A&P II / Chapter 18/ The Endocrine System:

The Endocrine System

Introduction

The endocrine system is a major regulatory system of the body, coordinating and integrating cellular activity through the release of hormones. It works closely with the nervous system to maintain homeostasis, regulate metabolism, growth, development, and reproduction.

Intercellular Communication

Mechanisms of Intercellular Communication

Cells communicate to coordinate activities and maintain homeostasis through several mechanisms:

• Direct Communication: Involves exchange of ions and molecules between adjacent cells via gap junctions. Example: cardiac muscle cells.

• Paracrine Communication: Chemical messengers transfer information to neighboring cells within the same tissue. Example: somatostatin from pancreatic cells.

• Autocrine Communication: Cells secrete messengers that affect themselves. Example: prostaglandins in smooth muscle cells.

• Endocrine Communication: Hormones are released into the bloodstream and act on distant target cells with specific receptors.

• Synaptic Communication: Neurons release neurotransmitters at synapses for rapid, targeted communication.

Comparison: Endocrine vs. Nervous System

• Nervous System: Fast, short-lived responses via neurotransmitters.

• Endocrine System: Slower, longer-lasting effects via hormones.

• Both systems use chemical messengers and negative feedback to regulate homeostasis.

Hormones: Structure, Transport, and Action

Classes of Hormones

Hormones are classified by their chemical structure:

• Amino Acid Derivatives (Biogenic Amines): Derived from tyrosine (e.g., thyroid hormones, catecholamines) or tryptophan (e.g., melatonin).

• Peptide Hormones: Chains of amino acids, including glycoproteins (TSH, LH, FSH) and short polypeptides (ADH, OXT, GH, PRL, insulin).

• Lipid Derivatives: Eicosanoids (from arachidonic acid) and steroid hormones (from cholesterol, e.g., androgens, estrogens, corticosteroids).

Transport and Inactivation of Hormones

• Hydrophilic hormones circulate freely and are quickly inactivated.

• Hydrophobic hormones (thyroid and steroid hormones) bind to transport proteins, forming a reserve in the bloodstream.

Mechanisms of Hormone Action

• Hormones bind to specific receptors on or in target cells.

• Extracellular receptors: For non-lipid soluble hormones; activate second messenger systems (e.g., cAMP, Ca2+).

• Intracellular receptors: For lipid-soluble hormones; directly affect gene expression and protein synthesis.

Second Messenger Systems

• cAMP Pathway: Hormone binds receptor → G protein activates adenylate cyclase → ATP converted to cAMP → cAMP activates kinases.

• Calcium Pathway: G protein activates phospholipase C → IP3 and DAG produced → Ca2+ released from stores → activates protein kinases.

Intracellular Hormone Binding

• Steroid hormones: Bind cytoplasmic/nuclear receptors, alter gene transcription.

• Thyroid hormones: Bind nuclear and mitochondrial receptors, increase ATP production and gene expression.

Control of Hormone Secretion

• Primarily regulated by negative feedback.

• Stimuli include humoral (blood composition), hormonal (other hormones), and neural (nervous system) triggers.

The Pituitary Gland (Hypophysis)

Structure and Location

The pituitary gland is located in the sella turcica, connected to the hypothalamus by the infundibulum. It has two lobes: anterior (adenohypophysis) and posterior (neurohypophysis).

Hypothalamic Control

• Hypothalamus synthesizes ADH and OXT, regulates anterior pituitary via releasing/inhibiting hormones, and controls adrenal medulla via neural input.

Anterior Lobe Hormones

• TSH (Thyroid-stimulating hormone): Stimulates thyroid hormone release.

• ACTH (Adrenocorticotropic hormone): Stimulates glucocorticoid release from adrenal cortex.

• FSH & LH (Gonadotropins): Regulate reproductive organs and hormone production.

• PRL (Prolactin): Stimulates mammary gland development and milk production.

• GH (Growth hormone): Stimulates growth, protein synthesis, and metabolism.

• MSH (Melanocyte-stimulating hormone): Stimulates melanin production.

Posterior Lobe Hormones

• ADH (Antidiuretic hormone): Promotes water retention by kidneys.

