Anatomy & Physiology TB

Chapter 17: The Endocrine System

Chapter Objectives

• Identify the contributions of the endocrine system to homeostasis.

• Discuss the chemical composition of hormones and mechanisms of hormone action.

• Summarize the production, regulation, and effects of hormones from major glands (pituitary, thyroid, parathyroid, adrenal, pineal).

• Discuss hormonal regulation of the reproductive system.

• Explain pancreatic endocrine cells' role in blood glucose regulation.

• Identify hormones from organs with secondary endocrine functions (heart, kidneys, etc.).

• Discuss common diseases related to endocrine dysfunction.

• Discuss embryonic development and aging effects on the endocrine system.

Overview of the Endocrine System

• Long-distance communication: The endocrine system communicates through chemical signals (hormones), contrasting the nervous system's electrical and chemical signals.

• Homeostasis: It influences functions like growth, metabolism, reproduction, and stress responses, maintaining internal balance.

• Hormones travel through the bloodstream to target cells, leading to specific physiological responses.

Types of Intercellular Communication

1. Nervous System:

   • Uses electrical (action potentials) and chemical (neurotransmitters) signaling.

   • Rapid responses for immediate changes in the environment, e.g., movement, sensation.

2. Endocrine System:

   • Utilizes chemical signaling via hormones released into the bloodstream.

   • Slower responses that can last longer; involved in long-term processes like growth and metabolism.

Characteristics of Endocrine Signaling

• Less specific than nervous signaling; same hormone can act on different tissues for various functions.

• Example: Oxytocin is involved in childbirth, breastfeeding, and emotional bonding.

• Integration of Systems: Rapid endocrine responses can be triggered by the nervous system, particularly under stress (fight-or-flight response).

Structures of the Endocrine System

• Endocrine Glands: Ductless glands that secrete hormones (pituitary, thyroid, adrenal glands, etc.).

• Pancreas: Exhibits both endocrine (insulin/glucagon secretion) and exocrine (digestion) functions.

• Other organs (hypothalamus, thymus, heart, etc.) also play roles in hormone production.

Types of Chemical Signaling

• Endocrine: Hormones travel through blood to distant cells.

• Autocrine: Chemicals act on the same cell that secretes them.

• Paracrine: Chemicals act on neighboring cells; e.g., histamine in local inflammation.

• Career Connection: Endocrinologists specialize in treating hormonal disorders (e.g., diabetes, thyroid disorders).

Hormones and Their Functions

Major Classes of Hormones

• Amino acid derivatives (amines, peptides, proteins) and Steroid hormones (lipid-derived).

• Hormones must bind to specific receptors on target cells to exert their effects.

• Feedback Mechanisms: Hormone levels are controlled by feedback loops, primarily negative feedback for homeostasis (e.g., glucocorticoids regulation).

Hormonal Actions on Cells

1. Intracellular Receptors: Steroid hormones bind inside cells, affecting gene expression.

2. Cell Membrane Receptors: Peptide and amino acid-derived hormones activate second messengers like cAMP for rapid response at the cell surface.

Factors Affecting Target Cell Response

• Receptor Regulation: Cells may upregulate/downregulate receptors based on hormone levels (downregulation in response to excess).

• Hormonal Interactions:

  • Permissive effect: One hormone enables another to act (e.g., thyroid hormones enhance reproductive hormones).

  • Synergistic effect: Combined effects of hormones amplify responses (e.g., FSH and estrogen).

  • Antagonistic effect: Opposing hormonal effects (e.g., insulin vs glucagon).

Regulation of Hormone Secretion

• Hormonal secretion initiated by feedback loops in response to internal changes (humoral, hormonal, neural stimuli).

• Example: High blood glucose stimulates insulin release; low levels stimulate glucagon secretion.

The Pituitary Gland and Hypothalamus

• The hypothalamus-pituitary complex coordinates endocrine functions, regulating hormones of other glands.

• Posterior Pituitary: Stores and releases oxytocin (uterine contractions, bonding) and ADH (water reabsorption).

• Anterior Pituitary: Produces several hormones; regulated by hypothalamic releasing/inhibiting hormones affecting growth, metabolism, reproduction.

The Thyroid Gland

• Location: Anterior to the trachea, below the larynx.

• Produces T3 and T4 (regulate metabolic rate), calcitonin (reduces blood Ca2+).

• Regulation: TSH from anterior pituitary maintains T3 and T4 levels; feedback loop in place to prevent excess.

The Adrenal Glands

• Location: On top of kidneys, divided into adrenal cortex (steroid hormones) and adrenal medulla (catecholamines like epinephrine).

• Functions: Cortisol (response to stress, blood glucose), aldosterone (regulates sodium/potassium), epinephrine/norepinephrine (fight-or-flight response).

The Endocrine Pancreas

• Pancreatic islets (alpha, beta, delta, PP cells) responsible for glucagon and insulin secretion; critical for blood glucose regulation.

• Glucagon: Increases glucose levels during fasting; Insulin: Facilitates glucose uptake post-meal.

Organs with Secondary Endocrine Functions

• Include heart (ANP); gastrointestinal tract (gastrin, secretin, CCK); kidneys (renin, calcitriol, EPO); skeleton (FGF23, osteocalcin); adipose tissue (leptin, adiponectin); skin (vitamin D); thymus (thymosins).

Development and Aging of the Endocrine System

• Endocrine glands arise from various embryonic tissues, reflecting distinct development pathways. Changes with age affect hormone levels (e.g., decreased GH, estrogen, testosterone) and can lead to disorders like hypothyroidism, diabetes.

Conclusion

Understanding the endocrine system is vital for recognizing how hormonal signaling affects body function, homeostasis, and overall health.

Physiology of the Endocrine System

• Hormonal Signaling: The endocrine system relies on hormones as chemical messengers that are released into the bloodstream. These hormones travel to target cells, where they bind to specific receptors and induce physiological responses.

• Types of Hormones:

  • Amino Acid Derivatives: Comprised of single amino acids, such as catecholamines (epinephrine, norepinephrine). These are typically involved in rapid responses.

  • Peptides and Proteins: Composed of short to long chains of amino acids (e.g., insulin, growth hormone). Often act via cell membrane receptors activating second messengers for quick responses.

  • Steroid Hormones: Derived from cholesterol (e.g., cortisol, sex hormones) that bind to intracellular receptors, altering gene expression and resulting in long-term changes in cell function.

• Homeostasis: Hormones primarily act through negative feedback mechanisms that maintain stability in the body's internal environment. For instance, the regulation of blood glucose levels involves insulin lowering glucose when levels are high, and glucagon increasing it when levels are low.

• Cellular Responses: The effect of hormones on target cells includes:

  • Intracellular Receptors: Steroid hormones bind to nuclear receptors, causing changes in gene regulation and protein synthesis.

  • Cell Membrane Receptors: Peptide hormones engage with receptors on the cell surface, initiating cascades that lead to quick cellular responses (e.g., cAMP pathway).

• Integration with Nervous System: The endocrine system can respond to stimuli from the nervous system, especially during stress. The hypothalamus orchestrates hormone release from the pituitary gland, which further regulates other endocrine glands, demonstrating a close interconnection between these two signaling systems.

• Development and Aging Context: The functionality of the endocrine system evolves throughout life. During development and aging, hormone levels fluctuate, contributing to physiological changes and potential disorders, such as hormonal deficiencies or heightened responses prevalent in aged populations.