Endocrine System Overview

Introduction to the Endocrine System

The endocrine system is a network of glands that secrete hormones directly into the bloodstream to regulate various physiological processes. These hormones control growth, metabolism, reproduction, and homeostasis.

• Endocrine glands are ductless and release hormones into the blood.

• Hormones are chemical messengers that act on target organs or tissues.

• Major endocrine glands include the pituitary gland, thyroid gland, adrenal glands, pancreas, gonads, and others.

Major Endocrine Glands and Their Hormones

Hypothalamus

The hypothalamus is a region of the brain that links the nervous and endocrine systems. It controls the pituitary gland and produces several releasing and inhibiting hormones.

• Secretes: ADH (antidiuretic hormone), OXT (oxytocin), and regulatory hormones that control the anterior pituitary.

Pituitary Gland

The pituitary gland, often called the "master gland," is located at the base of the brain and consists of anterior and posterior lobes. It regulates many other endocrine glands.

• Anterior pituitary hormones:

  • ACTH (adrenocorticotropic hormone): Stimulates adrenal cortex.

  • TSH (thyroid-stimulating hormone): Stimulates thyroid gland.

  • GH (growth hormone): Stimulates growth and metabolism.

  • PRL (prolactin): Stimulates milk production.

  • FSH (follicle-stimulating hormone): Stimulates gamete production.

  • LH (luteinizing hormone): Stimulates sex hormone production.

  • MSH (melanocyte-stimulating hormone): Affects skin pigmentation.

• Posterior pituitary hormones:

  • ADH (antidiuretic hormone): Regulates water balance.

  • OXT (oxytocin): Stimulates uterine contractions and milk ejection.

Thyroid Gland

The thyroid gland is located in the neck and produces hormones that regulate metabolism, growth, and development.

• Secretes: Thyroxine (T4), Triiodothyronine (T3), and Calcitonin.

• T3 and T4 increase metabolic rate and oxygen consumption.

• Calcitonin lowers blood calcium levels.

Parathyroid Glands

Small glands located on the posterior surface of the thyroid gland. They regulate calcium homeostasis.

• Secretes: Parathyroid hormone (PTH), which increases blood calcium levels.

Adrenal Glands

Located on top of each kidney, the adrenal glands consist of the cortex and medulla, each producing different hormones.

• Adrenal cortex:

  • Cortisol: Regulates metabolism and stress response.

  • Aldosterone: Regulates sodium and potassium balance.

  • Androgens: Contribute to sex characteristics.

• Adrenal medulla:

  • Epinephrine (adrenaline) and norepinephrine: Mediate fight-or-flight response.

Pancreas

The pancreas has both exocrine and endocrine functions. The endocrine portion consists of the islets of Langerhans, which regulate blood glucose.

• Alpha cells: Secrete glucagon (raises blood glucose).

• Beta cells: Secrete insulin (lowers blood glucose).

• Delta cells: Secrete somatostatin (inhibits insulin and glucagon secretion).

Gonads

The gonads (testes in males, ovaries in females) produce sex hormones that regulate reproduction and secondary sex characteristics.

• Testes: Produce androgens (mainly testosterone).

• Ovaries: Produce estrogens, progesterone, and inhibin.

Pineal Gland

The pineal gland is located in the brain and secretes melatonin, which regulates circadian rhythms.

Other Organs with Endocrine Functions

• Heart: Secretes atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) to regulate blood pressure.

• Thymus: Secretes thymosins for immune function.

• Adipose tissue: Secretes leptin (regulates appetite) and other hormones.

• Digestive tract: Produces hormones that regulate digestion and appetite.

• Kidneys: Secrete erythropoietin (EPO) for red blood cell production and calcitriol for calcium regulation.

Histology of Endocrine Glands

Pituitary Gland Histology

The pituitary gland is divided into anterior and posterior lobes, each with distinct histological features and hormone production.

• Anterior lobe (adenohypophysis): Contains chromophils (acidophils and basophils) and chromophobes.

• Posterior lobe (neurohypophysis): Contains nerve fibers and pituicytes; stores and releases ADH and oxytocin.

Thyroid Gland Histology

The thyroid gland is composed of spherical follicles filled with colloid, surrounded by follicular cells.

• Follicular cells: Produce T3 and T4.

• Colloid: Contains thyroglobulin, the precursor to thyroid hormones.

• Parafollicular (C) cells: Secrete calcitonin.

Adrenal Gland Histology

The adrenal gland has a cortex and medulla, each with unique histological zones and functions.

• Cortex: Divided into three zones:

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

  • Zona fasciculata: Secretes glucocorticoids (e.g., cortisol).

  • Zona reticularis: Secretes androgens.

• Medulla: Contains chromaffin cells that secrete epinephrine and norepinephrine.

Pancreas Histology

The pancreas contains clusters of endocrine cells called islets of Langerhans, surrounded by exocrine acini.

• Islets of Langerhans: Contain alpha, beta, and delta cells.

• Exocrine acini: Secrete digestive enzymes.

Summary Table: Major Endocrine Glands and Hormones

Key Terms and Definitions

• Hormone: A chemical messenger secreted by endocrine glands.

• Endocrine gland: A ductless gland that releases hormones into the bloodstream.

• Target organ: The organ or tissue affected by a specific hormone.

• Negative feedback: A regulatory mechanism in which a change in a physiological variable triggers a response that counteracts the initial change.