Introduction to the Endocrine System

Definition and Overview

• Endocrine System: Collection of cells, tissues, and glands that release chemical messengers into the bloodstream.
  • Includes explicit endocrine tissues (e.g., glands) and other organs (e.g., stomach) that perform endocrine functions.
• Hormones: Chemical messengers released into the bloodstream.
  • Different from neurotransmitters in the nervous system.

Comparison to the Nervous System

• Nervous System
  • Fast and direct communication.
  • Short-acting chemical messengers (neurotransmitters).
• Endocrine System
  • Slower communication (dependent on blood flow).
  • Indirect and longer-acting chemical messengers (hormones).

Types of Hormones

1. Protein/Peptide Hormones
   • More than 100 amino acids = Protein; less than 100 = Peptide.
   • Most abundant type.
   • Examples: Pituitary hormones (e.g., FSH, LH, ACTH, GH, Prolactin, TSH), Pancreatic hormones (e.g., Insulin, Glucagon).
2. Steroid Hormones
   • Derived from cholesterol.
   • Lipid-soluble.
   • Produced by adrenal cortex (e.g., Aldosterone, Cortisol, Androgens) and gonads (e.g., Estrogen, Progesterone, Testosterone).
3. Amino Acid-Derived Hormones
   • Derived from tyrosine.
   • Catecholamines (e.g., Adrenaline, Noradrenaline, Dopamine) from adrenal medulla.
   • Thyroid hormones (T3, T4) from thyroid gland.

Hormone Release Triggers

1. Neural Stimulus
   • Neurons trigger hormone release (e.g., Adrenaline from adrenal medulla).
2. Hormonal Stimulus
   • Hormones trigger release of other hormones (e.g., ACTH release).
   • Often includes 'tropic' hormones.
3. Humoral Stimulus
   • Nutrients or minerals trigger hormone release (e.g., Glucose triggers insulin from pancreas).

Hormone Transport in Bloodstream

• Peptides: Travel freely in bloodstream (hydrophilic).
• Steroids: Bound to plasma proteins (e.g., Albumin) for transport.
• Catecholamines: Transported freely.
• Thyroid Hormones: Require carrier proteins due to iodine.

Receptor Interaction

• Concentration: Hormones are in low concentration; high affinity and specificity needed for receptors.
• Receptor Locations:
  • Peptides/Catecholamines: Bind to membrane receptors.
  • Steroids: Bind to cytoplasmic receptors; involve transcription factors.
  • Thyroid Hormones: Use membrane transporters to reach nuclear receptors.

Key Takeaways

• Hormones are crucial for communication in the body, working alongside the nervous system.
• Understanding hormone types, triggers, transport, and receptor interactions is vital for grasping endocrine system functionality.