SOMAPL1A
The Endocrine System
OBJ 2663 — Vocabulary
23 questions — Define the medical vocabulary components related to the endocrine system.
OBJ 2664 — Endocrine vs Nervous
7 questions — Identify the similarities between the endocrine and nervous system.
OBJ 2665 — Hormone Classes
8 questions — Communicate the major chemical classes of hormones.
OBJ 2666 — Hormonal Action
9 questions — Identify the general mechanisms of hormonal action.
OBJ 2667 — Endocrine Control
8 questions — Communicate how endocrine organs are controlled.
OBJ 2668 — Glands & Hormones
13 questions — Communicate the location, hormones, and functions of all major endocrine glands.
OBJ 2669 — Hormone Interactions
7 questions — Communicate how hormones interact to produce coordinated physiological responses.
OBJ 2670 — Hormones & Growth
7 questions — Identify the hormones that are especially important to normal growth.
OBJ 2671 — Stress Response
8 questions — Communicate how the endocrine system responds to stress.
OBJ 2672 — Abnormal Production
7 questions — Communicate the results of abnormal hormone production or abnormal responses.
OBJ 2673 — Behavior & Aging
7 questions — Describe the effects of hormones on behavior and aging.
OBJ 2674 — System Integrations
5 questions — Communicate the functional relationships between the endocrine system and other body systems.
★ Final Score — SOMAPL1A
The Endocrine System
Martini, Ober, Bartholomew — Essentials of Anatomy & Physiology (Pearson, 2013) · Chapter 10
Word Roots & Meanings
Chapter 10 of Martini lists 23 vocabulary roots. Each root maps to a clinical or anatomical term used throughout the endocrine system. The test will ask you to connect a root to its meaning and to the term it generates.
| Root / Prefix | Meaning | Example Term & Connection |
|---|---|---|
| ad- | to or toward | Adrenal — glands located toward (on top of) the kidneys |
| andros | man | Androgen — male sex hormones (testosterone is the most important) |
| angeion | vessel | Angiotensin — hormone that stretches (constricts) blood vessels, raising blood pressure |
| corpus | body | Corpus luteum — the “yellow body” that forms from follicular cells after ovulation and secretes estrogens and progestins |
| diabetes | to pass through | Diabetes — conditions defined by excessive urine production (fluid “passes through” without adequate retention) |
| diourein | to urinate | Diuresis — increased urine production |
| erythros | red | Erythropoietin — hormone that stimulates production of red blood cells |
| infundibulum | funnel | Infundibulum — the funnel-shaped stalk connecting the pituitary gland to the hypothalamus |
| insipidus | tasteless | Diabetes insipidus — urine is dilute and tasteless (no glucose), unlike the sweet urine of diabetes mellitus |
| krinein | to secrete | Endocrine — “endo” (within) + “krinein” (to secrete) = secreting internally into the bloodstream |
| lac | milk | Prolactin — “pro” (before) + “lac” (milk) = hormone that stimulates milk production |
| mellitum | honey | Diabetes mellitus — urine is sweet due to glucose overflow when blood glucose exceeds kidney reabsorption capacity |
| natrium | sodium | Natriuretic — promoting sodium excretion in urine (ANP is natriuretic) |
| okytokos- | swift birth | Oxytocin — stimulates uterine contractions during labor and delivery |
| ouresis | making water | Polyuria — making excessive amounts of urine |
| para | beyond / beside | Parathyroid — glands embedded beside (in the posterior surface of) the thyroid |
| poiesis | making | Erythropoietin — “making red” (stimulates red blood cell production); thrombopoietin — “making clots” (stimulates platelet production) |
| pro- | before | Prolactin — “before milk” — prepares mammary glands for milk production |
| renes | kidneys | Suprarenal — “supra” (above) + “renes” (kidneys) = alternate name for adrenal glands |
| synergia | working together | Synergistic — hormones working together for a combined effect greater than either alone |
| teinein | to stretch | Angiotensin — “vessel stretching” hormone that constricts blood vessels to raise blood pressure |
| thyreos | an oblong shield | Thyroid — the thyroid cartilage of the larynx is shield-shaped; the thyroid gland lies just inferior to it |
| tropos | turning | Gonadotropins — hormones that “turn on” the gonads (FSH and LH) |
The Core Analogy
Nervous System = Telephone Company
Carries specific messages from one specific location to another. Effects are short-lived. Ideal for crisis management—leaping away from a speeding bus.
