Overview of the Endocrine System

Definitions and Key Concepts

• Endocrine system: The collection of glands that secrete hormones directly into the bloodstream to regulate body functions.

• Hormones: Chemical messengers produced by endocrine glands that travel through the blood to affect target cells/organs.

• Target cells: Cells with specific receptors that respond to particular hormones.

• Hormone receptor: A protein on or in a target cell that binds to a specific hormone, triggering a response.

Pure Endocrine Glands vs. Glands with Secondary Endocrine Function

• Pure endocrine glands: Pituitary, thyroid, parathyroid, adrenal (cortex and medulla), and pineal glands.

• Glands with secondary endocrine function: Pancreas, gonads (testes and ovaries), hypothalamus, thymus, heart, kidneys, digestive tract.

Functions of the Endocrine System

• Regulates metabolism, growth, development, tissue function, and mood.

• Works with the nervous system to coordinate body activities.

Comparison: Nervous System vs. Endocrine System

Endocrine vs. Exocrine Glands

• Primary difference: Endocrine glands secrete hormones into the bloodstream; exocrine glands secrete substances into ducts (e.g., sweat, saliva).

• An endocrine gland releases a hormone into the bloodstream.

Mechanism of Hormone Action (4 Steps)

1. Hormone is secreted by an endocrine gland.

2. Hormone travels through the bloodstream to target cells.

3. Hormone binds to specific receptors on/in target cells.

4. Target cell responds (e.g., changes in gene expression, enzyme activity).

Classes of Hormones

• Amino acid-based hormones: Includes peptides, proteins, and amines (e.g., insulin, growth hormone, epinephrine).

• Steroid hormones: Derived from cholesterol (e.g., cortisol, estrogen, testosterone).

Hypothalamus and Pituitary Gland

Hypothalamus

• Location: Inferior part of the diencephalon, below the thalamus.

• Structure: Small region of the brain, contains several nuclei.

• Connection to pituitary: The infundibulum connects the hypothalamus to the pituitary gland.

• Control of endocrine system:

  • Produces two hormones (antidiuretic hormone (ADH) and oxytocin) released from the posterior pituitary.

  • Produces regulatory hormones that stimulate or inhibit hormone release from the anterior pituitary.

Pituitary Gland (Hypophysis)

• Composed of two lobes: anterior lobe (adenohypophysis) and posterior lobe (neurohypophysis).

• Anterior lobe: Made of glandular epithelial cells.

• Vascular connection: The hypophyseal portal system delivers hormones from the hypothalamus to the anterior pituitary.

• Releasing hormones (from hypothalamus) stimulate, and inhibiting hormones suppress, anterior pituitary hormone release.

Hormones of the Anterior Pituitary

Posterior Pituitary

• Composed of axons of neurons whose cell bodies are in the hypothalamus.

• The hypothalamo-hypophyseal tract is the bundle of axons traveling through the infundibulum.

Thyroid Gland

Location and Anatomy

• Location: Anterior neck, inferior to the larynx, overlying the trachea.

• Macroscopic anatomy: Two lobes connected by an isthmus.

• Microscopic anatomy:

  • Follicle: Spherical structure; functional unit of the thyroid.

  • Follicular cells: Line the follicles; produce thyroid hormones.

  • Colloid: Gel-like substance in the center of follicles; stores thyroid hormone precursors.

  • Parafollicular (C) cells: Located between follicles; secrete calcitonin.

Primary Hormones Produced

• Thyroid hormones: Thyroxine (T4) and triiodothyronine (T3)

• Primary function: Regulate metabolism, growth, and development.

• Calcitonin:

  • Stimulus for release: High blood calcium levels.

  • Target cells: Bone cells (osteoblasts).

  • Effect: Lowers blood calcium by increasing calcium deposition in bones.

Parathyroid Gland

• Location: Posterior surface of the thyroid gland (usually 4 small glands).

• Gross anatomy: Small, oval-shaped glands.

• Hormone produced: Parathyroid hormone (PTH).

• Stimulus for release: Low blood calcium levels.

• Target cells/effects: Bone (stimulates osteoclasts), kidneys (increases calcium reabsorption), intestines (increases calcium absorption).

Adrenal Glands

Location and Anatomy

• Location: Superior to each kidney.

• Macroscopic anatomy: Pyramid-shaped glands with outer cortex and inner medulla.

• Microscopic anatomy:

  • Cortex: Composed of three regions; produces corticosteroids.

  • Medulla: Composed of chromaffin cells; produces catecholamines.

• Medulla hormones: Epinephrine and norepinephrine (fight-or-flight response).

Pancreas

Location and Anatomy

• Location: Posterior to the stomach, in the upper abdomen.

• Macroscopic anatomy:

  • Head: Nestled in the curve of the duodenum.

  • Body: Central portion.

  • Tail: Extends toward the spleen.

• Microscopic anatomy: Endocrine cells are grouped in islets of Langerhans (pancreatic islets).

• These hormones regulate blood glucose levels by negative feedback.

Hormone Imbalance

• Imbalances can cause disease.

• Most common endocrine disorder: Diabetes mellitus.

Gonads

• Male gonads: Testes

  • Primary hormone: Testosterone

  • Functions: Development of male secondary sex characteristics, sperm production

• Female gonads: Ovaries

  • Primary hormones: Estrogen and progesterone

  • Functions: Regulation of menstrual cycle, development of female secondary sex characteristics

Additional info: Some details (e.g., specific cell types, hormone names, and feedback mechanisms) were inferred based on standard anatomy and physiology content.