PhysioNotes3S25

The Endocrine System & Metabolism

• Definition: A system of cells, tissues, and organs with ductless (endocrine) glands that secrete chemicals called hormones directly into interstitial fluid and blood for regulating homeostasis.

Comparison to Nervous System

• Similarities:

  • Both systems serve regulatory functions.

  • Both transmit messages to coordinate functions.

  • Both activate target cells via specific receptor proteins.

• Differences:

  • Regulates using chemical hormones via the blood  instead of impulses via neurons.

    ❖ Causes changes in metabolic  activity rather than stimulation

    of muscles and glands.

    ❖ Can affect virtually all tissues of the body instead of just muscles and

    glands.

  • ❖ Has a slow, relatively relative effect instead of milliseconds like neurons.

    ➢ The two systems work together stimulating and inhibiting each other in feedback mechanisms.

Endocrine Glands

• Characteristics:

  • ➢ These are ductless glands that secrete their chemical products, called

    hormones, into the blood.

    ➢ The blood carries the hormones throughout the body and only target 

    cells  respond.

    ➢ Endocrine glands can be organs which function only to secrete hormones.

    e.g.pituitor, thyroid, parathyroid, pineal, adrenals 

    ➢ Or, endocrine glands can be organs with other functions that also contain

    endocrine tissue.

    e.g.pancreas, ovaries,testes, hypothamlus ,skin, kidney, stomach, heartHormones

• Precursor Types:

  • Prohormones: Relatively inactive precursor molecules modified to active form before release (e.g., inactive forms).

  • Perhormones: Inactive secretions converted by target cells into active form (e.g., T4 to T3).

Hormonal Activity and Concentration Effects

Concentration Effects

• Hormones are broken down quickly; their concentration in the blood aligns with their secretion rate.

  • Up-Regulation: Prolonged exposure to low hormone concentrations increases the number of receptors on target cells.

  • Down-Regulation: Prolonged exposure to high hormone concentrations decreases receptor numbers, leading to desensitization.

  • Pharmacological concentrations can result in artificial effects by binding to related receptor types.

Interaction Types

• Synergistic Interaction: Hormones work together for a cumulative effect.

  • Additive Synergy: All hormones have similar effects and add together.

  • Complementary Synergy: Different effects working together towards a common outcome.

• Antagonistic Interaction: One hormone counteracts another's effects.

• Permissive Effect: One hormone enhances the response of a target organ to a second hormone.

Hormone Categories

• Hormones are varied, including amines, polypeptides, glycoproteins, and steroids, categorized based on lipophilic or polar characteristics.

  • Lipophilic Hormones:

    • Pass through cell membranes (e.g., steroids, thyroid hormones).

    • Require plasma protein carriers in blood but can be taken orally.

  • Polar Hormones:

    • Cannot pass through cell membranes (e.g., polypeptides, glycoproteins).

    • Cannot be taken orally.

Mechanisms to Activate Target Cells

• Polar Hormone Mechanism (Cell Membrane Receptor Pathway): Bind to receptors in the cell membrane, activating second messengers (e.g., cyclic AMP, Ca2+, Tyrosine Kinase) which then trigger metabolic changes.

• Lipophilic Hormone Mechanism: Pass through cell membranes, bind to cytoplasmic receptors, transfer to nucleus, promoting transcription and leading to protein synthesis (e.g., thyroid hormone conversion from T4 to T3).

The Pituitary Gland

• Structure: Major endocrine gland with anterior and posterior portions secreting 8 hormones.

Anterior Pituitary

• Secretes 6 hormones:

  • Thyroid Stimulating Hormone (TSH): Targets thyroid gland for T4 production.

  • Adrenocorticotropic Hormone (ACTH): Targets adrenal cortex for glucocorticoid secretion.

  • Follicle-stimulating Hormone (FSH): Stimulates sex hormone secretion and growth.

  • Luteinizing Hormone (LH): Stimulates ovulation and sex hormone secretion.

  • Growth Hormone (GH): Promotes growth in muscles, bones.

  • Prolactin: Stimulates milk production post-childbirth.

Control of the Anterior Pituitary

• Regulated by hypothalamus through releasing/inhibiting hormones via the hypophyseal portal system.

  • Key Hormones:

    • Corticotropin-releasing Hormone (CRH): Stimulates ACTH.

    • Gonadotropin-releasing Hormone (GnRH): Stimulates FSH and LH.

    • Thyrotropin-releasing Hormone (TRH): Stimulates GH.

    • Growth Hormone-releasing Hormone (GHRH): Stimulates GH.

    • Somatostatin: Inhibits GH secretion.

    • Prolactin-releasing Hormone (PRH): Stimulates/inhibits prolactin secretion.

Posterior Pituitary

• Stores and secretes two hypothalamus-produced hormones:

  • Antidiuretic Hormone (ADH): Promotes water retention, arteriolar contraction.

  • Oxytocin: Stimulates uterine contractions and milk ejection reflex.

Metabolism and Energy Homeostasis

• Metabolism: Energy management through anabolic/catabolic reactions and energy conversion from food intake.

• Energy Units:

  • 4 Calories/gram for carbohydrates and proteins, 8 Calories/gram for fats.

  • Kilocalorie (C) = 1000 calories.

Caloric and Nutritional Needs

Essential Nutrients

• Food provides raw materials for metabolism;

  • Essential Amino Acids: 9; essential fatty acids: 2.

  • Vitamins: Water-soluble (e.g., B vitamins, C) and Fat-soluble (A, D, E, K).

  • Minerals: Necessary for enzyme functions (sodium, potassium, calcium, etc.).

Caloric Energy Expenditure Types

1. Basal Metabolic Rate (BMR): Energy used at rest.

2. Adaptive Thermogenesis: Energy for temperature adjustments.

3. Physical Activity: Energy used during activities.

• Total Metabolic Rate (TMR): Total energy used daily, influenced by physical activities and food intake.

Weight Management

• Weight gain: Excess caloric intake leads to fat storage; obesity defined as 20% over standard.

• Obesity Risks: Cardiovascular diseases, diabetes, and other health issues.

• Hormonal Regulation: Insulin and glucagon maintain glucose levels; glucagon promotes energy mobilization.

Metabolic Regulation: Key Hormones

Pancreatic Islets

• Insulin: Secreted by beta cells, promotes substrate uptake.

• Glucagon: Secreted by alpha cells, promotes energy release in response to low blood glucose.

Adrenal Glands

• Adrenal Medulla: Releases epinephrine/norepinephrine, stimulating metabolic responses.

• Adrenal Cortex:

  • Mineralocorticoids: e.g., aldosterone for Na+/K+ balance.

  • Glucocorticoids: e.g., cortisol for glucose metabolism during stress.

Stress Response

• General Adaptation Syndrome (GAS): Response to stressors by coordinating nervous and endocrine systems.

• Activation of sympathetic nervous system, resulting in energy substrate mobilization for stress response.

Thyroid Gland Regulation

• Hormones (T4, T3): Essential for normal CNS function, growth, and metabolic rates.

  • Hypothyroidism: Leads to low BMR, weight gain, excessive T4.

  • Hyperthyroidism: High BMR, weight loss, insufficient T4.

  • Calcitonin: Helps lower blood calcium levels.

Calcium and Phosphate Balance

• Bone Cell Activity: Osteoblasts form bone while osteoclasts resorb bone.

• Hormonal Regulation: Involves PTH, 1,25-dihydroxyvitamin D3, and calcitonin.

• Maintain calcium levels: PTH increases, calcitonin decreases blood calcium.