Endocrine System: Hormones, Glands, and Regulation – Study Notes

Endocrine System Overview

Introduction

The endocrine and nervous systems coordinate the activities of all body systems. The nervous system uses electrical signals and neurotransmitters, while the endocrine system uses hormones produced by endocrine structures to regulate physiological processes.

• Nervous system: Fast-acting, uses neurons and neurotransmitters.

• Endocrine system: Uses hormones released into the bloodstream to affect distant organs.

Types of Hormones

Classification of Hormones

Hormones are chemical messengers that can act locally or at distant sites in the body. They are classified based on their source, solubility, and mechanism of action.

• Local hormones: Act near their site of secretion (paracrine and autocrine).

• Endocrine hormones: Secreted into interstitial fluid, then absorbed into the bloodstream to act on distant target cells.

Solubility of Hormones

Chemical Classes

Hormones are divided into two broad chemical classes: lipid-soluble and water-soluble. This classification affects how hormones are transported and how they interact with target cells.

• Lipid-soluble hormones: Steroid hormones (derived from cholesterol), thyroid hormones (T3 and T4), and nitric oxide.

• Water-soluble hormones: Peptide and protein hormones, amine hormones, and eicosanoids (prostaglandins and leukotrienes).

Mechanism of Hormone Action

• Lipid-soluble hormones: Bind to receptors inside the cell (cytoplasm or nucleus), directly affecting gene expression.

• Water-soluble hormones: Bind to receptors on the cell surface, triggering second messenger systems (e.g., cAMP).

Transport and Action of Hormones

Transport Proteins

Lipid-soluble hormones require transport proteins in the blood for solubility and delivery to target cells. Water-soluble hormones travel freely in plasma.

Hormone-Receptor Interactions

• Up-regulation: Target cells increase receptor number in response to low hormone levels.

• Down-regulation: Target cells decrease receptor number in response to high hormone levels.

• Synergism: Multiple hormones produce greater effects together.

• Antagonism: One hormone opposes the action of another.

Endocrine Glands and Hormones

Major Endocrine Glands

Endocrine glands secrete hormones directly into the bloodstream. The main glands include:

• Pituitary gland

• Thyroid gland

• Parathyroid glands

• Adrenal glands

• Pineal gland

• Other organs with endocrine functions: hypothalamus, thymus, pancreas, ovaries, testes, kidneys, stomach, liver, small intestine, skin, heart, placenta

The Hypothalamus

The hypothalamus is the major link between the nervous and endocrine systems. It receives input from various regions and regulates pituitary gland activity.

The Pituitary Gland

• Infundibulum: Stalk connecting the pituitary to the hypothalamus.

• Divisions: Anterior pituitary (adenohypophysis) and posterior pituitary (neurohypophysis).

The Adenohypophysis (Anterior Pituitary)

The anterior pituitary is connected to the hypothalamus by a portal blood system, allowing regulatory hormones from the hypothalamus to control its activity.

• Hypophyseal portal system: Two capillary networks connecting hypothalamus and anterior pituitary.

• Releasing and inhibiting hormones: Secreted by hypothalamic neurosecretory cells.

Anterior Pituitary Hormones

Hypothalamus Hormone	Hormone released from Adenohypophysis	Major Function/Target
Growth hormone-releasing hormone (GHRH)	Growth hormone (GH)	Stimulates growth, cell reproduction, and regeneration
Corticotropin-releasing hormone (CRH)	Adrenocorticotropic hormone (ACTH)	Stimulates release of glucocorticoids from adrenal cortex
Gonadotropin-releasing hormone (GnRH)	Follicle-stimulating hormone (FSH), Luteinizing hormone (LH)	Regulates reproductive processes
Thyrotropin-releasing hormone (TRH)	Thyroid-stimulating hormone (TSH)	Stimulates thyroid hormone release
Additional info: Melanocyte-stimulating hormone (MSH)	Additional info: MSH	Additional info: Influences skin pigmentation

Hormone Regulation and Feedback

Control of Hormone Secretion

Hormone secretion is regulated by signals from the nervous system, chemical changes in the blood, and other hormones. Feedback mechanisms maintain homeostasis.

• Negative feedback: Hormone output reverses a particular stimulus (e.g., blood calcium regulation by parathyroid hormone).

• Positive feedback: Hormone output enhances or reinforces the stimulus (e.g., oxytocin during childbirth).

Posterior Pituitary (Neurohypophysis)

Hormones Released

• Oxytocin: Stimulates uterine contractions and milk ejection.

• Antidiuretic hormone (ADH): Regulates water reabsorption in kidneys and blood pressure.

Pituitary Gland Disorders

Acromegaly

• Caused by excess growth hormone (HGH) during adulthood.

• Symptoms: Enlargement of bones in face, jaw, hands, and feet.

Diabetes Insipidus

• Caused by insufficient ADH release.

• Symptoms: Excessive urination, inability to concentrate urine.

Thyroid Gland

Structure and Function

• Located inferior to the larynx, butterfly-shaped.

• Contains two lobes connected by an isthmus.

• Composed of spherical groups of follicular cells (thyroid follicles).

• Stores a 100-day supply of hormones in inactive form (colloid).

Thyroid Hormones

• TGB (Thyroglobulin): Glycoprotein used to synthesize thyroid hormones.

• Thyroid hormones: T3 (triiodothyronine) and T4 (thyroxine).

• Synthesis: Involves trapping iodide, oxidation, and coupling to form T3 and T4.

Thyroid Hormone Synthesis Steps

1. Iodide trapping by follicular cells.

2. Oxidation of iodide to iodine.

3. Iodination of tyrosine residues in thyroglobulin.

4. Coupling of T1 and T2 to form T3 and T4.

5. Release of hormones into the bloodstream.

Key Terms and Definitions

• Hormone: Chemical messenger secreted by endocrine glands.

• Endocrine gland: Ductless gland that releases hormones into the blood.

• Target cell: Cell with specific receptors for a hormone.

• Second messenger: Intracellular signaling molecule (e.g., cAMP).

• Negative feedback: Mechanism that reverses a change in a physiological variable.

Important Equations

• Hormone-Receptor Binding:

$H + R \rightarrow HR$

• Thyroid Hormone Synthesis:

$\text{Tyrosine} + n\,\text{Iodine} \rightarrow \text{T3 or T4}$

Examples and Applications

• Example: Parathyroid hormone (PTH) increases blood calcium by stimulating bone resorption and increasing calcium reabsorption in kidneys.

• Example: Oxytocin release during childbirth enhances uterine contractions (positive feedback).

Additional info: Some details about hormone mechanisms, feedback, and synthesis steps were expanded for clarity and completeness.