includes humoral regulation, neural regulation, and hormonal regulation.

Humoral regulation/reflex

Controlled by blood levels of ions or nutrients, where changes in blood chemistry trigger hormone release.

Humoral reflex examples

Low blood calcium → parathyroid hormone released; high blood glucose → insulin released; low blood glucose → glucagon released.

Neural regulation

Controlled by nerve impulses, where the nervous system directly stimulates hormone release, producing rapid, short-term hormonal responses.

Neural reflex example

Sympathetic stimulation of adrenal medulla releases epinephrine and norepinephrine, producing fight-or-flight response.

Hormonal regulation

One hormone controls the release of another, common in endocrine pathways, often involving pituitary hormones.

Endocrine axis

A multi-step hormonal control pathway typically including hypothalamus → pituitary → target gland, using negative feedback for regulation.

Hypothalamic control

Links nervous and endocrine systems by secreting releasing and inhibiting hormones that regulate anterior pituitary.

Releasing hormones

Secreted by hypothalamus to stimulate anterior pituitary hormone release, e.g., TRH, CRH, GnRH.

Inhibiting hormones

Secreted by hypothalamus to suppress pituitary hormone release, e.g., somatostatin inhibits growth hormone.

Pituitary gland role in control

Acts as the primary regulator of other endocrine glands, responding to hypothalamic signals and releasing tropic hormones.

Tropic hormones

Hormones that target other endocrine glands, stimulating secretion of other hormones, e.g., TSH, ACTH, FSH, LH.

Why tight hormonal control is necessary

Hormones have powerful, widespread effects; small imbalances cause major physiological disorders, and feedback prevents overstimulation or suppression.

Pituitary gland (overview)

Small, pea-sized endocrine gland located at the base of the brain, known as the 'master gland' that regulates many other endocrine glands.

Relationship between hypothalamus and pituitary

Hypothalamus controls pituitary function, acting as a link between nervous and endocrine systems using hormones and nerve signals.

Two lobes of the pituitary

Anterior pituitary (adenohypophysis) and posterior pituitary (neurohypophysis) with different embryological origins and control mechanisms.

Anterior pituitary characteristics

Glandular tissue that produces and secretes its own hormones, controlled by hypothalamic releasing and inhibiting hormones.

Hypophyseal portal system

Specialized blood vessel system connecting hypothalamus to anterior pituitary, allowing rapid hormone delivery and preventing dilution in systemic circulation.

Growth Hormone (GH)

Synthesized by the anterior pituitary, stimulated by growth hormone-releasing hormone (GHRH), and inhibited by somatostatin, targeting most tissues.

Thyroid-Stimulating Hormone (TSH)

Synthesized by the anterior pituitary, stimulated by thyrotropin-releasing hormone (TRH), targeting the thyroid gland to stimulate synthesis and release of T3 and T4.

Adrenocorticotropic Hormone (ACTH)

Synthesized by the anterior pituitary, stimulated by corticotropin-releasing hormone (CRH), targeting the adrenal cortex to stimulate secretion of cortisol.

Follicle-Stimulating Hormone (FSH)

Synthesized by the anterior pituitary, stimulated by gonadotropin-releasing hormone (GnRH), targeting ovaries and testes to stimulate follicle development and sperm production.

Luteinizing Hormone (LH)

Synthesized by the anterior pituitary, stimulated by gonadotropin-releasing hormone (GnRH), targeting ovaries and testes to trigger ovulation and testosterone secretion.

Prolactin (PRL)

Synthesized by the anterior pituitary, inhibited by dopamine, targeting mammary glands to stimulate milk production.

Melanocyte-stimulating hormone (MSH)

Synthesized by the anterior pituitary, targeting melanocytes in the skin to stimulate melanin production.

Posterior pituitary characteristics

Neural tissue that does not synthesize hormones but stores and releases hypothalamic hormones.

Where posterior pituitary hormones are synthesized

ADH and oxytocin are synthesized in the hypothalamus, transported down axons to posterior pituitary for storage and release.

Hormones released by posterior pituitary

Antidiuretic hormone (ADH) and oxytocin.

Antidiuretic Hormone (ADH)

Synthesized in the hypothalamus, stored and released by the posterior pituitary in response to increased blood osmolarity or dehydration, targeting kidneys.

Oxytocin

Synthesized in the hypothalamus, stored and released by the posterior pituitary in response to uterine stretch or nipple stimulation, targeting uterus and mammary glands.

Anterior vs posterior pituitary

Anterior: glandular, makes hormones, controlled by portal system; Posterior: neural, stores hormones, controlled by nerve impulses.

Thyroid gland (overview)

Largest pure endocrine gland located anterior to trachea, inferior to larynx, regulating metabolism, growth, and development.

Thyroid hormone (T3 & T4)

Secreted by follicular cells of the thyroid gland, derived from tyrosine and require iodine, regulating basal metabolic rate.

Iodine and thyroid hormone synthesis

Iodine is required to synthesize T₃ and T₄, obtained from diet; deficiency reduces hormone production, leading to increased TSH secretion.

