Chemical Coordination and Integration — NEET notes
Class 11 Biology · NCERT chapter 19 · 6 topics · free to read
Endocrine Glands & Hormones — The chemical messaging system
Endocrine glands are ductless glands that secrete hormones directly into the blood. Hormones are classified into peptide, steroid, and amino acid-derived types. They act on specific target organs via receptors — either on the cell membrane (water-soluble hormones, via second messengers like cAMP) or intracellularly (lipid-soluble steroids and thyroid hormones, acting directly on DNA).
Endocrine gland hormone via blood Target organ (specific receptor)
Endocrine vs Exocrine
Endocrine glands lack ducts and release hormones directly into the blood. Exocrine glands use ducts to deliver secretions to specific sites (e.g., salivary glands, sweat glands). Some organs like the pancreas are both — they're called heterocrine or mixed glands.
Your body has two types of glands: exocrine (with ducts — they deliver their products through tubes to a specific location) and endocrine (ductless — they dump hormones directly into the blood, which carries them everywhere). Think of exocrine glands as sending letters by courier (specific address), and endocrine glands as broadcasting on radio (message goes everywhere, only tuned-in receivers respond).
Ductless
Have ducts
Secrete hormones into blood
Secrete enzymes/other products into ducts
Act on distant target organs
Act locally at the site of secretion
E.g., pituitary, thyroid, adrenal
E.g., salivary, sweat, sebaceous glands
Chemical messengers = hormones
Products = enzymes, mucus, sweat, sebum
Pancreas is the classic heterocrine (mixed) gland — endocrine part (Islets of Langerhans → insulin, glucagon) + exocrine part (acinar cells → pancreatic juice with digestive enzymes)
Liver is the largest exocrine gland (bile secretion via bile duct)
Hormones act in very small quantities — even nanogram levels produce dramatic effects
The study of endocrine glands and hormones is called endocrinology
Types of Hormones
Hormones are chemically classified into three groups: peptide/protein hormones (insulin, GH, FSH), steroid hormones (cortisol, testosterone, estrogen — all derived from cholesterol), and amino acid-derived hormones (thyroid hormones T₃/T₄ from tyrosine, adrenaline from tyrosine).
Not all hormones are made of the same stuff. Their chemical nature determines everything — how they travel in blood, whether they can cross the cell membrane, and how they signal target cells.
Steroid hormones — derived from cholesterol. Lipid-soluble, CAN cross cell membrane. Examples: cortisol, aldosterone, testosterone, estrogen, progesterone
Amino acid-derived (amine) hormones — modified single amino acids (usually tyrosine). Examples: T₃ and T₄ (iodinated tyrosine derivatives), adrenaline and noradrenaline (catecholamines from tyrosine), melatonin (from tryptophan)
Steroid hormones travel in blood bound to carrier proteins (they're hydrophobic — can't dissolve freely in watery blood)
Peptide hormones dissolve freely in blood but can't enter cells — they need membrane receptors
Mechanism of Hormone Action
Water-soluble hormones (peptides, amines) bind membrane receptors and use second messengers (cAMP, IP₃, Ca²⁺) to relay signals inside the cell. Lipid-soluble hormones (steroids, T₃/T₄) enter the cell, bind intracellular/nuclear receptors, and directly regulate gene expression.
The golden rule: if the hormone can't enter the cell, it sends a message through the door (membrane receptor + second messenger). If it can enter, it walks right in and talks to the DNA directly. This divides hormones into two clean categories based on their mechanism.
