The Endocrine System: Structure, Function, and Hormones

Endocrine System

Overview

The endocrine system is a network of ductless glands and hormone-secreting tissues within various organs. It regulates physiological processes by releasing hormones—chemical messengers that travel through the bloodstream to target cells, where they bind to specific receptors and elicit cellular responses.

• Ductless glands: Release hormones directly into the blood, not through ducts.

• Target cell: Any cell with a receptor specific to a given hormone.

• Hormone action: Initiates changes in target cell activity, metabolism, or gene expression.

Hormone Classes

Steroid Hormones

Steroid hormones are synthesized from cholesterol and are lipid-soluble, allowing them to cross cell membranes easily.

• Examples: Testosterone, Estrogen, Vitamin D

• Mechanism: Bind to intracellular receptors, often influencing gene transcription.

Amino Acid-Derived Hormones

These hormones are synthesized from amino acids, primarily tyrosine.

• Thyroxine (T4): A thyroid hormone that regulates cellular metabolism, growth, and development.

• Epinephrine and Norepinephrine: Produced by the adrenal medulla; mediate the "fight or flight" response and are involved in sexual climax.

Peptide and Protein Hormones

Composed of chains of amino acids linked by peptide bonds, these hormones are water-soluble and act via cell surface receptors.

• Examples: Oxytocin, Antidiuretic hormone (ADH), Growth hormone (GH), Insulin

Endocrine Glands

Pituitary Gland

The pituitary gland is a small gland suspended from the hypothalamus and located in the sella turcica of the sphenoid bone. It has two distinct lobes:

• Anterior Pituitary (Adenohypophysis): Glandular tissue; hormone secretion is regulated by hypothalamic hormones.

• Posterior Pituitary (Neurohypophysis): Nervous tissue; stores and releases hormones produced by the hypothalamus.

Anterior Pituitary Hormones

• Growth Hormone (GH): Stimulates growth and metabolism.

• Thyroid Stimulating Hormone (TSH): Stimulates the thyroid gland to secrete thyroid hormones.

• Adrenocorticotropic Hormone (ACTH): Stimulates the adrenal cortex to secrete steroid hormones.

• Gonadotropins:

  • Luteinizing Hormone (LH): Stimulates ovulation and corpus luteum formation in ovaries; stimulates testosterone production in testes.

  • Follicle Stimulating Hormone (FSH): Stimulates sustentocytes in testes and follicle cells in ovaries.

• Prolactin (PRL): Stimulates milk production in mammary glands.

Posterior Pituitary Hormones

• Oxytocin: Stimulates uterine contractions and milk ejection during breastfeeding.

• Antidiuretic Hormone (ADH, Vasopressin): Enhances water reabsorption in the kidneys, reducing urine output.

Thyroid Gland

The thyroid gland is a butterfly-shaped gland located anterior to the trachea and inferior to the larynx.

• Follicles: Lined by follicular cells that secrete thyroxine (T4), which is converted to the active form (T3) in target cells.

• Parafollicular cells: Located between follicles; secrete calcitonin, which increases calcium uptake into bone, lowering blood calcium levels.

Parathyroid Glands

Usually four small glands embedded in the posterior thyroid. They secrete parathyroid hormone (PTH), which increases blood calcium levels by acting on bone and kidneys.

Adrenal Glands

Located on the superior surface of each kidney, each adrenal gland has two main parts:

• Adrenal Cortex (outer): Epithelial tissue; secretes only steroid hormones, with three layers:

Layer	Main Hormone	Function
Zona glomerulosa	Aldosterone	Regulates salt balance by acting on nephrons
Zona fasciculata	Cortisol	Regulates metabolism and stress response
Zona reticularis	Androgens	Converted to testosterone or estrogens in other tissues

• Adrenal Medulla (inner): Modified nervous tissue; synthesizes hormones for the fight or flight response:

  • Epinephrine (80%)

  • Norepinephrine (20%)

Endocrine Tissues

Some hormone-secreting cells are located in organs with other primary functions. These are not strictly endocrine glands but play important roles in hormone regulation.

Pancreatic Islets of Langerhans

• α-cells: Secrete glucagon, which increases blood glucose concentration.

• β-cells: Secrete insulin, which decreases blood glucose concentration.

Hypothalamus

• Produces hormones that regulate the anterior pituitary:

  • GHRH: Growth Hormone Releasing Hormone

  • GHIH (Somatostatin): Growth Hormone Inhibiting Hormone

  • GnRH: Gonadotropin Releasing Hormone (stimulates FSH and LH secretion)

• Produces hormones stored and released by the posterior pituitary (oxytocin and ADH).

Gonads

• Testes: Interstitial endocrine (Leydig) cells secrete testosterone.

• Ovaries: Granulosa cells of secondary and vesicular follicles secrete estrogen; corpus luteum secretes estrogen and progesterone.

Other Endocrine Tissues

• Duodenum: Secretes secretin and cholecystokinin (CCK).

• Kidney: Secretes renin and erythropoietin (EPO).

• Skin: Produces vitamin D.

• Heart: Secretes atrial natriuretic peptide (ANP).

• Other tissues may also have endocrine functions.

Summary Table: Major Endocrine Glands and Hormones

Gland/Tissue	Main Hormones	Primary Functions
Pituitary (anterior)	GH, TSH, ACTH, LH, FSH, PRL	Growth, metabolism, reproduction, lactation
Pituitary (posterior)	Oxytocin, ADH	Uterine contraction, milk ejection, water balance
Thyroid	T4 (thyroxine), calcitonin	Metabolism, calcium homeostasis
Parathyroid	PTH	Increases blood calcium
Adrenal cortex	Aldosterone, cortisol, androgens	Salt balance, stress response, sex hormones
Adrenal medulla	Epinephrine, norepinephrine	Fight or flight response
Pancreatic islets	Insulin, glucagon	Blood glucose regulation
Gonads	Testosterone, estrogen, progesterone	Reproduction, secondary sex characteristics
Other tissues	Secretin, CCK, renin, EPO, vitamin D, ANP	Digestive, renal, cardiovascular, and metabolic regulation

Additional info: The endocrine system works closely with the nervous system to coordinate and regulate body functions. Hormones typically act more slowly than nerve impulses but have longer-lasting effects.