Overview
This lecture introduces the endocrine system, explaining what hormones are, how they are released and transported, their types, and how they interact with receptors.
Endocrine System Basics
- The endocrine system consists of cells, tissues, and glands that release chemical messengers (hormones) into the bloodstream.
- Some organs (e.g., stomach) have both primary functions and additional endocrine functions.
- Hormones are chemical messengers that travel through the blood to target tissues with specific receptors.
Endocrine vs. Nervous System
- The nervous system communicates rapidly and directly via neurotransmitters, while the endocrine system communicates slowly and indirectly via hormones.
- Neurotransmitters act quickly and short-term; hormones act more slowly and have longer-lasting effects.
Types of Hormones
- Three main hormone categories: protein/peptide, steroid, and amino acid-derived (from tyrosine).
- Protein/peptide hormones (>100 amino acids = protein, <100 = peptide) are the most abundant and include all pituitary hormones, insulin, and glucagon.
- Steroid hormones are derived from cholesterol, are lipid soluble, and are produced in the adrenal cortex (aldosterone, cortisol, androgens) and gonads (estrogen, progesterone, testosterone).
- Amino acid-derived hormones include catecholamines (adrenaline, noradrenaline, dopamine) and thyroid hormones (T3, T4).
Hormone Release Stimuli
- Neural stimulus: neurons trigger hormone release (e.g., sympathetic stimulation of adrenal medulla for adrenaline).
- Hormonal stimulus: one hormone triggers release of another hormone (often with "tropic" in the name).
- Humoral stimulus: nutrients or minerals in the blood (e.g., glucose triggering insulin release, calcium triggering parathyroid hormone).
Hormone Transport in Blood
- Protein/peptide hormones travel freely in the blood (hydrophilic).
- Steroid hormones require carrier proteins (e.g., albumin) due to being lipid-soluble; free hormone is the active form.
- Catecholamines move freely; thyroid hormones need carrier proteins due to iodine making them non-water soluble.
Hormone Receptors and Target Cell Interaction
- Hormones are effective at very low concentrations (10^-7 to 10^-12 molar), requiring high affinity and specificity of receptors.
- Peptide hormones and catecholamines bind to cell membrane receptors.
- Steroid hormones bind to cytoplasmic receptors, affecting gene transcription in the nucleus.
- Thyroid hormones use membrane transporters, then bind to nuclear receptors to regulate gene expression.
Key Terms & Definitions
- Endocrine system — cells, tissues, and glands releasing hormones into the bloodstream.
- Hormone — chemical messenger released into blood, affecting distant target tissues.
- Protein/peptide hormone — hormone made of amino acids; water-soluble.
- Steroid hormone — hormone derived from cholesterol; lipid-soluble.
- Amino acid-derived hormone — hormone derived from tyrosine; includes catecholamines and thyroid hormones.
- Neural stimulus — neuron triggers hormone release.
- Hormonal stimulus — hormone triggers another hormone's release.
- Humoral stimulus — nutrient/mineral triggers hormone release.
- Affinity — strength of hormone-receptor binding.
- Specificity — receptor's preference for a particular hormone.
Action Items / Next Steps
- Review pituitary, adrenal, pancreatic, and thyroid hormone examples and their classifications.
- Understand and memorize three main hormone release stimuli.
- Study the mechanisms of hormone transport and receptor interaction.