Water-soluble hormones (peptides, catecholamines)
Lipid-soluble hormones (steroids, T₃/T₄)
Cannot cross lipid bilayer
Freely cross cell membrane
Bind receptor ON cell surface
Bind receptor INSIDE cell (cytoplasm/nucleus)
Generate second messengers (cAMP, IP₃, Ca²⁺)
Hormone-receptor complex binds DNA directly
Fast response (seconds to minutes)
Slow but long-lasting response (hours to days)
Example: adrenaline → cAMP cascade
Example: testosterone → gene activation
Second messenger concept — the hormone is the FIRST messenger (arrives at cell surface). It activates a second messenger inside the cell that actually does the work
cAMP (cyclic AMP) is the most famous second messenger. Hormone binds receptor → activates adenylate cyclase → converts ATP to cAMP → cAMP activates protein kinases → cellular response
Other second messengers: IP₃ (inositol triphosphate), DAG, Ca²⁺ ions
Steroid hormones + receptor complex acts as a transcription factor — it binds to specific DNA sequences and turns genes ON or OFF
T₃/T₄ are amine hormones that behave like steroids — they're lipid-soluble enough to cross membranes and bind nuclear receptors
Quick recall
Adrenaline triggers the fight-or-flight response within seconds. Does it use membrane receptors or intracellular receptors?
Membrane receptors with cAMP as second messenger. Adrenaline is a catecholamine (water-soluble amine) — it cannot cross the membrane. It binds to adrenergic receptors on the cell surface → activates adenylate cyclase → cAMP cascade → rapid glycogen breakdown, increased heart rate, etc.
Hypothalamus & Pituitary — The command center that controls all other glands
The hypothalamus links the nervous system to the endocrine system. It secretes releasing and inhibiting hormones that control the anterior pituitary (adenohypophysis), which secretes GH, TSH, ACTH, FSH, LH, and PRL. The posterior pituitary (neurohypophysis) stores and releases hypothalamic hormones ADH (antidiuretic) and oxytocin. The pituitary is called the 'master gland' but the hypothalamus is the real boss.
The hypothalamus is a small region at the base of the brain (part of the diencephalon). It contains neurosecretory cells that produce releasing hormones (RH) and inhibiting hormones (IH), which travel via the hypophyseal portal system to control the anterior pituitary.
The hypothalamus is the supreme commander of the endocrine system. It sits at the base of the brain and acts as a bridge between the nervous system (which processes information fast via electrical signals) and the endocrine system (which sends slower chemical messages via hormones). It contains specialized neurons called neurosecretory cells that can do something remarkable — they're neurons that secrete hormones.
Neurosecretory cells in the hypothalamus produce two types of hormones: Releasing Hormones (RH) that stimulate the anterior pituitary, and Inhibiting Hormones (IH) that suppress it
GnRH (Gonadotropin-Releasing Hormone) → stimulates FSH and LH release
TRH (Thyrotropin-Releasing Hormone) → stimulates TSH release
These hormones reach the anterior pituitary via the hypophyseal portal system — a special set of blood vessels connecting hypothalamus to pituitary
Anterior Pituitary (Adenohypophysis)
The anterior pituitary produces and secretes 6 key hormones: GH (growth), TSH (thyroid), ACTH (adrenal cortex), FSH (follicle/sperm), LH (ovulation/testosterone), and PRL (milk production). Most are tropic hormones — they control other endocrine glands.
The anterior pituitary (adenohypophysis) is the front lobe of the pituitary gland. Unlike the posterior pituitary, it actually synthesizes its own hormones (it doesn't just store hypothalamic hormones). It produces six major hormones, most of which are tropic hormones — meaning they control other endocrine glands.
GH (Growth Hormone / Somatotropin) — Stimulates growth of bones and tissues. Over-secretion in childhood → Gigantism. Over-secretion in adults → Acromegaly (enlarged jaw, hands, feet). Under-secretion in childhood → Pituitary dwarfism
TSH (Thyroid-Stimulating Hormone / Thyrotropin) — Stimulates the thyroid to produce T₃ and T₄. Controlled by TRH from hypothalamus
ACTH (Adrenocorticotropic Hormone) — Stimulates the adrenal cortex to secrete cortisol and other glucocorticoids. Controlled by CRH
FSH (Follicle-Stimulating Hormone) — In females: stimulates ovarian follicle development and estrogen production. In males: stimulates Sertoli cells → spermatogenesis
LH (Luteinizing Hormone) — In females: triggers ovulation, forms corpus luteum, stimulates progesterone. In males: stimulates Leydig cells → testosterone production
PRL (Prolactin) — Stimulates mammary gland development and milk production after childbirth. Unique: it's the only anterior pituitary hormone regulated mainly by INHIBITION (PIF/dopamine)
Quick recall
A patient has enlarged hands, feet, and protruding jaw but normal height. What's the condition and which hormone is involved?
