Endocrine System Overview

Introduction

• Endocrine vs. Exocrine Glands

  • Endocrine glands: Ductless, hormones released into the bloodstream.
  • Exocrine glands: Have ducts, secrete substances outside the body or into the digestive tract.

• Endocrine vs. Nervous System

  • Endocrine: Slow, hormonal communication, widespread effects.
  • Nervous: Fast, electrical communication, targeted effects.

Communication Mechanisms

1. Gap Junctions: Direct cell-to-cell communication via pores.
2. Neurotransmitters: Released by neurons, synaptic communication.
3. Paracrine (Local Hormones): Chemical secreted into tissue fluid affecting nearby cells.
4. Hormones: Chemical messengers secreted into the blood affecting distant cells.

Endocrine Glands Overview

• Brain: Pineal gland, hypothalamus, pituitary.
• Neck/Thorax: Thyroid, parathyroid, thymus.
• Abdomen/Pelvis: Adrenal glands, pancreas, gonads (testes/ovaries).

Endocrine System Details

Endocrine vs. Exocrine

• Endocrine: No ducts, bloodstream transport.
• Exocrine: Ducts present, secretion to surfaces or cavities.

The Nervous and Endocrine Systems

• Similarities: Chemicals can function both as neurotransmitters and hormones (e.g., epinephrine).
• Differences:
  • Speed: Nervous system is faster.
  • Scope: Endocrine system affects multiple organs.

Target Cells and Receptors

• Hormones affect only target cells with specific receptors.
• Receptors are specific and can become saturated.

Hypothalamus and Pituitary Gland

Structure

• Hypothalamus: W-shaped, central in the brain, connects to pituitary via the infundibulum.
• Pituitary Gland: Composed of anterior and posterior lobes with different embryonic origins and functions.

Hormone Production

• Hypothalamus: Produces 8 hormones.
  • 6 control anterior pituitary.
  • 2 stored in posterior pituitary (oxytocin, ADH).

Anterior Pituitary Hormones

• FSH & LH: Target gonads (gamete production, ovulation, testosterone secretion).
• TSH: Stimulates thyroid hormone secretion.
• ACTH: Stimulates adrenal cortex to secrete glucocorticoids.
• Prolactin: Stimulates milk synthesis.
• Growth Hormone: Stimulates growth and cell reproduction.

Posterior Pituitary Hormones

• Oxytocin: Stimulates uterine contractions, milk release, emotional bonding.
• ADH: Retains water by acting on kidneys.

Hormone Regulation

Regulation Mechanisms

• Negative Feedback: Most common; maintains hormone levels within a target range.
• Positive Feedback: Less common; amplifies a response (e.g., oxytocin during labor).

Hormone Transport and Action

• Hydrophilic Hormones: Travel freely in blood.
• Hydrophobic (Steroid) Hormones: Bind to transport proteins.

Steroid vs. Peptide Hormones

• Steroid Hormones: Derived from cholesterol, penetrate cell membranes, act within cells.
• Peptide/Monoamines: Amino acid-based, bind to surface receptors, use second messenger systems.

Endocrine Gland Functions

Thyroid and Parathyroid

• Thyroid Hormones (T3 & T4): Increase metabolic rate.
• Calcitonin: Lowers blood calcium.
• Parathyroid Hormone: Increases blood calcium.

Adrenal Glands

• Medulla: Epinephrine/norepinephrine (fight-or-flight response).
• Cortex: Produces mineralocorticoids (aldosterone), glucocorticoids (cortisol), sex steroids.

Pancreas

• Insulin: Lowers blood glucose.
• Glucagon: Raises blood glucose.

Gonads

• Ovaries: Estrogen, progesterone (regulate reproductive cycles).
• Testes: Testosterone (sperm production, secondary sex characteristics).

Hormone Interactions and Effects

Types of Hormone Interactions

• Synergistic: Hormones work together for a greater effect.
• Permissive: One hormone enhances the effect of another.
• Antagonistic: Hormones have opposite effects.

Signal Amplification

• Hormones trigger significant effects even in small concentrations.

Stress Response

• General Adaptation Syndrome: Alarm (fight-or-flight), Resistance (cortisol), Exhaustion (homeostasis breakdown).

Endocrine Disorders

Types

• Hyposecretion: Insufficient hormone production (e.g., diabetes insipidus).
• Hypersecretion: Excess hormone production (e.g., gigantism).

Diabetes Mellitus

• Type 1: Insulin deficiency, usually genetic.
• Type 2: Insulin resistance, often lifestyle-related.

Other Disorders

• Cushing Syndrome: Excess cortisol.
• Hyperparathyroidism: Excess parathyroid hormone.