Endocrine System: Key Terms, Glands, Hormones, and Disorders

Endocrine System Overview

Introduction

The endocrine system is a network of glands and organs that produce, store, and secrete hormones, which regulate various physiological processes throughout the body. Understanding the terminology, glands, hormones, and associated disorders is essential for Anatomy & Physiology students.

Key Terms and Concepts

Definitions and Functions

• Endocrinology: The scientific study of hormones and the endocrine organs.

• Neuroendocrine organ: An organ with neural functions that also produces and releases hormones (e.g., the hypothalamus).

• Hormones: Steroidal or amino acid-based molecules released into the blood that act as chemical messengers to regulate specific body functions.

• Autocrines: Short-distance chemical messengers that exert effects on the same cells that secrete them.

• Paracrines: Chemical messengers that act on nearby cells rather than traveling through the bloodstream.

• Steroids: Organic compounds functioning as signaling molecules and components of cell membranes.

• Eicosanoids: Signaling molecules derived from fatty acids.

• Target cell: Cells that respond to electrical or chemical signals, specifically to particular ligands.

• Adenylyl cyclase: Enzyme that makes a second messenger, cyclic AMP (cAMP), from ATP. $ATP \xrightarrow{\text{Adenylyl cyclase}} cAMP + PP_i$

• Protein kinases: Enzymes that phosphorylate proteins, resulting in activation or inactivation of the protein.

• Phosphodiesterase: Intracellular enzyme that breaks a phosphodiester bond, degrading cAMP or cGMP in second messenger systems.

• Phospholipase C: Enzyme that breaks down phosphatidylinositol 4,5-bisphosphate (PIP2) into second messengers. $\text{PIP}_2 \xrightarrow{\text{Phospholipase C}} \text{IP}_3 + \text{DAG}$

• Inositol triphosphate (IP3): Second messenger that triggers release of calcium ions from intracellular stores.

• Calmodulin: Protein that binds calcium and plays a crucial role in calcium signaling pathways.

• Negative feedback: Homeostatic mechanism where the output of a system reduces its own activity.

• Positive feedback: Mechanism where the output enhances the original stimulus.

• Up-regulation: Increase in the number of receptors on a cell in response to low levels of signaling molecules.

• Down-regulation: Decrease in the number of receptors in response to high levels of signaling molecules.

• Half-life: Time required to decrease the concentration of a hormone in the blood by half.

• Permissiveness: Hormone interaction in which one hormone cannot exert its full effects without another hormone.

• Synergism: Interaction in which two hormones produce amplified effects together.

Major Endocrine Glands and Structures

Hypothalamus and Pituitary Gland

• Hypophysis (Pituitary gland): Small, pea-sized gland below the hypothalamus.

• Infundibulum: Funnel-shaped structure connecting hypothalamus to pituitary.

• Neurohypophysis: Posterior lobe of the pituitary gland.

• Adenohypophysis: Anterior lobe of the pituitary gland.

• Hypothalamic-hypophyseal tract: Nerve fibers connecting hypothalamus to posterior pituitary, transporting oxytocin and vasopressin.

Feedback and Stimuli

• Releasing hormone: Hormone that stimulates the release of another hormone (e.g., GHRH).

• Inhibiting hormone: Hormone that inhibits the release of another hormone (e.g., GHIH).

• Humoral stimuli: Hormone release in response to changes in blood levels of ions or nutrients.

• Neural stimuli: Hormone release triggered by nerve impulses.

• Hormonal stimuli: Hormone release triggered by another hormone.

Endocrine Disorders and Conditions

Hypothalamic and Pituitary Disorders

• Osmoreceptors: Cells that detect changes in osmotic pressure and regulate vasopressin secretion.

• Diabetes insipidus: Condition causing excessive thirst and urination due to ADH deficiency.

• Syndrome of inappropriate ADH secretion: Excess ADH causes water retention.

• Acromegaly: Excess growth hormone in adulthood causes overgrowth of bones.

• Pituitary dwarfism: GH deficiency in childhood causes short stature.

• Gigantism: GH excess in childhood causes abnormal tall stature.

Thyroid and Parathyroid Disorders

• Thyroid gland: Regulates metabolism, growth, and development.

• Myxedema: Skin and tissue swelling due to hypothyroidism.

• Goiter: Enlarged thyroid gland.

• Graves' disease: Autoimmune hyperthyroidism.

• Parathyroid glands: Regulate blood calcium via parathyroid hormone (PTH).

• Hyperparathyroidism: Excess PTH causes high blood calcium.

• Hypoparathyroidism: Low PTH causes low blood calcium.