Acromegaly — caused by excess GH (Growth Hormone) secretion in adulthood. Since growth plates have already fused in adults, bones can't grow longer — instead, they thicken at the extremities (hands, feet, jaw, brow ridge).
Posterior Pituitary (Neurohypophysis)
The posterior pituitary does NOT synthesize hormones — it stores and releases two hormones made by hypothalamic neurons: ADH (antidiuretic hormone / vasopressin) which promotes water reabsorption in kidneys, and oxytocin which stimulates uterine contractions during childbirth and milk ejection.
The posterior pituitary (neurohypophysis) is fundamentally different from the anterior. It's not really a gland — it's more like a warehouse. It doesn't make any hormones. Instead, neurons in the hypothalamus synthesize ADH and oxytocin, package them into vesicles, and send them down long axons into the posterior pituitary where they're stored until needed.
ADH (Antidiuretic Hormone / Vasopressin) — Acts on kidney collecting ducts to promote water reabsorption → produces concentrated urine, prevents water loss. Also causes vasoconstriction (hence the name vasopressin)
Deficiency of ADH → Diabetes Insipidus — massive water loss through dilute urine (patient produces up to 20 L/day of very dilute urine). NOT related to blood sugar — completely different from diabetes mellitus
Oxytocin — Stimulates uterine smooth muscle contraction during labor (childbirth). Also triggers milk ejection (let-down reflex) from mammary glands during breastfeeding
Oxytocin works by positive feedback — contractions trigger more oxytocin release → stronger contractions → more oxytocin. This continues until delivery is complete
Both hormones are nonapeptides (9 amino acids each) and are structurally very similar
Thyroid & Parathyroid — Metabolism master and calcium commander
The thyroid gland produces T₃ (triiodothyronine) and T₄ (thyroxine) from iodine and tyrosine — they regulate BMR, growth, and development. Calcitonin from thyroid C-cells lowers blood calcium. The four parathyroid glands secrete PTH which raises blood calcium. Together, calcitonin and PTH maintain calcium homeostasis. Disorders include goitre, cretinism, myxedema, Graves' disease, and hypo/hyperparathyroidism.
PTH Ca^ 2+ | Calcitonin (TCT) Ca^ 2+
Thyroid Hormones (T₃ & T₄)
The thyroid gland is a butterfly-shaped gland in the neck. Its follicular cells produce T₃ and T₄ (both need iodine). T₃ is more active but T₄ is produced in greater quantity. They regulate BMR, protein synthesis, growth, development, and body temperature.
The thyroid gland sits in the front of the neck, just below the larynx. It's shaped like a butterfly with two lobes connected by an isthmus. It's composed of follicles — spherical structures lined by follicular cells that produce T₃ and T₄, with colloid (thyroglobulin) stored inside.
T₄ (Thyroxine / Tetraiodothyronine) — contains 4 iodine atoms. Major secretory product (~90% of thyroid output). Less active than T₃. Gets converted to T₃ in target tissues
T₃ (Triiodothyronine) — contains 3 iodine atoms. Only ~10% of thyroid output BUT 5-10x more biologically active than T₄
Both require iodine for synthesis — dietary iodine is trapped by thyroid follicular cells (iodine pump/trap)
TSH from anterior pituitary stimulates T₃/T₄ production. T₃/T₄ inhibit TSH via negative feedback
T₃/T₄ regulate BMR (basal metabolic rate), protein synthesis, carbohydrate metabolism, growth and development of brain and bones, body temperature regulation
T₃/T₄ are amino acid derivatives (from tyrosine + iodine) but behave like steroids — they're lipid-soluble and bind intracellular nuclear receptors
Thyroid Disorders
Hypothyroidism causes different problems at different ages: cretinism in infants, myxedema in adults. Hyperthyroidism includes Graves' disease (autoimmune). Goitre (enlarged thyroid) can occur in both hypo and hyper states. Iodine deficiency is the most common cause of endemic goitre.