Endocrine System = Postal Service
Hormones are like addressed letters distributed by the bloodstream. Effects are slow to appear but persist for days. Ideal for long-term regulation of metabolism, growth, and development.
What Is a Hormone?
A hormone is a chemical messenger released in one tissue and transported by the bloodstream to reach target cells in other tissues. Target cells possess specific receptors to “read” the message. Cells without the receptor ignore the hormone entirely—like bulk mail read only by those interested.
Four Shared Features
- Both release chemicals that bind to specific receptors on target cells.
- Both share chemical messengers—norepinephrine and epinephrine are hormones when released into the bloodstream and neurotransmitters when released across synapses.
- Both are regulated mainly by negative feedback control mechanisms.
- Both coordinate and regulate cell, tissue, organ, and system activities to maintain homeostasis.
1. Amino Acid Derivatives
Small molecules structurally similar to amino acids. Examples: epinephrine, norepinephrine, thyroid hormones (T3 and T4), melatonin. Structurally the simplest hormones.
2. Peptide Hormones
Chains of amino acids ranging from short polypeptides to small proteins. This is the largest class of hormones. Examples: ADH, oxytocin, growth hormone, prolactin, FSH, LH, TSH, ACTH, insulin, glucagon. Includes all hormones secreted by the hypothalamus, pituitary gland, heart, kidneys, thymus, digestive tract, and pancreas. Not lipid soluble—cannot cross the plasma membrane.
3. Lipid Derivatives
Steroid Hormones
Structurally similar to cholesterol. Released by reproductive organs and adrenal glands. Insoluble in water—bound to transport proteins in blood. Can diffuse through the lipid bilayer of the plasma membrane.
Eicosanoids
Derived from the 20-carbon fatty acid arachidonic acid. Include the prostaglandins. Coordinate local cellular activities and affect enzymatic processes (e.g., blood clotting) in extracellular fluids.
Target Cell Sensitivity
The presence or absence of a specific receptor determines whether a cell responds. Without the receptor, the hormone is ignored regardless of concentration. Hormones alter cellular operations by changing the identities, activities, locations, or quantities of important enzymes and structural proteins.
Mechanism A — Plasma Membrane Receptors (Nonsteroidal Hormones)
Used by epinephrine, norepinephrine, peptide hormones, and eicosanoids. These bind to receptor proteins on the plasma membrane surface (outer for peptides/E/NE; inner for eicosanoids).
Other second messengers include calcium ions and cyclic-GMP. Roughly 80 percent of prescription drugs target receptors coupled to G proteins.
Mechanism B — Intracellular Receptors (Steroid & Thyroid Hormones)
Steroid hormones diffuse through the lipid bilayer → bind to receptors in the cytoplasm or nucleus → hormone-receptor complex activates or inactivates specific genes → alters mRNA transcription rate → changes the structure or function of the cell. Example: testosterone stimulates production of enzymes and structural proteins in skeletal muscle.
Thyroid hormones cross the membrane via a transport mechanism → bind to receptors in the nucleus or on mitochondria. Nuclear binding activates genes and changes enzyme production. Mitochondrial binding increases the rate of ATP production.
Hormone Half-Life
A freely circulating hormone (not bound to a transport protein) remains functional for less than one hour—sometimes as little as two minutes. Steroid and thyroid hormones remain in circulation much longer because they are bound to transport proteins. As free hormone is removed, bound hormone releases to replace it.
