Endocrine System: Structure, Function, and Hormonal Regulation

Endocrine System Overview

Introduction to the Endocrine System

The endocrine system is a network of glands and tissues that produce and secrete hormones, which regulate various physiological processes to maintain homeostasis. Hormones act as chemical messengers, influencing growth, metabolism, reproduction, and other vital functions.

• Homeostasis: The endocrine system helps maintain internal balance by regulating the activity and growth of target cells and metabolic processes.

• Hormones: Specialized chemical messengers released into the extracellular fluid (ECF) and often transported via the bloodstream to distant target cells.

Chemical Categories of Hormones

Types of Hormones

Hormones are classified based on their chemical structure, which determines their solubility and mechanism of action.

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

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

• Eicosanoids: Biologically active lipids derived from fatty acids (e.g., prostaglandins, leukotrienes).

Endocrine vs. Nervous System

Comparison of Regulatory Systems

The endocrine and nervous systems both contribute to homeostasis but differ in their signaling mechanisms and response times.

• Endocrine System: Uses hormones; slower, longer-lasting effects.

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

Functions Regulated:

• Chemical composition and volume of interstitial fluid

• Metabolism and energy balance

• Muscle contraction (smooth and cardiac)

• Glandular secretions

• Immune system activities

• Growth, development, and reproduction

Hormones, Receptors, and Glands

Hormone Action and Target Cells

Hormones affect only those cells with specific receptors, either on the plasma membrane or inside the cell. The interaction between hormone and receptor initiates cellular changes.

• Endocrine glands: Ductless glands that secrete hormones directly into the ECF and bloodstream.

• Exocrine glands: Have ducts; secrete non-hormonal substances (e.g., sweat, saliva).

• Dedicated vs. mixed glands: Some organs (e.g., pancreas) have both endocrine and exocrine functions.

Types of Hormonal Signaling

Endocrine, Paracrine, and Autocrine

Hormones can act in different ways depending on their site of action:

• Endocrine: Hormones travel through the bloodstream to distant target cells.

• Paracrine: Hormones act on nearby cells within the same tissue.

• Autocrine: Hormones act on the same cell that secreted them.

Water-Soluble vs. Lipid-Soluble Hormones

Classification and Mechanism of Action

The solubility of a hormone determines its transport in blood and its mechanism of action at the target cell.

• Lipid-soluble hormones: Steroids, thyroid hormones, nitric oxide. Receptors are inside the cell (cytoplasm or nucleus). Mechanism: Direct gene activation.

• Water-soluble hormones: Amines, peptides, proteins, eicosanoids. Receptors are on the plasma membrane. Mechanism: Second messenger system.

Lipid-Soluble Hormones	Water-Soluble Hormones
Steroids (Aldosterone, Cortisol, Estrogen, Progesterone, Testosterone)	Amines (Melatonin, Epinephrine, Norepinephrine, Serotonin, Histamine)
Thyroid hormones (T3, T4)	Proteins/Peptides (Insulin, Glucagon, hGH, TSH, ACTH, FSH, LH, PRL, OT, ADH)
Nitric oxide (NO)	Eicosanoids (Prostaglandins, Leukotrienes)
Transported attached to proteins	Freely circulate in blood
Receptors inside cell	Receptors on plasma membrane
Direct gene activation	Second messenger system

Hormone Transport

Blood Transport Mechanisms

Hormones are transported in the blood according to their solubility:

• Water-soluble hormones: Circulate freely, unattached.

• Lipid-soluble hormones: Bound to transport proteins (e.g., alpha and beta globulins) to become temporarily water-soluble.

Mechanisms of Hormonal Action

Second Messenger System (Water-Soluble Hormones)

Water-soluble hormones bind to cell-surface receptors, triggering a cascade of intracellular events via second messengers.

• Hormone binds to plasma membrane receptor.

• Activates G-protein, which stimulates adenylate cyclase.

• Adenylate cyclase converts ATP to cyclic AMP (cAMP).

• cAMP activates protein kinases, which phosphorylate target proteins.

• Phosphodiesterase inactivates cAMP.

Key equation:

$\text{ATP} \xrightarrow{\text{adenylate cyclase}} \text{cAMP}$

Direct Gene Activation (Lipid-Soluble Hormones)

Lipid-soluble hormones diffuse into the cell and bind to intracellular receptors, directly influencing gene expression.

• Hormone diffuses through plasma membrane.

• Binds to receptor in cytoplasm or nucleus.

• Hormone-receptor complex binds to DNA, initiating transcription.

• mRNA directs synthesis of specific proteins.