Thyroid disorders are NEET gold — they appear almost every year. The key is knowing that hypothyroidism causes different diseases at different life stages, and goitre can appear in BOTH hypo and hyper conditions.
Simple/Endemic Goitre — enlarged thyroid due to iodine deficiency. Without iodine, T₃/T₄ can't be made → no negative feedback → TSH stays high → thyroid keeps growing. Common in hilly/landlocked areas far from sea
Cretinism — hypothyroidism in infants/children. Stunted growth, mental retardation, delayed sexual maturity, pot belly, protruding tongue. IRREVERSIBLE if not treated early
Myxedema — hypothyroidism in adults. Puffy face, sluggishness, low BMR, weight gain, cold intolerance, dry skin, slow heart rate. Treatable with thyroid hormone replacement
Hashimoto's thyroiditis — autoimmune hypothyroidism (immune system destroys thyroid tissue). Most common cause of hypothyroidism in iodine-sufficient areas
A child in a remote mountain village shows stunted growth, mental retardation, and a swollen neck. What's the condition and cause?
Cretinism caused by iodine deficiency (leading to hypothyroidism in infancy). The swollen neck is endemic goitre. Mountain regions lack seafood → dietary iodine deficiency → thyroid can't make enough T₃/T₄ → critical for brain and body development in children.
Calcitonin & PTH
Calcitonin (from thyroid C-cells/parafollicular cells) lowers blood Ca²⁺ by promoting calcium deposition in bones. PTH (from parathyroid glands) raises blood Ca²⁺ by stimulating bone resorption, renal reabsorption, and intestinal absorption (via vitamin D activation). They are antagonistic.
Blood calcium must stay in a narrow range (~9-11 mg/dL). Too low → muscle spasms and tetany. Too high → kidney stones and cardiac problems. Two hormones work as a seesaw to keep calcium perfectly balanced: calcitonin pushes it DOWN, PTH pushes it UP.
Calcitonin (TCT — Thyrocalcitonin) — Secreted by parafollicular cells (C-cells) of the thyroid (NOT follicular cells). Lowers blood Ca²⁺ by promoting calcium deposition into bones and inhibiting bone resorption
PTH (Parathormone) — Secreted by the 4 parathyroid glands (small, pea-sized, embedded on the posterior surface of thyroid). Raises blood Ca²⁺ through THREE mechanisms
PTH mechanism 1: Bone resorption — stimulates osteoclasts to break down bone → releases Ca²⁺ into blood
PTH mechanism 2: Renal reabsorption — promotes Ca²⁺ reabsorption from kidney tubules (less calcium lost in urine)
PTH mechanism 3: Intestinal absorption — activates vitamin D (→ calcitriol) in kidneys, which then increases Ca²⁺ absorption from the gut
Hypoparathyroidism → low Ca²⁺ → tetany (sustained muscle contraction/spasms due to increased nerve excitability)
Hyperparathyroidism → high Ca²⁺ → osteoporosis (excessive bone resorption), kidney stones, fatigue
Adrenal Glands — Stress responders sitting atop your kidneys
Each kidney has one adrenal (suprarenal) gland with two distinct regions. The outer adrenal cortex has three zones producing mineralocorticoids (aldosterone — Na⁺/K⁺ balance), glucocorticoids (cortisol — stress/metabolism), and sex steroids (androgens). The inner adrenal medulla produces catecholamines (adrenaline and noradrenaline) for the fight-or-flight response. Disorders: Addison's disease (cortex hypo), Cushing's syndrome (cortisol excess).
The adrenal cortex has three layers: zona glomerulosa (mineralocorticoids — aldosterone), zona fasciculata (glucocorticoids — cortisol), and zona reticularis (sex steroids — androgens). All cortical hormones are steroids derived from cholesterol.
The adrenal cortex makes up about 80% of the adrenal gland and is organized into three distinct zones, each producing a different class of steroid hormones. All adrenal cortical hormones are steroids — synthesized from cholesterol.