Primary Mechanism: Negative Feedback
A stimulus triggers hormone production whose direct or indirect effects reduce the intensity of the original stimulus.
Three Types of Stimuli That Trigger Hormone Secretion
Humoral Stimuli
Changes in extracellular fluid composition. Example: falling blood calcium → PTH released → calcium rises → PTH stops.
Hormonal Stimuli
Changes in circulating hormone levels. One hormone triggers another. Example: hypothalamus → anterior pituitary → thyroid.
Neural Stimuli
Neurotransmitter arrives at a neuroglandular junction. Example: sympathetic nerves → adrenal medulla → epinephrine.
The Hypothalamus — Highest Level of Endocrine Control
The hypothalamus links the nervous and endocrine systems. It controls endocrine activity in three ways:
The Hypophyseal Portal System
Regulatory hormones from the hypothalamus enter a capillary network near the infundibulum. These capillaries unite into portal veins that descend to the anterior pituitary and form a second capillary network. This portal system ensures all blood entering the portal vessels reaches anterior pituitary target cells before returning to general circulation—delivering regulatory hormones in concentrated form without systemic dilution.
Pituitary Gland (Hypophysis)
Location: Sella turcica of the sphenoid bone; hangs beneath the hypothalamus via the infundibulum. All nine hormones are peptides using cAMP as second messenger.
| Lobe | Hormone | Target | Effect |
|---|---|---|---|
| Anterior | TSH (thyrotropin) | Thyroid gland | Triggers thyroid hormone release. Controlled by TRH. |
| Anterior | ACTH (corticotropin) | Adrenal cortex | Stimulates glucocorticoid release. Controlled by CRH. |
| Anterior | FSH | Ovaries / Testes | Females: follicle/egg development, estrogen secretion. Males: sperm maturation via nurse cells. |
| Anterior | LH (ICSH in males) | Ovaries / Testes | Females: ovulation, corpus luteum, progestin secretion. Males: testosterone from interstitial cells. |
| Anterior | PRL (prolactin) | Mammary glands | Mammary gland development and milk production. |
| Anterior | GH (somatotropin) | All cells | Cell growth and protein synthesis. Liver releases somatomedins (IGFs). Also: glucose-sparing effect via lipid mobilization. |
| Anterior | MSH | Melanocytes | Increases melanin production. Virtually nonfunctional in healthy adults; active in fetal development, young children, pregnant women. |
| Posterior | ADH (vasopressin) | Kidneys | Decreases water loss in urine; causes vasoconstriction. Released when blood osmolarity rises or blood volume/pressure falls. Inhibited by alcohol. |
| Posterior | OXT (oxytocin) | Uterus, mammary glands; sperm duct, prostate | Females: uterine contractions during labor, milk let-down reflex. Males: smooth muscle contractions in sperm duct and prostate during emission. |
Thyroid Gland
Location: Anterior to trachea, just inferior to thyroid cartilage. Two lobes connected by the isthmus. Contains thyroid follicles—spheres lined by simple cuboidal epithelium with colloid storing hormones (more than a week’s supply). Requires iodine for hormone synthesis.
| Cell Type | Hormone | Effect |
|---|---|---|
| Follicular epithelial cells | Thyroxine (T4) — 90% of secretions; Triiodothyronine (T3) — more potent | Calorigenic effect: increases metabolic rate and oxygen consumption → more heat. Essential for normal skeletal, muscular, and nervous system development in children. |
| C cells (parafollicular) | Calcitonin (CT) | Lowers blood calcium: inhibits osteoclasts, stimulates calcium excretion by kidneys. Most important during childhood. Control is independent of hypothalamus/pituitary. |
Parathyroid Glands
Location: Two tiny pairs embedded in the posterior surfaces of the thyroid gland.