• Tetany: Involuntary muscle contractions due to low calcium.

Adrenal Gland Disorders

• Adrenal glands: Located atop kidneys, produce hormones for stress response.

• Adrenal medulla: Releases epinephrine and norepinephrine.

• Adrenal cortex: Produces steroid hormones.

• Corticosteroids: Anti-inflammatory and immunosuppressant hormones.

• Zona glomerulosa: Produces mineralocorticoids (e.g., aldosterone).

• Zona fasciculata: Produces glucocorticoids.

• Zona reticularis: Produces gonadocorticoids.

• Cushing's syndrome: Excess glucocorticoids cause muscle loss and bone weakness.

• Cushing's disease: ACTH-secreting pituitary tumor causes excess glucocorticoids.

• Addison's disease: Adrenal cortex hormone deficiency causes weight loss and skin bronzing.

• Adrenogenital syndrome: Genetic disorder affecting adrenal gland hormone production.

• Pheochromocytoma: Tumor of adrenal gland causing excess catecholamines.

Pancreatic Disorders

• Pancreatic islets (Islets of Langerhans): Clusters of cells producing insulin and glucagon.

• Alpha cells: Secrete glucagon.

• Beta cells: Secrete insulin.

• Diabetes mellitus: Chronic disorder with high blood sugar levels.

• Polyuria: Excessive urination.

• Polydipsia: Excessive thirst.

• Polyphagia: Excessive hunger.

• Ketones/ketone bodies: Molecules produced when fat is used for energy.

• Ketoacidosis: Dangerous complication of diabetes due to high ketone levels.

• Insulin resistance: Condition where more insulin is needed to maintain normal blood glucose.

• Hypoglycemia: Low blood glucose.

• Hyperglycemia: High blood glucose.

Other Endocrine Organs and Functions

Reproductive and Other Glands

• Placenta: Organ in pregnant uterus nourishing the fetus.

• Testes: Male glands producing sperm and testosterone.

• Ovaries: Female glands producing eggs, estrogen, and progesterone.

• Thymus: Lymphoid organ producing T cells for immunity.

• Galactorrhea: Abnormal milk production in non-pregnant individuals.

• Hirsutism: Excessive hair growth in females in male-like patterns.

Hormones to Know

Major Hormones and Their Functions

• Releasing/inhibiting hormones: Hypothalamic hormones that regulate pituitary hormone release.

• Oxytocin: Stimulates uterine contractions and milk ejection.

• Antidiuretic hormone (ADH): Regulates water balance and blood pressure.

• Growth hormone (GH): Stimulates general growth.

• Thyroid-stimulating hormone (TSH): Regulates thyroid hormone secretion.

• Adrenocorticotropic hormone (ACTH): Stimulates adrenal cortex to produce cortisol.

• Follicle-stimulating hormone (FSH): Stimulates follicle production in females and sperm production in males.

• Luteinizing hormone (LH): Stimulates ovulation and testosterone production.

• Gonadotropin-releasing hormone (GnRH): Stimulates pituitary to release LH and FSH.

• Prolactin-inhibiting hormone (PIH): Inhibits prolactin secretion.

• Prolactin: Stimulates breast development and milk production.

• GHRH: Stimulates growth hormone release.

• GHIH (somatostatin): Inhibits growth hormone release.

Table: Major Endocrine Glands, Hormones, and Functions

Gland	Hormone(s)	Main Function(s)
Hypothalamus	Releasing/inhibiting hormones	Regulate pituitary hormone release
Pituitary (anterior)	GH, TSH, ACTH, FSH, LH, Prolactin	Growth, metabolism, stress response, reproduction, lactation
Pituitary (posterior)	Oxytocin, ADH	Uterine contraction, milk ejection, water balance
Thyroid	Thyroid hormones (T3, T4), Calcitonin	Metabolism, calcium regulation
Parathyroid	PTH	Blood calcium regulation
Adrenal cortex	Corticosteroids, aldosterone, cortisol	Stress response, electrolyte balance
Adrenal medulla	Epinephrine, norepinephrine	Fight-or-flight response
Pancreas	Insulin, glucagon	Blood glucose regulation
Ovaries	Estrogen, progesterone	Female reproduction
Testes	Testosterone	Male reproduction

Example: Negative Feedback in Endocrine Regulation

• When blood glucose rises, the pancreas releases insulin, which lowers glucose levels. As glucose returns to normal, insulin secretion decreases, maintaining homeostasis.

Additional info:

• Some definitions and explanations have been expanded for clarity and completeness.

• Table entries inferred from standard endocrine system knowledge.