Zona Glomerulosa (outermost) → Mineralocorticoids, mainly Aldosterone. Regulates Na⁺ and K⁺ balance. Promotes Na⁺ reabsorption and K⁺ excretion in kidney tubules → water follows Na⁺ → increases blood volume and BP
Zona Fasciculata (middle, thickest) → Glucocorticoids, mainly Cortisol. The 'stress hormone.' Stimulates gluconeogenesis (making glucose from non-carb sources), suppresses immune response, anti-inflammatory, protein and fat catabolism
Zona Reticularis (innermost) → Sex steroids (Androgens) — small amounts of androgens (like DHEA). In females, adrenal androgens are the main source of androgens. In males, testes produce the bulk
ACTH from anterior pituitary primarily controls zona fasciculata and reticularis. Aldosterone secretion is mainly regulated by the Renin-Angiotensin-Aldosterone System (RAAS) and blood K⁺ levels
All three zones produce steroid hormones from cholesterol — they bind intracellular receptors and regulate gene expression
Adrenal Medulla
The adrenal medulla is the inner part of the adrenal gland. It's essentially modified nervous tissue (derived from neural crest cells). It produces catecholamines — adrenaline (epinephrine, ~80%) and noradrenaline (norepinephrine, ~20%) — which mediate the fight-or-flight response.
The adrenal medulla is unlike any other endocrine gland — it's actually modified neural tissue. The cells of the medulla (called chromaffin cells) are essentially modified postganglionic sympathetic neurons that lost their axons and became secretory. They're directly innervated by preganglionic sympathetic fibers.
Adrenaline (Epinephrine) — ~80% of medullary secretion. The classic 'fight-or-flight' hormone. Called the 'emergency hormone' or '3F hormone' (fight, fright, flight)
Noradrenaline (Norepinephrine) — ~20% of secretion. Also involved in emergency response. More potent vasoconstrictor than adrenaline
Both are catecholamines — amino acid derivatives (from tyrosine). Water-soluble → act via membrane receptors → cAMP second messenger pathway
Effects: Increased heart rate, increased BP, dilated pupils, bronchodilation (more air intake), glycogenolysis (glucose released from liver), blood diverted from gut to skeletal muscles, sweating, hair stands up (piloerection)
Medullary hormones are NOT essential for survival (sympathetic nervous system can compensate), unlike cortical hormones which ARE essential
Secretion is controlled by sympathetic nervous system (not by ACTH or any pituitary hormone)
Adrenal disorders are defined by which hormone is too much or too little. Since the cortex makes three types of hormones, disorders can be quite varied — but NCERT and NEET focus mainly on Addison's and Cushing's.
Addison's bronze skin explanation: Low cortisol → no negative feedback → pituitary produces excess ACTH. ACTH is made from the same precursor (POMC) as MSH (melanocyte-stimulating hormone) → excess MSH → darkening of skin
Pheochromocytoma — Tumor of adrenal medulla → excess adrenaline/noradrenaline → episodic severe hypertension, rapid heart rate, sweating, anxiety
Quick recall
A patient has moon face, central obesity, thin skin, and high blood sugar. What condition is this?
Cushing's Syndrome — excess cortisol. Moon face and buffalo hump are due to abnormal fat redistribution. High blood sugar = cortisol promotes gluconeogenesis. Thin skin = cortisol breaks down proteins including collagen.
Pancreas & Gonads — Blood sugar control and reproductive hormones
The endocrine pancreas (Islets of Langerhans) has alpha cells (glucagon — raises blood glucose), beta cells (insulin — lowers blood glucose), and delta cells (somatostatin). Insulin and glucagon maintain glucose homeostasis. Diabetes mellitus results from insulin deficiency (Type 1) or insulin resistance (Type 2). Testes produce testosterone (Leydig cells), ovaries produce estrogen (follicular cells) and progesterone (corpus luteum).