| Cell Type | Hormone | Effect |
|---|---|---|
| Chief cells | Parathyroid hormone (PTH) | Raises blood calcium — OPPOSITE of calcitonin: stimulates osteoclasts, inhibits osteoblasts, reduces urinary calcium excretion. Also stimulates kidneys to produce calcitriol → promotes calcium absorption from digestive tract. |
Adrenal Glands
Location: Yellow, pyramid-shaped glands on the superior border of each kidney. Two distinct regions:
| Region / Zone | Hormones | Effect |
|---|---|---|
| Cortex — Zona glomerulosa (outer) | Mineralocorticoids (aldosterone) | Retains sodium and water; promotes potassium excretion. Increases blood volume/pressure. Triggered by low Na+, low blood volume, high K+, or angiotensin II. |
| Cortex — Zona fasciculata (middle) | Glucocorticoids (cortisol, corticosterone, cortisone) | Speeds glucose synthesis and glycogen formation in liver. Skeletal muscle releases amino acids; adipose releases fatty acids. Glucose-sparing effect. Anti-inflammatory. Controlled by ACTH. |
| Cortex — Zona reticularis (inner) | Androgens | Small quantities; significance unclear in normal amounts. |
| Medulla | Epinephrine (75–80%) and Norepinephrine | Mobilizes glycogen and fat reserves; increases glucose availability and muscle ATP; increases heart rate and force. Effects peak at 30 seconds, linger for minutes. Controlled by sympathetic innervation. |
Pineal Gland
Location: Posterior portion of the roof of the third ventricle.
Melatonin — lowest during daylight, highest at night. Three proposed functions: (1) inhibits reproductive function (slows maturation of sperm, ova, reproductive organs); (2) antioxidant protecting CNS neurons from free radicals; (3) establishes circadian rhythms. Increased melatonin in winter darkness may cause seasonal affective disorder (SAD).
Pancreas
Location: J-shaped loop between stomach and proximal small intestine. Endocrine cells form pancreatic islets (islets of Langerhans)—only ~1% of all pancreatic cells.
| Cell Type | Hormone | Released When | Effect |
|---|---|---|---|
| Beta cells | Insulin | Blood glucose RISES | Stimulates glucose transport into cells; promotes glucose utilization, amino acid absorption, protein synthesis, triglyceride synthesis, glycogen formation. Exceptions: neurons, RBCs, kidney tubule epithelium, and intestinal epithelium lack insulin receptors. |
| Alpha cells | Glucagon | Blood glucose FALLS | Glycogen → glucose (liver/muscle); fat → fatty acids (adipose); amino acids → glucose (liver). All actions raise blood glucose. |
Normal blood glucose: 70–110 mg/dL. Parasympathetic stimulation enhances insulin release; sympathetic stimulation inhibits insulin and promotes glucagon.
Kidneys, Heart, Thymus, Gonads, Adipose
| Organ | Hormone(s) | Trigger | Effect |
|---|---|---|---|
| Kidneys | EPO (erythropoietin) | Low oxygen in kidney tissue | Stimulates red blood cell production by red bone marrow → increases blood volume and oxygen delivery. |
| Kidneys | Calcitriol (steroid) | PTH; requires vitamin D3 | Stimulates Ca2+ and PO43− absorption from digestive tract. Vitamin D3 synthesized in skin or from diet → liver → kidneys → calcitriol. |
| Kidneys | Renin (enzyme) | Declining blood volume/pressure | Starts renin-angiotensin system → angiotensin II → stimulates aldosterone and ADH → restricts salt/water loss → raises blood pressure and stimulates thirst. |
| Heart | ANP (atrial natriuretic peptide) | Excessive blood volume stretches right atrium | Opposes angiotensin II: promotes sodium and water loss at kidneys; inhibits renin, ADH, and aldosterone. Reduces blood volume and pressure. |
| Thymus | Thymosins | Ongoing | Development and maintenance of normal immune defenses. Max size (~40g) just before puberty; atrophies thereafter. By age 50: <12g. |
| Testes (interstitial cells) | Androgens (testosterone) | LH (ICSH) stimulation | Sperm production, male reproductive tract maintenance, secondary sex characteristics, protein synthesis, muscle growth, CNS development. |
| Testes (nurse cells) | Inhibin | FSH stimulation | Inhibits FSH secretion — negative feedback maintaining normal sperm production. |
