Endocrine System and Hormonal Regulation: Study Guide Notes

Chapter 7: Introduction to the Endocrine System

Basic Ways Hormones Act on Target Cells

Hormones are chemical messengers that regulate physiological processes by acting on specific target cells. Their mechanisms of action can be categorized as follows:

• Control of Enzyme Activity: Hormones can activate or inhibit enzymes within the cell, altering metabolic pathways.

• Control of Membrane Transport: Hormones can regulate the movement of ions or molecules across cell membranes.

• Control of Gene Expression: Hormones can influence transcription and translation, leading to changes in protein synthesis.

Example: Insulin increases glucose uptake by promoting the insertion of glucose transporters into the cell membrane.

Definition and General Function of Hormones

• Hormones: Chemical signals secreted by endocrine glands into the blood, affecting distant target cells.

• General Function: Regulation of growth, metabolism, reproduction, and homeostasis.

Cellular Mechanisms of Hormone Action

• Hormones bind to specific receptors on or in target cells, initiating signal transduction pathways.

• Responses depend on hormone type (peptide, steroid, or amine) and receptor location.

Classification of Hormones

• Peptide Hormones: Made from amino acids; water-soluble; act via membrane receptors.

• Steroid Hormones: Derived from cholesterol; lipid-soluble; act via intracellular receptors.

• Amine Hormones: Derived from tyrosine or tryptophan; include catecholamines and thyroid hormones.

Peptide Hormone Synthesis and Release

• Peptide hormones are synthesized as preprohormones, processed to prohormones, and then cleaved to active hormones.

• Stored in vesicles and released by exocytosis.

Comparison: Peptide vs. Steroid Hormones

Feature	Peptide Hormones	Steroid Hormones
Solubility	Water-soluble	Lipid-soluble
Receptor Location	Cell membrane	Intracellular
Storage	Vesicles	Not stored; synthesized on demand
Action Speed	Fast	Slow

Hormone Secretion and Regulation

• Hormone release is regulated by neural, hormonal, or humoral stimuli.

• Negative feedback is the primary mechanism for maintaining hormone levels.

Endocrine Glands and Hormone Examples

• Pituitary Gland: Releases growth hormone, ACTH, TSH, etc.

• Adrenal Cortex: Releases cortisol, aldosterone.

• Thyroid Gland: Releases thyroxine (T4), triiodothyronine (T3).

Hormone Pathways and Feedback Loops

• Simple Endocrine Reflex: Endocrine cell senses stimulus and releases hormone directly.

• Complex Endocrine Reflex: Involves multiple steps, often including the hypothalamus and pituitary.

• Negative Feedback: Hormone effects reduce further hormone release.

Hypothalamic-Pituitary Axis

• Anterior Pituitary: Controlled by hypothalamic releasing/inhibiting hormones via portal system.

• Posterior Pituitary: Releases hormones produced in hypothalamus (oxytocin, vasopressin).

Hormone Interactions

• Synergism: Combined effect of hormones is greater than sum of individual effects.

• Permissiveness: One hormone enables another to act.

• Antagonism: Hormones have opposing effects.

Chapter 23: Endocrine Control of Growth and Metabolism

Adrenal Cortex Hormones

The adrenal cortex secretes several steroid hormones, each produced in specific zones:

Zone	Hormone	Generalized Effect
Zona Glomerulosa	Aldosterone	Regulates sodium and potassium balance
Zona Fasciculata	Cortisol	Regulates metabolism, stress response
Zona Reticularis	Androgens	Sex hormone precursors

Cortisol Effects

• Cortisol: Has a catabolic effect; increases blood glucose, suppresses immune function, promotes protein and fat breakdown.

• Effects of Hypercortisolism: Cushing's syndrome (muscle wasting, fat redistribution, hypertension).

• Effects of Hypocortisolism: Addison's disease (fatigue, weight loss, hypotension).

Thyroid Hormones

• Thyroid Hormones: T3 (triiodothyronine) and T4 (thyroxine) regulate metabolism, growth, and development.

• Unique Features: Contain iodine; act via nuclear receptors; increase basal metabolic rate.

• Regulation: Controlled by TSH from anterior pituitary.

Thyroid Disorders

• Hyperthyroidism: Excess thyroid hormone; symptoms include weight loss, heat intolerance, tachycardia.

• Hypothyroidism: Deficient thyroid hormone; symptoms include weight gain, cold intolerance, bradycardia.

Calcium Homeostasis

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating bone resorption, kidney reabsorption, and activation of vitamin D.

• Calcitonin: Lowers blood calcium by inhibiting bone resorption.

• Vitamin D: Increases intestinal absorption of calcium.

Growth Hormone

• Growth Hormone (GH): Stimulates growth, protein synthesis, and increases blood glucose.

• Disorders: Gigantism (excess GH in children), acromegaly (excess GH in adults), dwarfism (deficient GH).

Bone Disorders

• Osteoporosis: Decreased bone mass and increased fracture risk.

• Paget's Disease: Abnormal bone remodeling.

Additional Info

• ACTH: Adrenocorticotropic hormone; stimulates cortisol release from adrenal cortex.

• Insulin: Lowers blood glucose by promoting cellular uptake.

• Glucagon: Raises blood glucose by promoting glycogen breakdown.