Key equation:

$\text{Hormone} + \text{Receptor} \rightarrow \text{Hormone-Receptor Complex} \rightarrow \text{DNA Activation} \rightarrow \text{mRNA} \rightarrow \text{Protein Synthesis}$

Regulation of Hormonal Secretion

Feedback Mechanisms

Hormone secretion is tightly regulated, primarily by negative feedback mechanisms.

• Negative feedback: Most hormones; increased hormone levels inhibit further secretion.

• Positive feedback: Rare; e.g., oxytocin during childbirth, LH surge during ovulation.

• Regulatory signals:

  • Hormonal stimulation (other hormones)

  • Humoral stimulation (blood chemistry changes)

  • Neural stimulation (nervous system signals)

Major Endocrine Organs and Hormones

Pituitary Gland (Hypophysis)

The pituitary gland is divided into two parts: anterior (adenohypophysis) and posterior (neurohypophysis).

• Anterior pituitary: Produces hGH, TSH, ACTH, FSH, LH, PRL.

• Posterior pituitary: Stores and releases OT (oxytocin) and ADH (antidiuretic hormone), produced by the hypothalamus.

Hormone	Production Site	Target Site	Action	Response	Disorder
hGH	Anterior pituitary	Bones, cartilage, liver	Stimulates growth, protein synthesis	Increased size, blood glucose	Hypo: dwarfism; Hyper: gigantism/acromegaly
TSH	Anterior pituitary	Thyroid gland	Stimulates thyroid hormone production	Increased metabolism	Hypo: hypothyroidism; Hyper: hyperthyroidism
ACTH	Anterior pituitary	Adrenal cortex	Stimulates cortisol production	Stress response	Hypo: Addison's disease; Hyper: Cushing's syndrome

Thyroid and Parathyroid Glands

• Thyroid hormones (T3, T4): Increase basal metabolic rate (BMR), stimulate glycolysis and lipolysis.

• Calcitonin (CT): Inhibits osteoclasts, lowers blood calcium.

• Parathyroid hormone (PTH): Stimulates osteoclasts, increases blood calcium.

Adrenal Glands

• Adrenal cortex: Produces corticosteroids (aldosterone, cortisol, androgens).

• Adrenal medulla: Produces catecholamines (epinephrine, norepinephrine) for fight-or-flight response.

Pancreas

• Insulin (beta cells): Lowers blood glucose by stimulating glycogenesis, glycolysis, lipogenesis, and protein synthesis.

• Glucagon (alpha cells): Raises blood glucose by stimulating glycogenolysis, gluconeogenesis, and lipolysis.

Hormone	Production Site	Target Site	Action	Response	Disorder
Insulin	Pancreas (beta cells)	Liver, muscle, adipose	Stimulates glucose uptake, storage	Decreased blood glucose	Hypo: Diabetes Mellitus
Glucagon	Pancreas (alpha cells)	Liver	Stimulates glucose release	Increased blood glucose	Hyper: Hyperglycemia

Selected Endocrine Disorders

Common Diseases

• Dwarfism: Hyposecretion of hGH in children; poor growth.

• Gigantism/Acromegaly: Hypersecretion of hGH; abnormal growth.

• Addison's disease: Hyposecretion of ACTH/cortisol; fatigue, low blood sugar.

• Cushing's syndrome: Hypersecretion of ACTH/cortisol; high blood sugar, obesity.

• Diabetes Mellitus: Hyposecretion of insulin; high blood glucose.

• Hypothyroidism: Poor growth, delayed reflexes, weight gain.

• Hyperthyroidism: Weight loss, goiter, bulging eyes.

Summary Table: Hormone Characteristics

Hormone	Stimulus	Production Site	Target Site	Action	Response	Disorder
hGH	Growth signals	Anterior pituitary	Bones, liver	Growth, metabolism	Increased size	Dwarfism, gigantism
Insulin	High blood glucose	Pancreas (beta cells)	Liver, muscle, adipose	Glucose uptake	Lowered blood glucose	Diabetes Mellitus
Glucagon	Low blood glucose	Pancreas (alpha cells)	Liver	Glucose release	Raised blood glucose	Hyperglycemia

Example: Insulin and Diabetes Mellitus

Insulin is secreted in response to high blood glucose. It promotes glucose uptake and storage, lowering blood glucose levels. Diabetes Mellitus results from hyposecretion or ineffective action of insulin, leading to chronic hyperglycemia.

Additional info: Some details, such as the full list of hormones and their actions, were inferred and expanded for clarity and completeness.