Human Endocrine System — Revision Notes

⚡ 30-Second Revision

Hypothalamus: — TRH, GnRH, CRH, GHRH (releasing); Somatostatin, Dopamine (inhibiting). Synthesizes ADH, Oxytocin.
Pituitary: — Anterior (GH, PRL, TSH, ACTH, LH, FSH); Posterior (releases ADH, Oxytocin).
Pineal: — Melatonin (sleep-wake cycle).
Thyroid: — $T_3$, $T_4$ (BMR, growth); Calcitonin ($\downarrow$ blood $Ca^{2+}$).
Parathyroid: — PTH ($\uparrow$ blood $Ca^{2+}$).
Thymus: — Thymosins (T-cell maturation).
Adrenal Cortex: — Cortisol (glucocorticoid), Aldosterone (mineralocorticoid), Androgens.
Adrenal Medulla: — Adrenaline, Noradrenaline ('fight or flight').
Pancreas (Islets): — Insulin ($\downarrow$ blood glucose), Glucagon ($\uparrow$ blood glucose), Somatostatin.
Gonads: — Testis (Testosterone); Ovary (Estrogen, Progesterone).
Heart: — ANF ($\downarrow$ BP).
Kidney: — Erythropoietin (RBC formation).
GI Tract: — Gastrin, Secretin, CCK, GIP.
Hormone Action: — Peptide/Catecholamine (membrane receptor, second messenger); Steroid/Thyroid (intracellular receptor, gene expression).
Regulation: — Negative feedback (most common), Positive feedback (e.g., Oxytocin).

2-Minute Revision

The human endocrine system is a network of ductless glands secreting hormones, which are chemical messengers, directly into the bloodstream. The hypothalamus is the neuro-endocrine control center, producing releasing and inhibiting hormones for the anterior pituitary, and synthesizing ADH and oxytocin for the posterior pituitary.

The anterior pituitary secretes GH, PRL, TSH, ACTH, LH, and FSH, while the posterior releases ADH (water reabsorption) and oxytocin (milk ejection, uterine contraction). The pineal gland secretes melatonin, regulating sleep cycles.

The thyroid gland produces $T_3$ and $T_4$ (metabolism, growth) and calcitonin (lowers blood calcium). Parathyroid glands secrete PTH (raises blood calcium). The thymus produces thymosins for T-cell maturation.

Adrenal glands have a cortex (cortisol, aldosterone) and medulla (adrenaline, noradrenaline for 'fight or flight'). The pancreas (Islets of Langerhans) secretes insulin (lowers blood glucose) and glucagon (raises blood glucose).

Gonads (testis/ovary) produce sex hormones. Hormones from the heart (ANF), kidney (erythropoietin), and GI tract also play crucial roles. Hormone action occurs via cell surface receptors (peptide/catecholamine) or intracellular receptors (steroid/thyroid), regulated primarily by negative feedback.

5-Minute Revision

The human endocrine system is a crucial regulatory network comprising ductless glands that release hormones into the bloodstream. These hormones act as chemical messengers, targeting specific cells or organs to regulate diverse physiological processes.

Key glands include the hypothalamus, which orchestrates the system by producing releasing and inhibiting hormones that control the anterior pituitary. It also synthesizes ADH and oxytocin, stored and released by the posterior pituitary.

The anterior pituitary, often called the 'master gland,' secretes Growth Hormone (GH) for growth, Prolactin (PRL) for milk production, Thyroid Stimulating Hormone (TSH) for thyroid activity, Adrenocorticotropic Hormone (ACTH) for adrenal cortex stimulation, and Gonadotropins (LH and FSH) for reproductive functions.

The posterior pituitary releases Antidiuretic Hormone (ADH) for water reabsorption in kidneys and Oxytocin for uterine contractions and milk ejection.

Other major glands include the pineal gland (melatonin for circadian rhythms), the thyroid gland ($T_3$ and $T_4$ for metabolism and growth, calcitonin for lowering blood calcium), and parathyroid glands (PTH for raising blood calcium).

The thymus produces thymosins, vital for T-lymphocyte maturation. The adrenal glands consist of a cortex (secreting glucocorticoids like cortisol for metabolism and stress, and mineralocorticoids like aldosterone for electrolyte balance) and a medulla (secreting adrenaline and noradrenaline for the 'fight or flight' response).

The pancreas contains Islets of Langerhans, with alpha cells secreting glucagon (raises blood glucose) and beta cells secreting insulin (lowers blood glucose).