Goiter

Enlargement of thyroid gland caused by iodine deficiency or chronic TSH stimulation, does not necessarily indicate hyperthyroidism.

Thyroid follicles

Structural and functional units of thyroid gland lined by follicular cells, surrounding a colloid-filled lumen, site of thyroid hormone synthesis.

Colloid

Gel-like substance inside follicles composed mainly of thyroglobulin, storing thyroid hormone precursors.

Calcitonin

Synthesized by parafollicular (C) cells of the thyroid gland, released when blood calcium levels are high, targeting bone tissue to inhibit osteoclast activity.

Thyroid disorders (general)

Result from excess or deficiency of thyroid hormones, disrupting metabolic balance, commonly involving feedback pathway dysfunction.

Hypothyroidism

Decreased T₃/T₄ levels leading to reduced metabolic rate, weight gain, cold intolerance, fatigue, and lethargy.

Hyperthyroidism

Excess T₃/T₄ levels leading to increased metabolic rate, weight loss, heat intolerance, nervousness, and irritability.

Parathyroid glands

Usually four small endocrine glands located on the posterior surface of thyroid gland, regulating blood calcium levels independently of thyroid hormone.

Primary role of parathyroid glands

Regulate blood calcium levels, maintaining calcium homeostasis as the most important regulators of calcium balance.

Parathyroid hormone (PTH)

Secreted by parathyroid glands, raises blood calcium levels, lowers blood phosphate levels, and acts on bones and kidneys.

PTH action on bone

Stimulates osteoblasts, which signal osteoclasts to break down bone matrix, releasing calcium and phosphate.

PTH action on kidneys

Increases calcium reabsorption and phosphate excretion, decreases calcium loss in urine, and stimulates activation of vitamin D.

Importance of calcium regulation

Required for muscle contraction, nerve impulse transmission, blood clotting, and bone strength; small changes have major effects.

Negative feedback of Blood Calcium

High blood calcium stimulates parafollicular cells to release calcitonin, inhibiting osteoclasts and promoting calcium excretion; low blood calcium stimulates parathyroid cells to release PTH.

Hypoparathyroidism

Insufficient PTH secretion leading to low blood calcium levels, causing tetany and muscle spasms.

Hyperparathyroidism

Excess PTH secretion leading to high blood calcium levels, causing bone demineralization, kidney stones, and muscle weakness.

Tetany

Caused by hypocalcemia, characterized by increased neuromuscular excitability and sustained muscle contractions.

Adrenal glands

Paired endocrine glands located on the superior surface of each kidney, involved in stress response and homeostasis.

Two regions of the adrenal gland

Adrenal cortex and adrenal medulla, differing in structure, origin, and hormones.

Adrenal cortex

Outer region of adrenal gland composed of glandular tissue, producing steroid hormones essential for long-term stress response.

Adrenal cortex organization

Divided into three distinct zones, each producing a specific hormone class.

Zona glomerulosa

Outermost layer of adrenal cortex producing mineralocorticoids, primarily aldosterone, regulating electrolyte balance.

Zona fasciculata

Middle layer of adrenal cortex producing glucocorticoids, primarily cortisol, regulating metabolism and involved in long-term stress response.

Zona reticularis

Innermost layer of adrenal cortex producing gonadocorticoids, secreting weak androgens contributing to puberty and sex drive.

Mineralocorticoids

Produced by zona glomerulosa, regulating sodium and potassium balance, affecting water retention and influencing blood pressure.

Aldosterone

Primary mineralocorticoid produced by adrenal cortex, targeting kidneys to increase sodium reabsorption and water retention.

Aldosterone vs ADH

Aldosterone regulates sodium balance; ADH regulates water balance, with both increasing blood volume and pressure.

Glucocorticoids

Produced by zona fasciculata, regulating metabolism of carbohydrates, fats, and proteins, helping the body cope with stress.

Cortisol

Primary glucocorticoid synthesized by zona fasciculata, increasing blood glucose through gluconeogenesis and suppressing inflammation.

Regulation of cortisol

Controlled by hypothalamic-pituitary-adrenal axis, where CRH stimulates ACTH release, which stimulates cortisol secretion.

Gonadocorticoids

Produced by zona reticularis, more significant in females, supplementing sex hormones from gonads and contributing to puberty.

Adrenal medulla

Inner region of adrenal gland derived from neural tissue, part of sympathetic nervous system, producing catecholamines.

Catecholamines

Hormones released during acute stress, including epinephrine and norepinephrine, producing rapid physiological responses.

Regulation of adrenal medulla hormones

Controlled by sympathetic nervous system, where nerve impulses trigger hormone release rapidly.

Epinephrine (adrenaline)

Catecholamine hormone produced by adrenal medulla, released during acute stress, increasing heart rate and blood glucose levels.

Norepinephrine

Catecholamine produced by adrenal medulla that also functions as a neurotransmitter, causing vasoconstriction and increasing blood pressure.