Overview
This lecture introduces the endocrine system, emphasizes the functions and structures of endocrine glands, explains hormone types, and describes hormone release and regulation.
Introduction to the Endocrine System
- The endocrine system consists of glands that secrete hormones to regulate body functions.
- Endocrine glands make hormones that act on specific target tissues via the bloodstream.
- Some brain regions act as endocrine glands, but terminology can get complex; for simplicity, all are called glands and their products hormones.
Endocrine vs. Exocrine Glands
- Glands are clusters of cells that secrete products.
- Exocrine glands release products onto body surfaces or into ducts (e.g., sweat, oil, salivary, and mammary glands).
- Endocrine glands lack ducts and release hormones directly into the blood.
- Some organs (pancreas, gonads) have both exocrine and endocrine functions.
Hormones and Their Actions
- A hormone is a chemical messenger made by an endocrine gland that affects specific cells (target cells) with matching receptors.
- Hormones only affect cells with specific receptors, altering activities like membrane permeability, protein synthesis, enzyme activity, or cell division.
Types of Hormones
- Steroid hormones can enter cells and influence the DNA in the nucleus.
- Peptide (protein) hormones bind to cell surface receptors and trigger intracellular signaling cascades.
Hormone Release Mechanisms
- Humoral control: Hormone release triggered by changes in blood fluid composition (e.g., calcium levels).
- Neural control: The nervous system directly stimulates hormone release (e.g., adrenal medulla releases epinephrine).
- Hormonal control: One hormone stimulates the release of another hormone (e.g., pituitary hormones regulating other glands).
Regulation of Hormone Levels
- Negative feedback mechanisms regulate hormone release, similar to a thermostat maintaining set conditions.
Key Terms & Definitions
- Endocrine gland — gland that secretes hormones directly into the bloodstream.
- Hormone — a chemical messenger regulating activity in target cells.
- Exocrine gland — gland that releases substances through ducts to body surfaces.
- Steroid hormone — lipid-based hormone that enters cells to affect DNA.
- Peptide hormone — protein-based hormone acting on cell surface receptors.
- Negative feedback — process where a system self-regulates by reducing output when a set point is reached.
Action Items / Next Steps
- Review the list of major endocrine glands in the body.
- Watch the posted video explaining negative feedback using a thermostat analogy.
- Prepare to study specific hormones and their effects in upcoming lessons.