Endocrine System

Chapter 10: The Endocrine System

Introduction

  • Contributors:

    • William C. Ober, M.D. (Art Coordinator and Illustrator)

    • Claire E. Ober, R.N. (Illustrator)

    • Kathleen Welch, M.D. (Clinical Consultant)

    • Kevin Petti, Ph.D. (Contributor)

  • Textbook: Essentials of Anatomy & Physiology, Eighth Edition, Pearson

Intercellular Communication

  • Purpose: Maintains homeostasis

  • Mechanism:

    • Achieved mainly through chemical messages

    • Long-distance communication coordinated by endocrine and nervous systems

  • Comparison:

    • Nervous System:

    • Faster response

    • Specific destinations

    • Short-lived effects

    • Endocrine System:

    • Slower response

    • Releases hormones into bloodstream

    • Bind to target cells

    • Longer-lasting effects

Nervous and Endocrine Systems Comparison

  • Shared features:

    • Release of chemicals that bind to specific receptors on target cells

    • Share chemical messengers (e.g., epinephrine and norepinephrine)

    • Both regulated by negative feedback mechanisms

    • Coordinate and regulate activities of other cells, tissues, organs, and systems to maintain homeostasis

The Endocrine System

  • Overview:

    • Includes all endocrine cells and tissues

    • Consists of glandular, secretory cells

    • Secretions enter extracellular fluid

  • Types of hormones:

    • Local hormones (e.g., prostaglandins) affect adjacent cells

    • Hormones: Released from one tissue and transported via bloodstream to target cells in other tissues

Figure 10-1: Organs and Tissues of the Endocrine System

  • Key organs and their functions:

    • Hypothalamus: Production of ADH, oxytocin, and regulatory hormones

    • Pineal Gland: Produces melatonin

    • Pituitary Gland: Releases several hormones (e.g., ACTH, TSH, GH)

    • Thyroid Gland: Produces thyroid hormones (T4, T3)

    • Adrenal Glands: (adrenal medulla, and adrenal cortex)

    • Medulla: Epinephrine, norepinephrine

    • Cortex: Cortisol, aldosterone, androgens

    • Pancreas: Insulin, glucagon

    • Ovary: Estrogens, progesterone

    • Parathyroid Glands: Parathyroid hormone (PTH)

    • Secondary Endocrine Functions: Organs such as heart (ANP), thymus (thymosins), kidneys (EPO, calcitriol), testes (androgens)


Three Groups of Hormones

  • Classification based on chemical structure:

    1. Amino Acid Derivatives:

    • Structurally similar to amino acids

    • E.g., epinephrine, norepinephrine, thyroid hormones, melatonin

    1. Peptide Hormones:

    • Chains of amino acids

    • E.g., ADH, oxytocin, hypothalamic, pituitary, and pancreatic hormones

    1. Lipid Derivatives:

    • Steroid Hormones: Similar to cholesterol, bound to transport proteins

      • Released by reproductive organs and adrenal glands

    • Eicosanoids: Derived from arachidonic acid, affect local cellular functions, include prostaglandins

Hormone Action

  • Sensitivity: Target cell sensitivity to hormone depends on the presence of receptors

  • Hormones alter target cell operations including:

    • Types

    • Activities

    • Locations

    • Quantities of structural proteins and enzymes

  • Receptor Location:

    • Plasma membrane or inside the cell

Plasma Membrane Receptors

  • Required for non-lipid soluble hormones:

    • E.g., epinephrine, norepinephrine, peptide hormones, eicosanoids

    • These hormones cannot diffuse through the plasma membrane

    • Must bind to receptors on the outside

    • First messengers activate second messengers in the cytoplasm, linked by a G protein

Second Messenger Systems

  • Amplification: A small number of hormone molecules activate thousands of second messengers

  • Examples of second messengers:

    • Cyclic-AMP

    • Calcium ions

    • Cyclic-GMP

Cyclic-AMP Second Messenger System

  • Mechanism:

    • First messenger activates G protein

    • G protein activates enzyme adenylate cyclase

    • ATP is converted to second messenger (cAMP)

    • cAMP activates kinase enzymes, leading to phosphorylation

  • Effect varies depending on the protein affected


Intracellular Receptors

  • Used for lipid-soluble hormones

    • E.g., thyroid and steroid hormones

  • Mechanism:

