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.