Anatomy & Physiology: Endocrine System Notes

Overview of the Endocrine System

• Endocrine System
  • Regulates long-term processes: growth, development, and reproduction.
  • Uses chemical messengers (hormones) to relay information between cells.

Hormone Transport

• The endocrine system consists of ductless glands that synthesize and secrete hormones.
• Hormone Action
  • Released into the blood and transported to target cells with specific receptors.
  • Hormones bind to receptors and induce responses in target cells.
  • Transport path: hormones → interstitial fluid → blood → target cells.

Comparison: Endocrine vs Nervous System

• Commonalities
  • Both release ligands (chemical messengers).
• Differences
  • Endocrine transmits via blood; nervous uses neurons.
  • Hormones reach any cell with the right receptor; nervous system has specific pathways.
  • Endocrine has longer reaction times and effects last longer (minutes to weeks).

General Functions of the Endocrine System

• Regulation of Development, Growth, and Metabolism
  • Hormonal influence on cell division and differentiation in embryos; regulates metabolism (both anabolism and catabolism).
• Maintaining Homeostasis
  • Regulates blood solute concentrations (glucose, ions) and blood volume.
• Controlling Digestive Processes
  • Impact on secretory processes and material movement in the digestive tract.
• Controlling Reproductive Activities
  • Affects reproductive system development and sexual behavior expression.

Major Endocrine Glands

• Contains epithelial tissue within a connective tissue framework.
• Endocrine glands:
  • Pituitary, pineal, thyroid, parathyroid, and adrenal glands.
  • Other organs/tissues with endocrine cells: hypothalamus, skin, thymus, heart, liver, stomach, pancreas, small intestine, kidneys, gonads.

Stimulation of Hormone Synthesis and Release

• Initiating release can occur through:
  • Hormonal Stimulation: Hormone release triggered by another hormone.
  • Humoral Stimulation: Changes in nutrient/ion levels trigger hormone release.
  • Nervous Stimulation: Hormone release triggered by neuron stimulation.

Types of Hormones

• Steroids:
  • Lipid-soluble molecules derived from cholesterol (e.g., cortisol, estrogen).
• Biogenic Amines (Monoamines):
  • Modified amino acids; includes catecholamines and thyroid hormone.
• Proteins:
  • Most hormones are water-soluble chains of amino acids.

Local Hormones

• Local Hormones: Do not circulate in the blood; can act on releasing cell (autocrine) or nearby cells (paracrine).
• Eicosanoids: Local hormones formed from fatty acids; example includes prostaglandins, which prompt pain/inflammation.

Hormone Transport in Blood

• Lipid-soluble hormones:: Require carrier proteins to transport due to being nonpolar.
• Water-soluble hormones: Mostly travel freely in blood; few may use carrier proteins to prolong their life.

Levels of Circulating Hormones

• Blood Concentration:
  • Dependent on synthesis rate and elimination.
  • Hormone release increases blood concentration and vice versa.

Hormone Action Mechanism

• Lipid-Soluble Hormones:
  • Diffuse across the target cell membrane; bind to intracellular receptors.
• Water-Soluble Hormones:
  • Utilize membrane receptors, initiating a signal transduction pathway; often involve G-proteins and second messengers (like cAMP).

Target Cells and Receptor Interaction

• Degree of Cellular Response: Varies based on receptor number and interaction with other hormones.
• Receptor Modification:
  • Up-Regulation: Increases receptor number and sensitivity; occurs with low hormone levels.
  • Down-Regulation: Decreases number and sensitivity; occurs with high hormone levels.

Interactions Between Hormones

• Synergistic Interactions: Hormones work together for greater effect.
• Permissive Interactions: One hormone requires the presence of another to function.
• Antagonistic Interactions: One hormone opposes another's effects.

Hypothalamus and Pituitary Gland Structure

• Hypothalamus: Controls the pituitary gland, which in turn regulates many endocrine organs.
  • Pituitary Anatomy: lies inferior to the hypothalamus, connected by the infundibulum.
• Posterior Pituitary: Releases hormones such as Antidiuretic Hormone (ADH) and Oxytocin.
  • Functions of ADH: Decreases urine production, increases thirst, constricts blood vessels.
  • Functions of Oxytocin: Uterine contractions, milk ejection, emotional bonding.

Anterior Pituitary Hormones and Regulation

• Releasing Hormones from Hypothalamus:
  • TRH, PRH, GnRH, CRH, GHRH.
• Inhibiting Hormones:
  • PIH and GIH.
• Hormones and Their Functions:
  • TSH: Stimulates thyroid hormone secretion.
  • PRL: Stimulates milk production.
  • ACTH: Stimulates glucocorticoid release from adrenal cortex.
  • FSH/LH: Regulate gamete development and hormone secretion.
  • GH: Stimulates growth through IGFs from the liver.

Growth Hormone (GH)

• Regulated by GHRH and GHIH from the hypothalamus; influences varied growth processes.
• Effects of GH: Stimulates nutrient release, increases protein synthesis, and cell division.

Thyroid Hormone (TH)

• Produced by the thyroid, TH affects metabolism, body temperature regulation, and contributes to energy levels.
• Calcitonin: Produced by parafollicular cells; lowers blood calcium levels by inhibiting osteoclast activity.

Adrenal Glands

• Adrenal Medulla: Releases epinephrine/norepinephrine in response to stress.
• Adrenal Cortex: Produces corticosteroids; regulates various body functions including blood sugar and electrolyte levels.

Hormonal Disorders

• Diabetes Mellitus: Impaired glucose uptake; results in significant health issues.
  • Type 1: Lack of insulin production; Type 2: insulin resistance.
• Cushing’s Syndrome: Excess cortisol leads to various health issues.
• Addison’s Disease: Chronic adrenal insufficiency; causes fatigue, weight loss.

Aging and the Endocrine System

• Secretory activity of the endocrine glands tends to decrease with aging, affecting hormone levels and system functionality.