Insulin Blood Glucose | Glucagon Blood Glucose
Islets of Langerhans
About 1-2% of the pancreas is endocrine tissue organized as Islets of Langerhans. Alpha cells (~20%) secrete glucagon, beta cells (~70%) secrete insulin, and delta cells secrete somatostatin. These three cell types work together to maintain blood glucose within a narrow range.
The pancreas is the classic mixed gland — 98% of it is exocrine (acinar cells making digestive enzymes), while the remaining 1-2% forms tiny clusters called Islets of Langerhans that function as endocrine tissue. There are about 1-2 million islets scattered throughout the pancreas.
Alpha (α) cells (~20%) → Secrete Glucagon — raises blood glucose by stimulating glycogenolysis (glycogen → glucose in liver) and gluconeogenesis (making new glucose from amino acids/fats)
Beta (β) cells (~70%) → Secrete Insulin — the ONLY hormone that lowers blood glucose. Promotes glucose uptake by cells, glycogenesis (glucose → glycogen), lipogenesis (glucose → fat), and protein synthesis
Delta (δ) cells → Secrete Somatostatin — inhibits both insulin and glucagon secretion (local regulatory role). Same as GHIH from hypothalamus
Insulin is the only hypoglycemic hormone (lowers glucose). Multiple hormones raise it (glucagon, cortisol, GH, adrenaline) — the body has many backup systems to prevent low glucose (brain needs it!)
Diabetes Mellitus
Diabetes mellitus is a metabolic disorder characterized by chronic hyperglycemia due to insulin deficiency or resistance. Type 1 (insulin-dependent, autoimmune destruction of beta cells, juvenile onset) vs Type 2 (non-insulin-dependent, insulin resistance, adult onset, most common). Symptoms: polyuria, polydipsia, polyphagia, glycosuria.
Diabetes mellitus literally means 'sweet urine' (mellitus = honey-sweet) — a condition known since ancient times when physicians would taste urine to diagnose it. It occurs when the body can't properly use glucose, causing it to accumulate in the blood and spill into urine.
Insulin-Dependent Diabetes Mellitus
Non-Insulin-Dependent Diabetes Mellitus
Autoimmune destruction of β-cells
Insulin resistance (cells don't respond to insulin)
Little or NO insulin produced
Insulin IS produced but target cells are resistant
Juvenile onset (usually < 30 years)
Adult onset (usually > 40 years, increasingly younger)
~5-10% of diabetes cases
~90-95% of diabetes cases (most common)
Treatment: insulin injections (mandatory)
Treatment: diet, exercise, oral drugs, sometimes insulin
Patients are usually lean
Patients are often overweight/obese
Classic triad of symptoms: Polyuria (excessive urination — glucose in urine pulls water osmotically), Polydipsia (excessive thirst — dehydration from polyuria), Polyphagia (excessive hunger — cells can't use glucose, so body 'thinks' it's starving)
Glycosuria — glucose in urine (normally kidneys reabsorb all glucose; when blood glucose exceeds ~180 mg/dL — the renal threshold — glucose spills into urine)
Diabetic ketoacidosis (DKA) — mainly in Type 1. Without insulin, body burns fat → ketone bodies accumulate → blood becomes acidic → can be fatal
Gestational diabetes — diabetes during pregnancy. Usually resolves after delivery but increases risk of Type 2 later
Gonadal Hormones
Testes have Leydig (interstitial) cells that produce testosterone under LH stimulation — responsible for male secondary sexual characters, spermatogenesis, and muscle development. Ovaries have follicular cells producing estrogen (female secondary sexual characters, follicle development) and corpus luteum producing progesterone (prepares uterus for pregnancy, maintains pregnancy).
The gonads are dual-purpose organs — they produce both gametes (reproductive function) and hormones (endocrine function). The hormones they produce are all steroids (derived from cholesterol), and they drive everything related to sexual development, reproduction, and secondary sexual characteristics.
