The Endocrine System: Structure, Function, and Regulation

The Endocrine System

Introduction to the Endocrine System

The endocrine system is a major regulatory system of the body, composed of glands and tissues that produce hormones. These chemical messengers coordinate and control various physiological processes to maintain homeostasis.

• Endocrine cells and tissues produce about 30 different hormones.

• Hormones control and coordinate body processes such as metabolism, growth, and reproduction.

Intercellular Communication

Mechanisms of Intercellular Communication

• Direct communication: Exchange of ions and molecules between adjacent cells via gap junctions; occurs between cells of the same type and is rare.

• Paracrine communication: Chemical signals transfer information from cell to cell within a single tissue.

• Autocrine communication: Messages affect the same cells that secrete them (e.g., prostaglandins in smooth muscle).

• Endocrine communication: Endocrine cells release hormones into the bloodstream, affecting distant target cells and altering metabolic activities of many organs.

• Synaptic communication: Neurons release neurotransmitters at synapses, leading to rapid, targeted responses (ideal for crisis management).

Target Cells and Hormones

• Target cells have specific receptors to bind and respond to hormones.

• Hormones can change the types, quantities, or activities of enzymes and structural proteins in target cells, affecting long-term processes like growth and development.

Hormones

General Properties

• Both the endocrine and nervous systems use chemical messengers and are regulated mainly by negative feedback.

• They function to preserve homeostasis by coordinating and regulating activities.

Endocrine vs. Exocrine

• Endocrine cells release secretions into extracellular fluid (not ducts), while exocrine cells secrete into ducts.

• Endocrine organs are scattered throughout the body.

Classes of Hormones

• Amino acid derivatives

• Peptide hormones

• Lipid derivatives

Mechanisms of Hormone Action

• Hormones may alter genetic activity, protein synthesis, or membrane permeability in target cells.

• Hormone receptor: A protein molecule that binds a specific hormone, determining the cell's sensitivity to that hormone.

Mechanism of Action Depends on Hormone Nature

• Catecholamines and peptide hormones: Not lipid soluble; bind to extracellular receptors and activate second messengers.

• Steroid and thyroid hormones: Lipid soluble; diffuse across plasma membrane and bind to intracellular receptors, affecting gene expression and protein synthesis.

Hormone Secretion and Regulation

• Mainly controlled by negative feedback: a stimulus triggers hormone production, which then reduces the stimulus.

• Control mechanisms:

  • Humoral stimuli: Changes in blood composition (e.g., ions, nutrients).

  • Hormonal stimuli: Arrival or removal of another hormone.

  • Neural stimuli: Arrival of neurotransmitters at neuroglandular junctions.

The Pituitary Gland (Hypophysis)

Anatomy and Structure

• Lies within the sella turcica, connected to the hypothalamus by the infundibulum.

• Divided into anterior (adenohypophysis) and posterior (neurohypophysis) lobes.

Hormones of the Anterior Lobe

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

• Adrenocorticotropic hormone (ACTH): Stimulates adrenal cortex.

• Prolactin (PRL): Stimulates mammary gland development and milk production.

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

• Gonadotropins:

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

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

• Melanocyte-stimulating hormone (MSH): Stimulates melanin production (pars intermedia; mostly nonfunctional in adults).

Hormones of the Posterior Lobe

• Antidiuretic hormone (ADH): Reduces water loss by kidneys.

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

Hypothalamic Control

• Hypothalamus produces releasing hormones (RH) and inhibiting hormones (IH) that regulate anterior pituitary secretion.

• Secretion is controlled by negative feedback.

The Thyroid Gland

Anatomy and Structure

• Lies inferior to the thyroid cartilage of the larynx; consists of two lobes connected by an isthmus.

• Composed of thyroid follicles (lined by cuboidal epithelium) and parafollicular (C) cells.

Thyroid Hormones

• Thyroglobulin: Protein precursor stored in follicles; contains tyrosine residues for hormone synthesis.

• Thyroxine (T4): Contains four iodine atoms.

• Triiodothyronine (T3): Contains three iodine atoms.

• Regulated by TSH from the anterior pituitary; absence of TSH leads to inactivity of thyroid follicles.

Functions of Thyroid Hormones

• Affect nearly every cell in the body by increasing metabolic rate and oxygen consumption.

• Essential for normal development of skeletal, muscular, and nervous systems in children.

• Increase heart rate, stimulate red blood cell formation, and enhance sympathetic nervous system sensitivity.

Calcitonin (CT)

• Produced by C cells; lowers blood calcium levels by inhibiting osteoclast activity and increasing calcium excretion by kidneys.

Summary Table: Major Endocrine Glands and Hormones

Gland	Hormone(s)	Main Function(s)
Pituitary (anterior)	TSH, ACTH, PRL, GH, FSH, LH, MSH	Regulates other endocrine glands, growth, reproduction, lactation, pigmentation
Pituitary (posterior)	ADH, OXT	Water balance, uterine contraction, milk ejection
Thyroid	T3, T4, CT	Metabolic rate, development, calcium homeostasis

Additional info: The notes above cover the first half of the Endocrine System chapter, focusing on the structure, function, and regulation of the endocrine system, pituitary gland, and thyroid gland. For a complete review, students should also study the adrenal glands, pancreas, and secondary endocrine functions.