Overview
This lecture introduces the endocrine system, focusing on hormones, their types, mechanisms of action, and regulation, emphasizing their significance in body communication and homeostasis.
Endocrine System Overview
- The endocrine system consists of ductless glands that release hormones directly into the bloodstream.
- Hormones are chemical messengers that affect many body functions including blood sugar, calcium, water balance, stress responses, reproduction, growth, and physical characteristics.
- Endocrinology is the study of endocrine glands and hormones.
Hormone Function and Mechanism
- Hormones act on specific target cells that have the appropriate receptors ("locks") for each hormone ("key").
- Only a small amount of hormone is needed for a significant effect due to their potency.
- Hormones travel in the blood to reach target tissues.
Types of Hormones
- Two major classes: steroid (lipid-based) and non-steroid (protein/peptide-based) hormones.
- Steroid hormones are lipid-soluble and can cross cell membranes to act directly on nuclear DNA (direct gene activation).
- Examples of steroid hormones: estrogen, progesterone, testosterone, cortisol, aldosterone, triiodothyronine, thyroxine.
- Protein (non-steroid) hormones are water-soluble and cannot cross the cell membrane; they act via surface receptors and second messengers (e.g., cyclic AMP).
- Examples include catecholamines (like epinephrine), insulin, vasopressin.
Hormone Action Details
- Steroid hormones: enter cells, bind nuclear receptors, activate genes, and cause protein synthesis.
- Non-steroid hormones: bind to surface receptors, trigger second messengers, activate/inhibit enzymes, alter membrane permeability, or stimulate protein synthesis.
- Steroid response is proportional to hormone-receptor complex formed; non-steroid hormones can have amplification effects (one hormone triggers many reactions).
Prostaglandins
- Prostaglandins are lipid-based chemicals that act locally, are not hormones, cannot be stored, and do not travel in blood.
- Involved in inflammation, blood clotting, and reproductive processes.
- Aspirin blocks prostaglandins, reducing pain, fever, and blood clotting.
Hormone Regulation
- Releasing and inhibiting factors from the hypothalamus control the anterior pituitary gland.
- Negative feedback mechanisms maintain hormone balance and homeostasis.
- Positive feedback (rare) amplifies responses, e.g., oxytocin during labor.
- Neural control can directly stimulate glands, especially during fight-or-flight (adrenaline release).
Key Terms & Definitions
- Hormone — Chemical messenger secreted by glands to regulate body functions.
- Endocrinology — Study of ductless glands and hormones.
- Steroid hormones — Fat-based hormones that cross cell membranes and act directly on DNA.
- Protein/Peptide hormones — Water-based hormones that act via cell surface receptors and second messengers.
- Prostaglandins — Local lipid-based chemicals involved in inflammation and other processes, not true hormones.
- Negative feedback — Regulatory mechanism that maintains homeostasis by counteracting changes.
- Positive feedback — Self-amplifying mechanism, rare in the body.
- Second messenger (cAMP) — Intracellular signaling molecule activated by non-steroid hormones.
Action Items / Next Steps
- Review the two hormone classes and their mechanisms.
- Understand negative and positive feedback with hormone examples.
- Prepare to learn specific hormones and their functions in upcoming material.