| Ovaries (follicle cells) | Estrogens; Inhibin | FSH stimulation | Estrogens: egg maturation, uterine lining growth, female secondary sex characteristics. Inhibin: suppresses FSH. |
| Ovaries (corpus luteum) | Estrogens + Progestins (progesterone) | After ovulation | Progesterone: accelerates fertilized egg movement through uterine tubes; prepares uterus for embryo implantation; mammary gland enlargement. |
| Adipose tissue | Leptin (peptide) | After eating | Binds hypothalamic neurons → satiation, appetite suppression. Must be present for normal GnRH and gonadotropin levels — very low body fat disrupts menstruation and fertility. |
Extracellular fluids contain a mixture of hormones whose concentrations change daily and hourly. When a cell receives instructions from two hormones simultaneously, four outcomes are possible:
| Interaction Type | Definition | Example |
|---|---|---|
| Antagonistic | Two hormones have opposing effects; net result is the balance between them. | PTH vs calcitonin (blood calcium). Insulin vs glucagon (blood glucose). |
| Synergistic (Additive) | Two hormones working together produce a result greater than either alone—sometimes greater than the sum of individual effects. | GH + glucocorticoids → glucose-sparing effect greater than either hormone alone. |
| Permissive | One hormone must be present for a second to produce its effects. The first does not produce the effect itself—it enables the second. | Epinephrine has no apparent effect on energy consumption UNLESS thyroid hormones are present at normal concentrations. |
| Integrative | Two hormones produce different but complementary results in a tissue or organ—coordinating diverse physiological systems. | Calcitriol promotes Ca2+ absorption from the gut; PTH promotes Ca2+ release from bone. Different actions, same goal: maintain calcium homeostasis. |
Normal growth requires cooperation among several endocrine organs. Six hormones are especially important:
| Hormone | Role in Growth | Consequence of Absence |
|---|---|---|
| Growth Hormone (GH) | Stimulates cell growth and protein synthesis, especially in children. Supports muscular and skeletal development. Acts primarily via somatomedins (IGFs) released from the liver. In adults: maintains blood glucose and mobilizes lipid reserves. | Undersecretion → pituitary dwarfism. Oversecretion before puberty → gigantism. Oversecretion after puberty → acromegaly. An adult with GH deficiency but normal thyroxine, insulin, and glucocorticoids will have no significant physiological problems. |
| Thyroid Hormones | Required for normal growth throughout development. | Absent in first year → nervous system fails to develop normally → mental retardation (cretinism). Declining before puberty → skeletal development stops. |
| Insulin | Enables passage of glucose and amino acids across plasma membranes. Growing cells need both fuel and building materials. | Without insulin → cells cannot access nutrients → growth fails regardless of other hormone levels. |
| PTH and Calcitriol | Promote calcium absorption for building bone matrix. | Without adequate levels → bones enlarge but are poorly mineralized, weak, flexible. In children: rickets—limb bones bend under body weight. |
| Reproductive Hormones | Androgens and estrogens affect osteoblast activity, drive sex-specific growth spurts at puberty, and cause closure of epiphyseal cartilages (ending further height increase). Androgens stimulate protein synthesis and muscle growth in males. | Sex-related differences in skeletal proportions and secondary sex characteristics arise from different target profiles of androgens vs estrogens. |
Definition of Stress
Any condition that threatens homeostasis: (1) physical (illness, injury); (2) emotional (depression, anxiety); (3) environmental (extreme heat/cold); (4) metabolic (acute starvation).
Phase 1 — Alarm Phase (“Fight or Flight”)
Dominant hormone: Epinephrine (adrenal medulla), driven by general sympathetic activation. Immediate, short-term response.
