Chapter 17

Chapter 17: The Endocrine System Overview

17.1 Overview of the Endocrine System

• Expected Learning Outcomes:

  • Define the terms "hormone" and "endocrine system".

  • Identify several organs associated with the endocrine system.

  • Contrast endocrine glands with exocrine glands.

  • Describe similarities and differences between the nervous and endocrine systems.

Mechanisms of Cellular Communication

• Four principal mechanisms:

  • Gap junctions:

  • Pores in cell membranes that allow signaling molecules, nutrients, and electrolytes to flow from one cell to another.

  • Neurotransmitters:

  • Released from neurons to traverse synaptic clefts and communicate with adjacent cells.

  • Paracrines:

  • Chemicals secreted into tissue fluids to target nearby cells.

  • Hormones:

  • Chemical messengers transported through the bloodstream to elicit physiological responses in other tissues and organs.

Definition and Structure of the Endocrine System

• Endocrine System:

  • Comprises glands, tissues, and cells that produce hormones.

• Endocrinology:

  • The scientific study of the endocrine system and its disorders.

• Endocrine Glands:

  • Organs that produce hormones.

  • Examples include the pituitary gland, pineal gland, thyroid gland, parathyroid glands, and adrenal glands.

  • Neuroendocrine Organ: While primarily a nervous structure, the hypothalamus also serves endocrine functions.

  • Some organs serve both exocrine and endocrine functions:

  • Examples: Pancreas, gonads, and placenta.

  • Other tissues and organs producing hormones:

  • Examples: Skin, thymus, kidneys, heart, stomach and small intestine, adipose tissue, osseous tissue.

Comparison of Endocrine and Exocrine Glands

• Exocrine Glands:

  • Possess ducts that transport secretions to an epithelial surface or mucosa of the digestive tract.

  • Produce non-hormonal substances (e.g., sweat, saliva).

  • Considered "external secretions" with extracellular effects (e.g., food digestion).

• Endocrine Glands:

  • Lack ducts; have dense capillary networks allowing easy uptake of hormones into the bloodstream.

  • Produce hormones regarded as "internal secretions".

  • Cause intracellular effects by altering target cell metabolism.

Interaction between Nervous and Endocrine Systems

• Both systems work together to integrate the activity of body cells, with hormones having a slower but longer-lasting response compared to the nervous system.

• Several chemicals function as both hormones and neurotransmitters:

  • E.g., norepinephrine, dopamine, and antidiuretic hormone.

• Both systems can influence each other:

  • Neurotransmitters may act on glands; hormones can affect neurons.

Target Cells and Receptor Dynamics

• Target Cells/Organs:

  • Organs or cells possessing receptors for specific hormones that enable them to respond accordingly.

• Some target cells contain enzymes that activate circulating hormones into their active forms.

17.4 Hormones and Their Actions

• Expected Learning Outcomes:

  • Classify hormones by their chemical structure.

  • Describe the synthesis and transportation of hormones.

  • Explain mechanisms of hormonal action on target cells.

  • Discuss regulation of sensitivity to hormones in target cells.

  • Detail hormone interactions when stimulating the same target cells.

  • Describe hormonal clearance from circulation.

Hormone Chemistry

1. Three Chemical Classes of Hormones:

   • Steroid Hormones:

     • Derived from cholesterol.

     • Example: Estrogens from gonads, cortisol from adrenal cortex.

   • Monoamines (Biogenic Amines):

     • Formed from amino acids.

     • Examples: Catecholamines (dopamine, epinephrine, norepinephrine), melatonin, thyroid hormone.

   • Peptide Hormones:

     • Composed of chains of amino acids.

     • Examples: Pituitary hormones, insulin, and hypothalamic hormones.

Hormone Synthesis

• Steroids:

  • Synthesized from cholesterol, differing mainly in functional groups on the steroid backbone.

• Peptides:

  • Synthesized via transcription of genes to mRNA, followed by assembly on ribosomes.

  • Example: Proinsulin is modified to insulin by removing a connecting peptide.

• Monoamines:

  • Created from specific amino acids such as tryptophan (melatonin) and tyrosine (thyroid hormones).

  • Thyroid hormone synthesis involves iodination of tyrosine residues within thyroglobulin.

Hormone Secretion

• Fluctuations in hormone secretion are characterized by:

  • Rhythmic Patterns:

  • Circadian (daily) rhythms, monthly cycles (e.g., female ovarian cycle), or stimulus-induced release.

  • Three Stimuli:

  • Neural Stimuli:

    • Activation by nerve fibers, e.g., adrenal medulla releases epinephrine during stress.

  • Hormonal Stimuli:

    • Hormones stimulating other endocrine glands.

