Chapter 18: The Endocrine System

An Introduction to the Endocrine System

The endocrine system consists of cells and tissues that produce hormones, which are chemical messengers that regulate and coordinate various physiological processes to maintain homeostasis. It works closely with the nervous system and is primarily regulated by negative feedback mechanisms.

• Hormones: About 30 different hormones are produced by endocrine cells and tissues.

• Negative Feedback: Most hormone secretion is regulated by negative feedback, where a stimulus triggers hormone production that reduces the intensity of the original stimulus.

18-1 Intercellular Communication

Mechanisms of Intercellular Communication

Homeostasis is maintained through communication between cells, primarily via the nervous and endocrine systems. There are five main mechanisms:

1. Direct Communication: Exchange of ions and molecules between adjacent cells through gap junctions. Example: Cardiac muscle cells use gap junctions for action potential propagation.

2. Paracrine Communication: Cells release paracrines into extracellular fluid to affect neighboring cells. Example: Somatostatin from pancreatic cells inhibits insulin release.

3. Autocrine Communication: Cells secrete autocrines that affect the same cell type. Example: Prostaglandins cause smooth muscle contraction.

4. Endocrine Communication: Endocrine cells release hormones into the bloodstream, affecting distant target cells with specific receptors.

5. Synaptic Communication: Neurons release neurotransmitters at synapses for rapid, targeted responses.

Comparison: Endocrine System vs. Nervous System

18-2 Hormones

Classes of Hormones

Hormones are classified based on their chemical structure:

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

2. Peptide Hormones: Chains of amino acids; includes glycoproteins (e.g., TSH, LH, FSH) and polypeptides/proteins (e.g., insulin, GH, prolactin).

3. Lipid Derivatives:

   • Eicosanoids: Derived from arachidonic acid (e.g., prostaglandins, leukotrienes).

   • Steroid Hormones: Derived from cholesterol (e.g., testosterone, estrogen, calcitriol).

Control of Hormone Secretion

• Humoral Stimuli: Changes in extracellular fluid composition (e.g., blood glucose stimulates insulin release).

• Hormonal Stimuli: Arrival/removal of another hormone (e.g., TSH stimulates thyroid hormone release).

• Neural Stimuli: Neurotransmitter arrival at neuroglandular junctions (e.g., hypothalamus stimulates adrenal medulla).

Transport and Receptors

• Free Hormones: Circulate unbound, short-lived.

• Bound Hormones: Attached to carrier proteins, longer-lasting (e.g., thyroid and steroid hormones).

• Hormone Receptors: Specificity of hormone action depends on presence of receptors, which may be on the plasma membrane (extracellular) or inside the cell (intracellular).

Mechanisms of Hormone Action

• Extracellular Receptors: Bind non-lipid soluble hormones (e.g., catecholamines, peptides). Use second messengers (e.g., cAMP, Ca2+) for signal amplification.

• Intracellular Receptors: Bind lipid-soluble hormones (e.g., steroids, thyroid hormones). Directly affect gene transcription and protein synthesis.

Regulation of Hormone Receptors

• Down-Regulation: High hormone levels decrease receptor numbers (e.g., insulin resistance).

• Up-Regulation: Low hormone levels increase receptor numbers.

18-3 The Pituitary Gland

Structure and Control

• Located in the sella turcica, connected to the hypothalamus by the infundibulum.

• Divided into anterior lobe (adenohypophysis) and posterior lobe (neurohypophysis).

• Controlled by hypothalamic hormones via the hypophyseal portal system.

Hormones of the Anterior Lobe (Adenohypophysis)

• Tropic Hormones: Stimulate other endocrine glands.

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

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

  • FSH (Follicle-Stimulating Hormone): Stimulates gamete production.

  • LH (Luteinizing Hormone): Triggers ovulation and androgen production.

• Non-Tropic Hormones:

  • Prolactin (PRL): Stimulates milk production.

  • Growth Hormone (GH): Stimulates growth, protein synthesis, and mobilizes energy stores.