Testosterone (from Leydig/interstitial cells of testes, stimulated by LH) — Male sex hormone. Promotes spermatogenesis, development of male secondary sexual characters (deep voice, facial hair, muscle mass, bone density, Adam's apple), male libido
Estrogen (mainly estradiol, from ovarian follicular/granulosa cells, stimulated by FSH) — Female sex hormone. Promotes development of female secondary sexual characters (breast development, fat distribution, wider hips), regulates menstrual cycle, ovarian follicle maturation
Progesterone (from corpus luteum after ovulation, stimulated by LH) — 'Pregnancy hormone.' Prepares endometrium for implantation, maintains pregnancy, inhibits uterine contractions during pregnancy, develops mammary glands for lactation
All three are steroid hormones → cross cell membrane → bind intracellular nuclear receptors → regulate gene expression
Androgens (including testosterone) in small amounts are produced by the adrenal cortex in BOTH males and females
FSH stimulates Sertoli cells (in males — support spermatogenesis) and granulosa cells (in females — estrogen production)
Quick recall
After ovulation, which structure in the ovary produces progesterone, and what triggers its formation?
The corpus luteum (formed from the ruptured Graafian follicle after ovulation). LH surge triggers ovulation and then transforms the remaining follicular cells into the corpus luteum, which secretes progesterone to prepare the uterine lining for possible implantation.
Other Endocrine Glands & Hormones — Pineal, thymus, GI, heart, and kidney
The pineal gland secretes melatonin (regulates circadian rhythm, inhibits reproductive function). The thymus secretes thymosin (T-cell maturation, immunity — degenerates with age). The GI tract produces several hormones: gastrin (HCl secretion), secretin (bicarbonate from pancreas), CCK (enzyme-rich pancreatic juice + bile), GIP (insulin stimulation). The heart secretes ANF (lowers blood pressure), and the kidney produces erythropoietin (RBC production).
ANF Blood Pressure | Erythropoietin RBC Production
Pineal Gland
The pineal gland is a small, pea-sized gland located on the dorsal side of the forebrain. It secretes melatonin, which regulates the circadian rhythm (sleep-wake cycle), and has inhibitory effects on reproductive function and pigmentation in some animals.
The pineal gland is a tiny cone-shaped structure in the brain, sometimes called the 'third eye' because in some lower vertebrates it actually has photoreceptor cells. In humans, it's the body's internal clock manager — producing melatonin in response to darkness.
Melatonin — the only significant hormone of the pineal gland. Derived from serotonin (which is derived from tryptophan)
Melatonin production is high in darkness, low in light. Light inhibits melatonin synthesis. This is why bright screens at night mess up your sleep — they suppress melatonin
Circadian rhythm regulation — melatonin signals the body that it's nighttime → promotes sleep, lowers body temperature, lowers alertness
Inhibits reproductive function — melatonin has anti-gonadotropic effects. In seasonal breeders (like sheep, deer), changing day length → changing melatonin → controls breeding season
Pineal gland is prominent in children and degenerates/calcifies with age (pineal calcification is visible on X-rays of older adults)
Also involved in skin pigmentation in lower vertebrates (melatonin lightens skin in frogs by causing melanin aggregation in melanophores)
Thymus
The thymus gland is located behind the sternum (mediastinum). It secretes thymosin, which is crucial for T-lymphocyte differentiation and maturation. The thymus is most active in childhood and progressively degenerates (involutes) after puberty, replaced by fatty tissue.
The thymus is a temporary endocrine gland — it does its most important work in childhood, training the immune system, and then gradually shuts down after puberty. Think of it as a military training academy that's most active when recruits (T-cells) need training, then slowly closes its doors.
Thymosin — the major hormone. Promotes differentiation and maturation of T-lymphocytes (T-cells) which are critical for cell-mediated immunity
The thymus is where immature T-cells from bone marrow come to 'graduate' — they learn to recognize self vs non-self antigens. The 'T' in T-cells stands for Thymus
Most active in childhood — reaches maximum size at puberty, then undergoes involution (shrinks and is replaced by fat). By age 50-60, very little functional thymic tissue remains
Despite involution, the T-cells already trained and released into circulation provide lifelong immunity — the thymus essentially trained enough soldiers before retiring
Thymosin also stimulates antibody production by enhancing B-cell activity indirectly
Absence of thymus (DiGeorge syndrome) → severe immunodeficiency due to lack of T-cells
Quick recall
A patient has severely compromised cell-mediated immunity since birth, with very low T-cell counts. Which gland might be absent or non-functional?