  • Humoral Stimuli:

    • Blood-borne substances influencing hormone secretions, such as glucose and calcium levels.

Hormone Transport and Receptor Interaction

• Transport in Blood:

  • Hormones travel in blood (mainly water).

  • Monoamines and peptides are usually hydrophilic, while steroids and thyroid hormones are hydrophobic and bind to transport proteins.

  • Unbound (free) hormones can diffuse to target cells.

• Receptor Binding:

  • Target cells have specific receptors that activate metabolic pathways.

  • Receptor Specificity and Saturation:

  • Specificity ensures hormones only bind to their exact receptors.

  • Saturation occurs when all receptors are occupied.

Mechanisms of Hormone Action

• Modes of Action:

  • Water-Soluble Hormones:

  • Bind to plasma membrane receptors, activating second messengers; cannot enter cells directly.

  • Lipid-Soluble Hormones:

  • Enter cells, bind intracellular receptors, and activate gene transcription; slower effects compared to water-soluble hormones.

Signal Amplification and Modulation of Sensitivity

• Signal Amplification (Cascade Effect):

  • A single hormone can produce a large cascade of effects through enzyme synthesis.

• Modulation of Sensitivity:

  • Adjusting receptor numbers alters cellular sensitivity to hormones.

  • Up-Regulation: Increase in receptors leads to heightened sensitivity.

  • Down-Regulation: Reduction of receptors leads to decreased sensitivity.

Hormonal Interactions

• Interactive Effects:

  • Hormones can interact with each other in various combinations:

  • Synergistic Effects:

    • Two or more hormones act together to amplify effects (e.g., FSH and testosterone).

  • Permissive Effects:

    • One hormone increases target organ responsiveness to another.

  • Antagonistic Effects:

    • One hormone opposes the action of another (e.g., insulin vs glucagon).

Hormone Clearance

• Hormonal Deactivation:

  • Hormones must be cleared from the bloodstream after action; generally processed by the liver and kidneys.

  • Metabolic Clearance Rate (MCR): The rate at which hormones are eliminated from circulation, typically described with a half-life (the time taken to reduce hormone concentration by half).

17.2 The Hypothalamus and Pituitary Gland

• Expected Learning Outcomes:

  • Describe the anatomical relationship between the hypothalamus and pituitary gland.

  • Differentiate between the anterior and posterior pituitary lobes.

  • Identify hormones produced by the hypothalamus and each lobe of the pituitary, along with their functions.

  • Explain hypothalamic control of the pituitary and target organs.

  • Describe growth hormone's effects.

Anatomy of the Hypothalamus and Pituitary Gland

• Hypothalamus:

  • Shaped like a flattened funnel, forming the floor of the third ventricle of the brain.

  • Regulates various functions: water balance, thermoregulation, sex drive, and childbirth, largely mediated through the pituitary gland.

• Pituitary Gland (Hypophysis):

  • Size and shape similar to a kidney bean; located in the sella turcica of the sphenoid bone.

  • Comprises anterior (adenohypophysis) and posterior (neurohypophysis) lobes, with distinct embryonic origins and functions.

• Connective Structure:

  • The pituitary is connected to the hypothalamus via the infundibulum.

Anterior Pituitary Hormones

• Six Major Hormones Secreted:

  • Follicle-Stimulating Hormone (FSH): Stimulates gonadal hormone secretion and gamete production.

  • Luteinizing Hormone (LH): Promotes ovulation and testosterone secretion.

  • Thyroid-Stimulating Hormone (TSH): Stimulates thyroid hormone secretion.

  • Adrenocorticotropic Hormone (ACTH): Stimulates the adrenal gland to secrete glucocorticoids.

  • Prolactin (PRL): Promotes milk production in the mammary glands.

  • Growth Hormone (GH): Stimulates growth, mitosis, and cellular differentiation.

Posterior Pituitary Hormones

• Two Key Hormones:

  • Oxytocin (OT): Regulates labor contractions and milk secretion.

  • Antidiuretic Hormone (ADH): Enhances water retention in kidneys.

Control of Pituitary Secretion

• Control Mechanisms:

  • Rates of secretion are variable, controlled by hypothalamus, feedback from target organs, and cerebral influence.

  • Negative Feedback: High hormone levels from target organs inhibit hypothalamic or pituitary hormones.

  • Positive Feedback: Examples include oxytocin during childbirth, which enhances contractions.

Growth Hormone (GH) Analysis

• GH has significant effects, especially on cartilage, bone, and muscle tissue.

• Indirect Effects: via insulin-like growth factors (IGF-I and IGF-II), which increase cell growth and division.

• Variability in Secretion: Influenced by factors like sleep, exercise, and age.

