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PREFINAL-ANAPHY

Chapter 17: The Endocrine System

Overview of the Endocrine System

  • Nervous System vs. Endocrine System

    • Both systems facilitate communication within the body.

    • Nervous System: Uses electrical signals (neurotransmitters) for fast communication.

    • Endocrine System: Utilizes hormones (chemical signals) that travel through the bloodstream, affecting many cells.

Functions of the Endocrine System

  • Maintains homeostasis by regulating:

    • Nutrient and calorie usage.

    • Waste secretion.

    • Blood pressure and osmolarity.

    • Growth, fertility, and sex drive.

    • Sleep and lactation.

Signaling Types

  • Hormones are chemical messengers released into the blood.

  • Types of Signaling:

    • Endocrine Signaling: Hormones affect distant cells via the bloodstream.

    • Paracrine Signaling: Hormones affect nearby cells.

    • Autocrine Signaling: Hormones affect the same cell that released them.

Glands of the Endocrine System

  • Endocrine Glands: Include the pituitary, thyroid, parathyroid, adrenal, and pineal glands.

    • Produce hormones and lack ducts. Secretion occurs directly into blood or interstitial fluid.

  • Other Organs with Endocrine Functions:

    • Hypothalamus, thymus, heart, kidneys, stomach, intestine, liver, fat tissue, ovaries, testes.

Target Cells and Hormonal Action

  • Hormones can reach nearly every cell but only affect target cells (have specific receptors).

  • Binding to receptors initiates signaling pathways inside target cells.

Comparison of Nervous and Endocrine Systems

  • Nervous system effects are rapid and short-lived; endocrine effects are more prolonged and widespread.

Hormones

Types of Hormones

  • Steroid Hormones:

    • Lipid-based, pass easily through cell membranes.

    • Examples: Testosterone, estrogens.

  • Amine Hormones:

    • Modified amino acids, typically water-soluble.

    • Examples: Melatonin, epinephrine.

  • Peptide and Protein Hormones:

    • Chains of amino acids.

    • Examples: Insulin, antidiuretic hormone (ADH).

Hormone Production & Release

  • Steroid Hormones: Synthesized on demand from cholesterol.

  • Peptide Hormones: Synthesized and stored until released.

Hormone Receptors

  • Types of receptors:

    • Intracellular Receptors: For lipid-soluble hormones (steroid, thyroid).

    • Membrane-bound Receptors: For water-soluble hormones; these activate second messenger systems.

Second Messenger Systems

  • Hormone binding activates a cascade of intracellular effects via second messengers like cAMP (cyclic adenosine monophosphate).

  • Amplification allows small amounts of hormone to have significant effects.

Endocrine Control Mechanisms

Regulation of Hormone Secretion

  • Negative Feedback Loops: Commonly regulate hormonal levels (e.g., high levels inhibit secretion).

  • Positive Feedback: E.g., oxytocin during childbirth.

Factors Influencing Hormone Release

  • Chemical levels, nervous system stimulation, tropic hormones from other endocrine glands.

The Hypothalamus and Pituitary Gland

Structure and Function

  • The hypothalamus connects to the pituitary gland, dictating its hormone release through the hypothalamic-hypophyseal portal system.

  • Posterior Pituitary: Stores and releases hormones (ADH, oxytocin) produced in the hypothalamus.

  • Anterior Pituitary: Secretes hormones regulated by hypothalamic hormones (e.g., growth hormone, prolactin).

The Thyroid Gland

Location and Function

  • Located anterior to the trachea. Produces thyroid hormones (T3, T4) affecting metabolism.

  • Calcitonin: Lowers blood calcium levels by inhibiting osteoclasts.

Hormonal Regulation

  • Negative feedback mechanism influences T3 and T4 production.

Adrenal Glands

Structure

  • Located atop each kidney, composed of cortex and medulla.

  • Cortex: Secretes corticosteroids (e.g., cortisol, aldosterone).

  • Medulla: Secretes catecholamines (e.g., epinephrine).

Pancreas

Function

  • Has both endocrine (insulin, glucagon) and exocrine functions (digestive enzymes).

  • Regulates blood glucose levels through insulin and glucagon.

Other Endocrine Organs

  • Thymus: Produces thymosins which support T lymphocyte maturation.

  • Heart: Secretes atrial natriuretic peptide.

  • Gastrointestinal Tract: Endocrine cells stimulate digestion.

  • Adipose Tissue: Secretes hormones like leptin that regulate appetite.

Conclusion

  • The endocrine system plays a crucial role in maintaining homeostasis and regulating various bodily functions through a complex network of hormones and their target cells.