• OXT (Oxytocin): Stimulates uterine contractions and milk ejection.

The Thyroid Gland

Structure and Hormones

The thyroid gland is located inferior to the larynx, consists of two lobes connected by an isthmus, and contains follicles filled with colloid. Parafollicular (C) cells are found between follicles.

Thyroid Hormone Synthesis and Regulation

• Thyroglobulin and iodide are used to synthesize T3 (triiodothyronine) and T4 (thyroxine).

• TSH stimulates hormone synthesis and release.

• Thyroid hormones are transported in blood bound to proteins (TBG, transthyretin, albumin).

Functions of Thyroid Hormones

• Increase metabolic rate and ATP production.

• Essential for growth and development, especially of the nervous, skeletal, and muscular systems.

• Increase heart rate, blood pressure, and sensitivity to sympathetic stimulation.

• Stimulate red blood cell formation and respiratory sensitivity.

Calcitonin

• Produced by C cells; lowers blood calcium by increasing excretion and decreasing absorption.

• Important during childhood and for reducing bone loss during pregnancy and starvation.

Parathyroid Glands

Structure and Hormones

Four small glands on the posterior thyroid. Principal cells secrete parathyroid hormone (PTH) in response to low blood calcium.

Effects of Parathyroid Hormone (PTH)

• Increases blood calcium by stimulating osteoclasts, increasing kidney reabsorption, and promoting calcitriol synthesis for increased intestinal absorption.

Adrenal Glands

Structure and Regions

Located superior to the kidneys, each adrenal gland has an outer cortex and inner medulla.

Adrenal Cortex Hormones

• Zona glomerulosa: Mineralocorticoids (aldosterone) regulate sodium and potassium balance.

• Zona fasciculata: Glucocorticoids (cortisol, corticosterone) regulate glucose metabolism and have anti-inflammatory effects.

• Zona reticularis: Androgens (sex hormones) influence development and secondary sex characteristics.

Adrenal Medulla Hormones

• Secretes catecholamines (epinephrine and norepinephrine) in response to sympathetic stimulation.

• Effects include increased heart rate, blood pressure, energy mobilization, and enhanced muscular performance.

Pineal Gland

Structure and Function

Located in the epithalamus, the pineal gland contains pinealocytes that produce melatonin, which regulates circadian rhythms, inhibits reproductive functions, and acts as an antioxidant.

Pancreas

Structure and Function

The pancreas is both an exocrine and endocrine gland. The exocrine portion secretes digestive enzymes, while the endocrine portion (islets of Langerhans) regulates blood glucose.

Pancreatic Hormones

• Insulin (beta cells): Lowers blood glucose by promoting uptake, storage, and utilization of glucose.

• Glucagon (alpha cells): Raises blood glucose by stimulating glycogen breakdown and gluconeogenesis.

• Somatostatin (delta cells): Inhibits insulin and glucagon secretion.

• Pancreatic polypeptide (PP cells): Regulates pancreatic enzyme secretion and gallbladder contraction.

Secondary Endocrine Functions

Other Organs with Endocrine Roles

• Kidneys: Produce calcitriol (calcium homeostasis), erythropoietin (RBC production), and renin (blood pressure regulation).

• Heart: Produces natriuretic peptides (ANP, BNP) to lower blood pressure and volume.

• Thymus: Produces thymosins for lymphocyte development.

• Gonads: Testes produce androgens and inhibin; ovaries produce estrogens, progesterone, and inhibin.

• Adipose tissue: Produces leptin, which regulates appetite and reproductive function.

Hormone Interactions and Stress Response

Types of Hormone Interactions

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

• Synergistic: Additive effects.

• Permissive: One hormone enables another to act.

• Integrative: Different but complementary effects.

General Adaptation Syndrome (GAS)

• Alarm phase: Immediate, fight-or-flight response (epinephrine dominant).

• Resistance phase: Prolonged stress; glucocorticoids, GH, and thyroid hormones mobilize energy reserves.

• Exhaustion phase: Homeostatic breakdown, organ failure, potentially fatal.

Aging and the Endocrine System

• Most hormone levels remain stable with age, but reproductive hormones decline and tissue responsiveness may decrease.