The Thymus gland. This describes DiGeorge Syndrome — congenital absence of thymus → no T-cell maturation → severely compromised cell-mediated immunity while humoral (B-cell/antibody) immunity may be partially intact.
GI Tract Hormones
The gastrointestinal tract is the largest endocrine organ by cell number. Key hormones: gastrin (from stomach G-cells, stimulates HCl), secretin (from duodenum S-cells, stimulates pancreatic bicarbonate), CCK/cholecystokinin (from duodenum I-cells, stimulates enzyme-rich pancreatic juice and bile release), and GIP (from small intestine K-cells, stimulates insulin secretion).
Your gut is not just a food processor — it's the largest endocrine organ in the body (by total number of hormone-producing cells). These gut hormones coordinate digestion by telling different organs exactly when to secrete what.
Gastrin — Secreted by G-cells in the stomach (pyloric region). Stimulus: food entering stomach. Action: stimulates HCl secretion by parietal cells and pepsinogen secretion by chief cells. Also promotes gastric motility
Secretin — Secreted by S-cells of duodenum. Stimulus: acidic chyme entering duodenum. Action: stimulates pancreas to release bicarbonate-rich (alkaline) pancreatic juice to neutralize acid. Also inhibits gastric acid secretion. First hormone ever discovered (Bayliss & Starling, 1902)
CCK (Cholecystokinin) — Secreted by I-cells of duodenum. Stimulus: fats and proteins in duodenum. Action: stimulates gallbladder contraction (bile release) + enzyme-rich pancreatic juice. Also promotes satiety (feeling full)
GIP (Gastric Inhibitory Peptide) — Secreted by K-cells of small intestine. Stimulus: glucose and fat in duodenum. Action: stimulates insulin secretion from β-cells (this is why oral glucose triggers more insulin than IV glucose — the 'incretin effect'). Also inhibits gastric motility and acid secretion
Heart & Kidney as Endocrine Organs
The heart's atrial wall produces ANF (Atrial Natriuretic Factor) which lowers blood pressure by promoting Na⁺ excretion and vasodilation — it antagonizes aldosterone. The juxtaglomerular cells of the kidney produce erythropoietin (EPO) which stimulates red blood cell production in bone marrow in response to hypoxia.
Not just traditional endocrine glands make hormones — some unexpected organs do too. The heart and kidneys are primarily known for pumping blood and filtering it, but they also produce crucial hormones that regulate blood pressure and red blood cell production.
ANF (Atrial Natriuretic Factor / ANP — Atrial Natriuretic Peptide) — Produced by atrial wall cells of the heart when atria are stretched (high blood volume/pressure). Actions: promotes Na⁺ excretion in urine (natriuresis) → water follows → reduces blood volume → lowers BP. Also causes vasodilation
ANF is a natural antagonist of aldosterone — aldosterone retains Na⁺ (raises BP), ANF excretes Na⁺ (lowers BP). They form another hormonal seesaw
Erythropoietin (EPO) — Produced by juxtaglomerular cells of the kidney in response to hypoxia (low oxygen in blood). Stimulates erythropoiesis (RBC production) in red bone marrow
High altitude → low O₂ → more EPO → more RBCs → better O₂ carrying capacity. This is why athletes train at high altitude — natural EPO boost
EPO is also the notorious doping hormone in sports — synthetic EPO was used by cyclists (Lance Armstrong scandal) because more RBCs = more oxygen = more endurance
Quick recall
An athlete moves to a high-altitude training camp. After 3 weeks, blood tests show elevated RBC count. Which hormone caused this?
Erythropoietin (EPO) from the kidneys. High altitude → low pO₂ → kidney JG cells detect hypoxia → increase EPO secretion → stimulates red bone marrow → more RBC production → elevated RBC count (polycythemia of altitude).