17.3 Other Endocrine Glands

• Expected Learning Outcomes:

  • Describe the structure and location of the remaining endocrine glands.

  • Name hormones produced, secretion stimuli, and functions of these glands.

  • Discuss hormonal roles by organs and tissues outside classical endocrine glands.

Pineal Gland

• Location: Attached to the roof of the third ventricle.

• Function: Synthesizes and secretes melatonin, regulating circadian rhythms and sleep.

Thyroid Gland

• Location: Below the larynx adjacent to the trachea.

• Structure: Two lobes connected by isthmus; contains thyroid follicles that produce thyroid hormones (TH), primarily thyroxine (T4) and triiodothyronine (T3).

• Function: TH increases metabolic rate, appetite, and growth hormone secretion.

Parathyroid Glands

• Structure: Four small glands situated behind the thyroid gland.

• Function: Secretes parathyroid hormone (PTH) to regulate calcium levels in the blood, promoting calcium absorption in the intestines and kidneys, and bone resorption.

Adrenal Glands

• Location: Sit atop the kidneys; consist of two parts:

  • Adrenal Medulla: Secretes catecholamines (epinephrine, norepinephrine) in response to stress.

  • Adrenal Cortex: Produces corticosteroids from three layers: zona glomerulosa (mineralocorticoids), zona fasciculata, and zona reticularis (glucocorticoids and androgens).

Pancreatic Islets (Islets of Langerhans)

• Function: Regulates blood glucose levels through hormone secretion (glucagon and insulin).

• Alpha Cells: Secrete glucagon to raise blood glucose levels.

• Beta Cells: Secrete insulin to lower blood glucose.

• Delta Cells: Produce somatostatin to regulate insulin and glucagon.

Gonads (Ovaries and Testes)

• Function: Secrete sex hormones (estrogen, progesterone in ovaries; testosterone in testes), which regulate reproduction, secondary sexual characteristics, and reproductive cycles.

Endocrine Functions of Other Organs

• Kidneys: Produce erythropoietin for red blood cell formation, and renin for blood pressure regulation.

• Heart: Releases atrial natriuretic peptide (ANP) to regulate blood pressure.

• Skin: Synthesizes vitamin D from cholesterol precursor.

• Thymus: Produces thymosins important for T lymphocyte development.

• Adipose Tissue: Produces leptin, which regulates appetite, and resistin, which antagonizes insulin action.

17.5 Stress and Adaptation

• Expected Learning Outcomes:

  • Define stress physiologically and discuss adaptation mechanisms.

Understanding Stress

• Defined as situations disrupting homeostasis, threatening well-being.

• Stressors: injury, exercise, grief, etc.

• General Adaptation Syndrome (GAS): Body's stress response occurring in three stages:

  1. Alarm Reaction:

  • Mediated by the sympathetic nervous system; releases norepinephrine and epinephrine.

  1. Stage of Resistance:

  • Dominated by cortisol to supply alternative energy.

  1. Stage of Exhaustion:

  • Potential physiological failure and death if stress continues too long.

Biological Implications of Stress

• Chronic stress can cause detrimental effects on the immune system, cardiovascular health, and can lead to metabolic diseases.

17.6 Eicosanoids and Other Signaling Molecules

• Expected Learning Outcomes:

  • Define eicosanoids and their functions.

Understanding Eicosanoids

• Derived from arachidonic acid; include leukotrienes, prostacyclin, prostaglandins, and thromboxanes.

• Function in inflammation, blood clotting, and modulating physiological processes.

Anti-Inflammatory Drugs

• Steroidal Drugs: (e.g., cortisol) inhibit eicosanoid production from the membrane.

• NSAIDs: (e.g., aspirin) inhibit cyclooxygenase to reduce inflammation and pain.

17.7 Endocrine Disorders

• Expected Learning Outcomes:

  • Explore causes and examples of hormone imbalances.

General Hormonal Disorders

• Hyposecretion: Inadequate hormone release due to destruction or dysfunction.

• Hypersecretion: Excessive hormone production due to tumors or autoimmune disorders.

Specific Disorders

• Pituitary Disorders: e.g., gigantism, acromegaly.

• Thyroid Disorders: e.g., hypothyroidism, goiter.

• Adrenal Disorders: e.g., Cushing's syndrome, adrenal insufficiency.

• Diabetes Mellitus:

  • Disorders impairing insulin function leading to glucose metabolism dysfunction, resulting in symptoms like polyuria, polydipsia, and fatigue. Types include Type 1 and Type 2 diabetes.

• Diabetes Pathophysiology:

  • Leads to long-term complications involving neuropathy, cardiovascular issues, and renal damage.