- Increases mental alertness and energy use by all cells
- Mobilizes glycogen and lipid reserves
- Changes circulation patterns (shunts blood to active muscles, heart, brain)
- Reduces digestive activity and urine production
- Increases sweat gland secretion, heart rate, and respiratory rate
Phase 2 — Resistance Phase
Dominant hormones: Glucocorticoids (cortisol, corticosterone). Begins if stress lasts longer than a few hours. Lipid reserves sufficient for weeks to months.
- Mobilizes remaining energy reserves: lipids from adipose, amino acids from skeletal muscle
- Conserves glucose: peripheral tissues (except neural) break down lipids for energy
- Elevates blood glucose: liver synthesizes glucose from amino acids, lipids, other carbohydrates
- Maintains blood volume: conservation of salts and water; loss of potassium and hydrogen ions
Also involved: growth hormone, glucagon, mineralocorticoids (with ADH), and the renin-angiotensin system.
Phase 3 — Exhaustion Phase
When resistance ends, homeostatic regulation breaks down. Without immediate corrective actions, failure of one or more organ systems is fatal. Signs of collapse:
- Exhaustion of lipid reserves
- Cumulative structural or functional damage to vital organs
- Inability to produce glucocorticoids
- Failure of electrolyte balance
Endocrine disorders fall into two categories: inadequate hormonal effects (hyposecretion, or normal secretion but defective receptor) and excessive hormonal effects (hypersecretion, or normal secretion but hypersensitive receptor).
| Disorder | Cause | Key Features |
|---|---|---|
| Diabetes Insipidus | Posterior pituitary fails to release adequate ADH, OR kidneys fail to respond to ADH. | Massive urine output (polyuria); constant thirst (polydipsia). Severe cases: up to 10 L/day → fatal dehydration. Urine is dilute (insipidus = tasteless). |
| Diabetes Mellitus | Type 1: Inadequate insulin production (beta cells destroyed). Type 2: Normal/elevated insulin but peripheral cells no longer respond (insulin resistance). | Blood glucose overwhelms kidney reabsorption → glucose in urine (glycosuria) → excessive urine (polyuria). Type 1: juvenile-onset, requires insulin therapy. Type 2: maturity-onset, associated with obesity, managed with diet/weight loss/oral agents. |
| Acromegaly | GH overproduction AFTER puberty (epiphyseal cartilages fused). | Cartilaginous areas and bone shapes enlarge. Broad facial features, enlarged lower jaw, hands, feet. Cannot grow taller. |
| Pituitary Dwarfism | GH undersecretion before puberty. | Bones do not elongate normally → short stature with normal proportions. |
| Gigantism | GH oversecretion BEFORE puberty (epiphyseal cartilages still open). | Bones elongate dramatically → abnormally tall stature. |
| Cretinism | Thyroid hormone insufficiency in infancy. | Inadequate skeletal and nervous system development; metabolic rate up to 40% below normal; mental retardation if untreated. |
| Goiter | Inadequate dietary iodine → cannot synthesize T3/T4 → TSH stimulation continues → follicles distend. | Enlarged thyroid gland; can interfere with breathing and swallowing. |
| Addison Disease | Hyposecretion of corticosteroids (especially glucocorticoids). | Cannot mobilize energy reserves or maintain blood glucose. Skin pigment changes: elevated ACTH (structurally similar to MSH) stimulates melanocytes. |
| Cushing Disease | Hypersecretion of glucocorticoids. | Excessive lipid mobilization and redistribution: adipose accumulates in cheeks (“moon face”) and base of neck (“buffalo hump”). Protein breakdown, muscle wasting. |
| Myxedema | Hyposecretion of thyroid hormones in adults. | Subcutaneous swelling, hair loss, dry skin, low body temperature, muscle weakness, slowed reflexes. |
| Hypogonadism | Abnormally low gonadotropin production (FSH and LH). | Children will not undergo sexual maturation. Adults cannot produce functional sperm or ova. |
Hormones and Behavior
Brain regions that affect behavior are sensitive to hormonal stimulation. Even normal changes in circulating hormone levels produce behavioral changes.