Gonads (testis in males, ovary in females) produce sex hormones like testosterone, estrogen, and progesterone, regulating reproduction and secondary sexual characteristics. Additionally, organs like the heart (Atrial Natriuretic Factor - ANF, lowers blood pressure), kidney (Erythropoietin - stimulates RBC formation), and gastrointestinal tract (e.g., Gastrin, Secretin, CCK, GIP) also secrete hormones.

Hormones act via two main mechanisms: water-soluble hormones (peptides, catecholamines) bind to cell surface receptors, activating second messengers (e.g., cAMP), leading to rapid responses. Lipid-soluble hormones (steroids, thyroid hormones) diffuse across the membrane, binding to intracellular receptors to alter gene expression, resulting in slower, sustained effects.

Hormone levels are tightly regulated by feedback mechanisms, primarily negative feedback, to maintain homeostasis. Disorders arise from hypo- or hyper-secretion, leading to conditions like diabetes, thyroid disorders, and adrenal imbalances.

Prelims Revision Notes

The Human Endocrine System is a crucial topic for NEET, focusing on ductless glands and their hormone secretions.

Key Glands and Hormones:

Hypothalamus: — Produces releasing (e.g., GnRH, TRH) and inhibiting (e.g., Somatostatin, Dopamine) hormones that control the anterior pituitary. Synthesizes ADH and Oxytocin, released by posterior pituitary.
Pituitary Gland (Hypophysis):

* Anterior Pituitary (Adenohypophysis): GH (growth), PRL (milk production), TSH (thyroid stimulation), ACTH (adrenal cortex stimulation), LH (gonadotropin), FSH (gonadotropin). * Posterior Pituitary (Neurohypophysis): Releases ADH (vasopressin - water reabsorption), Oxytocin (uterine contraction, milk ejection).

Pineal Gland: — Melatonin (circadian rhythm, sleep-wake cycle).
Thyroid Gland: — Thyroxine ($T_3$, $T_4$ - BMR, growth, CNS development); Calcitonin (lowers blood $Ca^{2+}$).
Parathyroid Glands: — Parathyroid Hormone (PTH - raises blood $Ca^{2+}$).
Thymus Gland: — Thymosins (T-lymphocyte maturation, immunity).
Adrenal Glands:

* Cortex: Glucocorticoids (Cortisol - metabolism, anti-inflammatory), Mineralocorticoids (Aldosterone - electrolyte balance), Adrenal Androgens. * Medulla: Adrenaline (Epinephrine), Noradrenaline (Norepinephrine) - 'fight or flight' response.

Pancreas (Islets of Langerhans): — Alpha cells (Glucagon - increases blood glucose); Beta cells (Insulin - decreases blood glucose); Delta cells (Somatostatin - inhibits insulin/glucagon).
Gonads:

* Testis: Testosterone (male secondary sexual characteristics, spermatogenesis). * Ovary: Estrogen (female secondary sexual characteristics, follicular development); Progesterone (maintains pregnancy, uterine preparation).

Hormones from Other Organs:

Heart: — Atrial Natriuretic Factor (ANF) - decreases blood pressure.
Kidney: — Erythropoietin - stimulates RBC formation.
GI Tract: — Gastrin, Secretin, Cholecystokinin (CCK), Gastric Inhibitory Peptide (GIP).

Mechanism of Hormone Action:

Peptide/Protein Hormones & Catecholamines: — Water-soluble, bind to membrane receptors, use second messengers (e.g., cAMP). Rapid, short-lived effects.
Steroid Hormones & Thyroid Hormones: — Lipid-soluble, diffuse across membrane, bind to intracellular receptors (cytoplasmic/nuclear), alter gene expression. Slower, long-lasting effects.

Regulation: Primarily negative feedback loops maintain homeostasis. Positive feedback (e.g., oxytocin during childbirth) is less common.

Disorders: Know the symptoms and causes of hypo- and hyper-secretion for major hormones (e.g., Diabetes Mellitus, Goitre, Cretinism, Acromegaly, Addison's disease, Cushing's syndrome).

Vyyuha Quick Recall

To remember the major endocrine glands (from top to bottom): He Plays Piano To Please The Audience Perfectly Greatly.

Hypothalamus
Pituitary
Pineal
Thyroid
Parathyroid
Thymus
Adrenal
Pancreas
Gonads (Testis/Ovary)