Endocrine System Overview

Major Functions of the Endocrine System

The endocrine system is a collection of glands and tissues that secrete hormones to regulate various physiological processes. It works alongside the nervous system to maintain homeostasis and coordinate body functions.

• Regulation of metabolism: Controls the rate of chemical reactions in cells.

• Growth and development: Influences cell division, differentiation, and maturation.

• Homeostasis: Maintains internal balance of water, electrolytes, and nutrients.

• Reproduction: Regulates sexual development and reproductive cycles.

• Response to stress: Manages adaptation to physical and emotional stressors.

Key Terms and Definitions

Hormone, Endocrine Gland, Endocrine Tissue (Organ), Target Cell

• Hormone: A chemical messenger secreted into the blood by endocrine cells that affects distant target cells.

• Endocrine gland: A ductless gland that releases hormones directly into the bloodstream (e.g., pituitary, thyroid).

• Endocrine tissue (organ): Organs or tissues with endocrine functions but not classified as glands (e.g., pancreas, heart).

• Target cell: A cell with specific receptors for a hormone, enabling it to respond to that hormone.

Comparison: Nervous System vs. Endocrine System

Control of Body Function

• Nervous system: Uses electrical impulses and neurotransmitters; rapid, short-lived responses; localized effects.

• Endocrine system: Uses hormones transported in blood; slower onset, longer-lasting effects; widespread influence.

Example: The nervous system triggers muscle contraction in milliseconds, while the endocrine system regulates growth over years.

Chemical Classes of Hormones

Major Classes

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

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

• Eicosanoids: Derived from arachidonic acid; act locally (e.g., prostaglandins).

Transport in Blood

• Amino acid-based hormones: Usually water-soluble; travel freely in plasma.

• Steroid hormones: Lipid-soluble; require transport proteins in blood.

• Eicosanoids: Act locally; not typically transported in blood.

Hormone Receptors

• Cell membrane receptors: For water-soluble hormones (amino acid-based); cannot cross plasma membrane.

• Intracellular receptors: For lipid-soluble hormones (steroids); diffuse through membrane to bind inside cell.

Mechanisms of Hormone Action

• Water-soluble hormones: Bind to membrane receptors; activate second messenger systems (e.g., cAMP).

• Lipid-soluble hormones: Bind to intracellular receptors; directly regulate gene expression.

Example: Epinephrine (water-soluble) triggers cAMP production; cortisol (lipid-soluble) alters gene transcription.

Control of Hormone Secretion

Types of Stimuli

• Humoral stimuli: Changes in blood levels of ions or nutrients (e.g., calcium, glucose).

• Neural stimuli: Nerve fibers stimulate hormone release (e.g., adrenal medulla by sympathetic nerves).

• Hormonal stimuli: Hormones stimulate other endocrine glands (e.g., hypothalamic hormones on pituitary).

Feedback Mechanisms

• Negative feedback: Most common; hormone release decreases as its effects are achieved (e.g., insulin lowers blood glucose).

• Positive feedback: Less common; hormone release increases in response to its effects (e.g., oxytocin during childbirth).

Anatomy of the Endocrine System

Hypothalamus, Anterior Pituitary, and Posterior Pituitary

• Hypothalamus: Located in the brain; links nervous and endocrine systems; produces releasing and inhibiting hormones.

• Anterior pituitary (adenohypophysis): Receives signals from hypothalamus via blood vessels; secretes tropic hormones.

• Posterior pituitary (neurohypophysis): Stores and releases hormones made by hypothalamus (e.g., ADH, oxytocin).

Anatomical relationships: The pituitary gland sits below the hypothalamus, connected by the infundibulum (pituitary stalk).

Key Hormone Terms

• Releasing hormone: Stimulates secretion of another hormone (e.g., TRH stimulates TSH release).

• Inhibiting hormone: Inhibits secretion of another hormone (e.g., somatostatin inhibits GH release).

• Tropic hormone: Hormone that regulates the function of other endocrine glands (e.g., ACTH, TSH).

Hypothalamic Control of Pituitary Hormones

• Anterior pituitary: Hypothalamus secretes releasing/inhibiting hormones into hypophyseal portal system; regulates anterior pituitary hormone secretion.

• Posterior pituitary: Hypothalamic neurons synthesize hormones (ADH, oxytocin); transported down axons and released from posterior pituitary.

Major Endocrine Glands and Hormone-Producing Cells

Identification and Function

• Pituitary gland: Anterior (various tropic hormones), Posterior (ADH, oxytocin).

• Thyroid gland: Follicular cells (thyroid hormones), parafollicular cells (calcitonin).

• Parathyroid gland: Chief cells (parathyroid hormone).

• Adrenal gland: Cortex (zona glomerulosa – aldosterone; zona fasciculata – cortisol; zona reticularis – androgens), Medulla (chromaffin cells – epinephrine, norepinephrine).

• Testes: Interstitial cells (testosterone).

• Ovary: Follicular cells (estrogen), corpus luteum (progesterone).

• Pancreas: Alpha cells (glucagon), beta cells (insulin).

• Kidney: Erythropoietin, renin.

• Thymus: Thymosins (in children).

• Heart: Atrial natriuretic peptide (ANP).

• Placenta: Human chorionic gonadotropin (hCG), others.

Hormone Table: Source, Stimulus, and Action

Additional info: Table entries are representative; students should be able to complete a similar table for all major hormones.