  • Melanocyte-Stimulating Hormone (MSH): Stimulates melanin production (mainly in pregnancy/disease).

Hormones of the Posterior Lobe (Neurohypophysis)

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

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

18-4 The Thyroid Gland

Structure and Function

• Located inferior to the thyroid cartilage; consists of two lobes connected by an isthmus.

• Thyroid Follicles: Produce thyroid hormones (T3 and T4) stored in colloid.

• C (Parafollicular) Cells: Produce calcitonin, which lowers blood Ca2+ levels.

Thyroid Hormone Synthesis and Effects

1. TSH stimulates iodide uptake and hormone synthesis in follicle cells.

2. Thyroglobulin stores T3 and T4 in colloid.

3. Hormones released into blood, mostly bound to plasma proteins.

4. Thyroid hormones increase metabolic rate, heart rate, and are essential for growth and development.

18-5 Parathyroid Glands

Structure and Function

• Four small glands on the posterior thyroid.

• Parathyroid Hormone (PTH): Increases blood Ca2+ by stimulating osteoclasts, enhancing kidney reabsorption, and promoting calcitriol synthesis for increased GI absorption.

18-6 Adrenal Glands

Structure

• Located atop each kidney; consist of outer cortex and inner medulla.

Adrenal Cortex

Adrenal Medulla

• Produces epinephrine (80%) and norepinephrine (20%) under sympathetic control.

• Effects: Mobilizes energy reserves, increases heart rate and contractility, prepares body for 'fight or flight.'

18-7 Pineal Gland

• Located in the epithalamus; contains pinealocytes that produce melatonin in response to darkness.

• Melatonin regulates circadian rhythms and may be involved in seasonal affective disorder (SAD).

18-8 Pancreas

Structure and Function

• Both exocrine (digestive enzymes) and endocrine (hormones) functions.

• Pancreatic Islets: Contain four cell types:

  • Alpha (α) cells: Produce glucagon (raises blood glucose).

  • Beta (β) cells: Produce insulin (lowers blood glucose).

  • Delta (δ) cells: Produce somatostatin (inhibits GH and insulin).

  • PP cells: Produce pancreatic polypeptide (regulates pancreatic secretions).

Diabetes Mellitus

• Type 1: Autoimmune destruction of β cells; requires insulin therapy.

• Type 2: Insulin resistance; often associated with obesity; may be managed with lifestyle changes.

• Complications: Kidney failure, blindness, cardiovascular disease, neuropathy, tissue damage.

18-9 Organs With Secondary Endocrine Functions

• Intestines: Secrete hormones for digestive coordination.

• Kidneys: Produce calcitriol (Ca2+ absorption), erythropoietin (RBC production), and renin (RAAS system for blood pressure).

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

• Thymus: Secretes thymosins for lymphocyte development.

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

• Adipose Tissue: Produces leptin, which regulates appetite and reproductive hormone synthesis.

18-10 Hormone Interactions

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

• Synergistic: Additive effects (e.g., GH and glucocorticoids).

• Permissive: One hormone enables another's effect (e.g., thyroid hormones for epinephrine action).

• Integrative: Different but complementary effects (e.g., PTH and calcitriol on Ca2+).

Hormonal Response to Stress (General Adaptation Syndrome, GAS)

1. Alarm Phase: Immediate, sympathetic response; epinephrine dominant; mobilizes energy reserves.

2. Resistance Phase: Prolonged stress; glucocorticoids dominant; mobilizes lipids and amino acids.

3. Exhaustion Phase: Energy reserves depleted; homeostasis fails; organ failure possible.

Hormones and Behavior

• Hormones influence behavior, cognition, memory, and emotional states.

Aging and the Endocrine System

• Most hormone levels remain stable with age, except reproductive hormones (decline leads to decreased fertility) and thymosins (decline leads to reduced immune function).

Additional info: This summary expands on the original notes with definitions, examples, and context for hormone mechanisms, disorders, and regulatory feedback. Tables have been recreated for clarity. For equations related to hormone feedback or metabolic effects, see advanced physiology texts.