Endocrine System: Structure, Function, and Hormonal Regulation

Endocrine System Overview

Definition and Function

The endocrine system is a network of glands that secrete hormones directly into the bloodstream to regulate various physiological processes. It works closely with the nervous system to maintain homeostasis, growth, metabolism, reproduction, and electrolyte balance.

• Hormone: Long-distance chemical messenger secreted into blood that stimulates a response in target cells.

• Target cell: Cell with receptors for a specific hormone; without receptors, no response occurs.

• Endocrine gland: Ductless gland that secretes hormones into blood/lymph; produces intracellular effects.

• Neuroendocrine cells: Neurons that release hormones (e.g., hypothalamus).

• Exocrine gland: Have ducts, secrete onto surfaces/cavities; extracellular effects (e.g., sweat, saliva).

Major Endocrine Organs

• Hypothalamus: Master control; links nervous and endocrine systems; regulates pituitary function.

• Pituitary gland (hypophysis): Anterior (glandular) and posterior (neural) lobes.

• Thyroid gland: Produces thyroid hormone (T3/T4); regulates metabolism.

• Parathyroid glands: Secrete parathyroid hormone (PTH); regulates Ca2+.

• Adrenal glands: Cortex (mineralocorticoids, glucocorticoids, sex steroids); medulla (epinephrine, norepinephrine).

• Pineal gland: Produces melatonin; regulates circadian rhythms.

• Pancreas: Endocrine (insulin, glucagon) and exocrine (digestive enzymes) functions.

• Gonads: Ovaries and testes; produce sex hormones.

Hormones: Types, Mechanisms, and Effects

Classification of Hormones

• Amino acid-based hormones (hydrophilic): Water-soluble, bind membrane receptors, act via second messengers.

• Steroids (hydrophobic): Lipid-soluble, enter cells, bind intracellular receptors, alter gene activity.

• Eicosanoids: Local signals; autocrine or paracrine effects.

Hormone Transport and Action

• Water-soluble hormones: Free in blood, bind membrane receptors, use second messengers.

• Lipid-soluble hormones: Bound to proteins, diffuse into cells, alter gene expression.

Hormone Effects on Target Cells

• Change membrane permeability

• Activate enzymes

• Stimulate protein synthesis

• Induce secretion

• Stimulate mitosis

Hormone-Receptor Interactions

• Affinity: Strength of hormone-receptor binding.

• Permissiveness: One hormone needed for another (e.g., thyroid + sex hormones).

• Synergism: Combined hormones amplify effects (e.g., glucagon + epinephrine).

• Antagonism: One hormone opposes the action of another.

Regulation of Hormone Secretion

• Up-regulation: More receptors when hormone is low.

• Down-regulation: Fewer receptors when hormone is high.

• Negative feedback: Maintains stable hormone levels.

Hormone Signaling Mechanisms

cAMP Signaling Pathway

• Mechanism: Hormone → receptor → G protein → adenylate cyclase → cAMP → protein kinases

$\text{Hormone} \rightarrow \text{Receptor} \rightarrow \text{G protein} \rightarrow \text{Adenylate cyclase} \rightarrow \text{cAMP} \rightarrow \text{Protein kinases}$

Phospholipase C (PLC) / Calcium Mechanism

• PLC splits PIP2 → DAG (activates PKC) + IP3 (Ca2+ release, activates kinases)

$\text{PIP}_2 \xrightarrow{PLC} \text{DAG} + \text{IP}_3$

Insulin Receptor Mechanism

• Tyrosine kinase receptor; directly phosphorylates targets.

Endocrine vs. Nervous System

Comparison Table

Feature	Nervous System	Endocrine System
Signal Type	Electrical impulses, neurotransmitters	Chemical messengers (hormones)
Speed	Fast, short duration	Slow, long duration
Location of Effects	Local, specific	Widespread, systemic

Major Endocrine Disorders and Hormones

Thyroid Disorders

• Hypothyroidism: Low thyroid hormone; symptoms include cretinism, myxedema, goiter.

• Hyperthyroidism: Excess thyroid hormone; symptoms include Graves' disease.

Parathyroid Hormone (PTH)

• Regulates blood calcium levels.

• Targets bone, kidney, and intestine.

Adrenal Disorders

• Cushing's syndrome: Excess glucocorticoids; fat redistribution, hyperglycemia.

• Addison's disease: Low aldosterone/cortisol; weight loss, hypotension.

• Adrenogenital syndrome (AGS): Excess adrenal androgens; masculinization.

Diabetes Mellitus

• Type 1: Autoimmune, no insulin.

• Type 2: Insulin resistance, linked to obesity.

• Symptoms: Polyuria, polydipsia, polyphagia, glycosuria, ketonuria, ketoacidosis.

Pineal Gland and Circadian Rhythms

• Melatonin: Sleep-inducing hormone; peaks at night.

• Biological clock: Suprachiasmatic nucleus in hypothalamus synchronizes circadian rhythms.

Summary Table: Major Hormones and Their Functions

Hormone	Source	Main Function
TSH	Pituitary (anterior)	Stimulates thyroid hormone release
ACTH	Pituitary (anterior)	Stimulates adrenal cortex
FSH/LH	Pituitary (anterior)	Regulate gonadal function
ADH	Pituitary (posterior)	Water retention in kidneys
Oxytocin	Pituitary (posterior)	Uterine contraction, milk ejection
Insulin	Pancreas (beta cells)	Lowers blood glucose
Glucagon	Pancreas (alpha cells)	Raises blood glucose
Melatonin	Pineal gland	Regulates sleep/wake cycle

Additional info:

• Some content inferred from context and standard Anatomy & Physiology curriculum, such as the summary tables and expanded explanations of hormone mechanisms.

• Disorders and hormone functions are described with examples for exam preparation.