CHPT 17 Functional Organization of the Endocrine System

Functional Organization of the Endocrine System

Overview of the Endocrine System

• Definition: Composed of ductless glands (endocrine) that secrete hormones directly into the bloodstream.
• Functions:
  • Regulates metabolism (e.g., glucose homeostasis, blood pressure).
  • Manages electrolyte balance and water regulation (blood pH, Na⁺, K⁺, Ca²⁺).
  • Stimulates growth and development (bone growth, muscle enlargement).
  • Controls reproduction through hormone regulation in both sexes.

Comparison of Nervous and Endocrine Systems

• Nervous System:

  • Communicates via electrical impulses and neurotransmitters.
  • Fast response (1-10 msec) at specific target cells.
  • Effect stops quickly after stimulus cessation.

• Endocrine System:

  • Communicates through hormones in blood for general distribution.
  • Slower response (seconds to days), may continue long after stimulus.
  • Response is based on hormone concentration amplitude.

Major Endocrine Glands

• List of glands to memorize:
  • Hypothalamus
  • Pineal gland
  • Pituitary gland
  • Thyroid
  • Thymus
  • Parathyroids (posterior part of thyroid)
  • Adrenals
  • Pancreas (islets)
  • Ovaries (female)
  • Testes (male)

Hormone Types and Mechanism of Action

• Hormones: Chemical messengers that regulate bodily functions.
• Types:
  • Water-soluble hormones: proteins, peptides.
  • Lipid-soluble hormones: steroids, amino acid derivatives.
• Receptors:
  • Hormones must bind to specific receptors to work.
  • Cell Interaction categories:
  • Extracellular (membrane-bound receptors).
  • Intracellular (nuclear receptors).

Transport of Hormones

• Free Hormones: Activate target cells quickly, blood level fluctuates.
• Bound Hormones:
  • Circulate longer, provide stable hormone levels.
  • Use binding proteins for transport.

Hormonal Regulation Mechanisms

• Control Mechanisms:
  • Humoral Control: Substances (e.g., glucose, calcium) trigger secretion.
  • Neural Control: Nerve impulses stimulate hormone release (e.g., adrenal medulla releasing epinephrine).
  • Hormonal Control: Hormones from one gland stimulate secretion from another (tropic hormones).

Feedback Mechanism

• Negative Feedback:
  • Hormone secretion is inhibited by its own effect, ensuring balance.
  • Example: Thyroid hormones inhibit their own production.
• Positive Feedback:
  • Hormone secretion is amplified by its effect, self-perpetuating.
  • Example: Oxytocin during labor.

Upregulation and Downregulation

• Receptor Sensitivity:
  • Upregulation: Increased receptors lead to heightened hormone response (e.g., oxytocin receptors in pregnancy).
  • Downregulation: Decreased receptors due to chronic exposure results in reduced response (e.g., insulin in type 2 diabetes).

Target Tissue and Specificity

• Binding Site: The specific region on a receptor where a hormone binds to elicit a response.
• Drug Interactions:
  • Agonist: Mimics hormone action (e.g., asthma inhalers mimic epinephrine).
  • Antagonist: Inhibits hormone action (e.g., anti-stroke drugs inhibit epinephrine effects).

Hormone Interactions

• Types of Interactions:
  • Permissive: One hormone enhances the effect of another.
  • Example: Thyroid hormones support epinephrine and norepinephrine for heart function.
  • Synergistic: Two hormones produce a combined effect.
  • Example: Estrogen and progesterone in reproductive functions.
  • Antagonistic: Opposing effects on the same target (e.g., PTH increases blood calcium, calcitonin decreases).

Endocrine System Integration with the Nervous System

• Pituitary Gland:
  • Connected to hypothalamus, regulating numerous body functions and integrating endocrine responses.
  • Divisions: Anterior (adenohypophysis) and Posterior (neurohypophysis).

Secretion & Regulation of Pituitary Hormones

• Posterior Pituitary:
  • Hormones are synthesized in hypothalamus, transported to the posterior pituitary where they are stored and released into circulation.
• Anterior Pituitary:
  • Regulated by releasing/inhibiting hormones from hypothalamus and secretes hormones that influence other glands.

Key Hormones of the Hypothalamus

• Growth hormone–releasing hormone (GHRH): stimulates growth hormone secretion.
• Thyrotropin-releasing hormone (TRH): stimulates thyroid-stimulating hormone secretion.
• Corticotropin-releasing hormone (CRH): stimulates adrenocorticotropic hormone secretion.
• Gonadotropin-releasing hormone (GnRH): stimulates luteinizing hormone and follicle-stimulating hormone secretion.