Precocious (premature) puberty—sex hormones produced as early as age 5 or 6: the child develops adult secondary sex characteristics AND becomes aggressive and assertive. This demonstrates that teenage behaviors attributed to peer pressure can have a direct physiological basis in sex hormones acting on the CNS.
In adults, changes in circulating hormone mixture can significantly affect intellectual capabilities, memory, learning, and emotional states.
Seasonal Affective Disorder (SAD)—increased melatonin secretion during prolonged winter darkness at high latitudes. Characterized by changes in mood, eating habits, and sleeping patterns.
Hormones and Aging
The endocrine system undergoes relatively few functional changes with age compared to other systems.
| Change | Detail |
|---|---|
| Most dramatic decline | Concentration of reproductive hormones (testosterone, estrogens, progestins). Drives most age-related skeletal and reproductive changes. |
| Unchanged with age | TSH, thyroid hormones, ADH, PTH, prolactin, and glucocorticoids remain within normal limits. |
| Reduced tissue responsiveness | Some tissues become less responsive to stimulation. Elderly secrete less GH and insulin after a carbohydrate-rich meal. Peripheral tissues become less responsive to glucocorticoids and ADH. |
| Downstream consequences | Reduced GH and other tropic hormones → reductions in bone density and muscle mass (sarcopenia, osteoporosis). |
The endocrine system provides long-term regulation and adjustment of homeostatic mechanisms that affect all body functions. For every organ system, the endocrine system adjusts metabolic rates, substrate utilization, and regulates growth and development.
| System | Endocrine → That System | That System → Endocrine |
|---|---|---|
| Integumentary | Sex hormones stimulate sebaceous glands, influence hair growth, fat distribution, and apocrine sweat glands. PRL stimulates mammary gland development. Adrenal hormones alter dermal blood flow. MSH stimulates melanocyte activity. | Protects superficial endocrine organs. Epidermis synthesizes vitamin D3 (precursor to calcitriol). |
| Skeletal | Skeletal growth regulated by GH, thyroid hormones, and reproductive hormones. Calcium mobilization regulated by PTH and calcitonin. Sex hormones speed growth at puberty, close epiphyseal cartilages, and help maintain bone mass in adults. | Protects endocrine organs in brain (cranium), chest (rib cage), and pelvic cavity (pelvic girdle). |
| Muscular | Hormones adjust muscle metabolism, energy production, and growth. Regulate calcium and phosphate levels. GH and testosterone speed skeletal muscle growth. | Skeletal muscles provide mechanical protection for some endocrine organs. |
| Nervous | Hormones affect neural metabolism and brain development. Regulate fluid and electrolyte balance. Reproductive hormones influence CNS development and behaviors. | Hypothalamus directly controls pituitary secretions and indirectly controls other endocrine organs. Controls adrenal medullae via sympathetic innervation. Secretes ADH and oxytocin. |
| Cardiovascular | Epinephrine, NE, and other hormones increase cardiac output and blood pressure. Aldosterone and ADH regulate blood volume. ANP reduces blood volume and pressure. | Bloodstream distributes all hormones to their target cells. Heart produces ANP. |
| Lymphatic / Immune | Glucocorticoids have anti-inflammatory effects; suppress immune responses. Thymosins develop and maintain immune defenses. | Thymus is itself an endocrine organ (thymosins). |
| Reproductive | LH and FSH from anterior pituitary drive gonadal function. Testosterone, estrogens, and progestins govern all aspects of reproductive anatomy, function, and behavior. | Testes and ovaries are endocrine organs producing androgens, estrogens, progestins, and inhibin. During pregnancy, the placenta functions as a temporary endocrine organ. |