    • Hormones cross plasma membrane, form hormone-receptor complex

    • Activates or inactivates specific genes

    • Alters mRNA transcription rate, changing cell structure/function

Hormone Secretion and Distribution

  • Hormones rapidly enter bloodstream; freely circulating hormones are short-lived

  • Inactivation:

    • Hormones bind to receptors and diffuse to target cells

    • Absorbed and broken down in liver and kidney

    • Inactivated by enzymes in plasma/interstitial fluid

  • Hormones bound to transport proteins stay in circulation longer

Control of Endocrine Activity

  • Negative Feedback Control: Hormonal secretion based on three stimuli:

    1. Humoral Stimuli: Changes in extracellular fluid composition

    2. Hormonal Stimuli: Changes in circulating hormone levels

    3. Neural Stimuli: Neural stimulation of neuroglandular junctions

Hypothalamus and Endocrine Control

  • Provides the highest level of endocrine control

  • Regulates nervous and endocrine systems in three ways

The Pituitary Gland

  • Also called hypophysis

  • Structure: Protected by the sella turcica, hangs from the hypothalamus via infundibulum

  • Hormones:

    • Secretes nine hormones, all unique peptides or small proteins

    • Uses cAMP second messenger mechanism

The Anterior Lobe of the Pituitary Gland

  • Contains endocrine cells surrounded by a complex capillary bed (hypophyseal portal system)

  • Two capillary networks linked by blood vessels

  • Regulatory hormones diffuse onto target cells in the anterior lobe, many of which are tropic hormones (target other endocrine glands)

The Seven Anterior Lobe Hormones

  1. Thyroid-stimulating hormone (TSH)

  2. Adrenocorticotropic hormone (ACTH)

  3. Follicle-stimulating hormone (FSH)

  4. Luteinizing hormone (LH)

  5. Prolactin (PRL)

  6. Growth hormone (GH)

  7. Melanocyte-stimulating hormone (MSH)


Thyroid-stimulating Hormone (TSH)

  • Also called thyrotropin

  • Released in response to thyrotropin-releasing hormone (TRH) from hypothalamus

  • Targets thyroid gland; triggers release of thyroid hormones

  • Increases in thyroid hormones cause decrease in TRH and TSH secretion

Adrenocorticotropic Hormone (ACTH)

  • Also called corticotropin

  • Released in response to corticotropin-releasing hormone (CRH) from hypothalamus

  • Targets adrenal cortex; stimulates secretion of glucocorticoids

  • Increases in glucocorticoids cause decrease in ACTH and CRH secretion

Gonadotropins

  • Released in response to gonadotropin-releasing hormone (GnRH) from hypothalamus

  • Target the male and female gonads: testes in males and ovaries in females

  • Includes two hormones:

    • Follicle-stimulating hormone (FSH)

    • Luteinizing hormone (LH)

Follicle-stimulating Hormone (FSH)

  • Promotes follicle development in females

  • Promotes sperm production in males

  • Cells of ovaries and testes produce inhibin which inhibits FSH production through negative feedback

Luteinizing Hormone (LH)

  • Induces ovulation and secretion of estrogens and progesterone in females

  • In males, also known as interstitial cell-stimulating hormone (ICSH) which stimulates interstitial cells in testes to produce androgens (e.g., testosterone)

  • Estrogens and progesterone inhibit GnRH, decreasing levels of LH

Prolactin (PRL)

  • Released in response to prolactin-releasing factor (PRF) from hypothalamus

  • Targets mammary glands in females; stimulates development and milk production during pregnancy and nursing

  • Circulating PRL stimulates prolactin-inhibiting hormone (PIH) inhibiting PRF

Growth Hormone (GH)

  • Also called human growth hormone (hGH) or somatotropin

  • Regulated by growth hormone-releasing hormone (GH-RH) and growth hormone-inhibiting hormone (GH-IH) from the hypothalamus

  • Stimulates cell growth and replication, especially in skeletal muscle and chondrocytes

  • Indirect action: Stimulates liver to release somatomedins (insulin-like growth factors, IGFs) influencing protein synthesis

Direct Actions of Growth Hormone

  • In Epithelial and Connective Tissues: Stimulates stem cell division and differentiation

  • In Adipose Tissue: Breaks down stored fats and releases fatty acids, sparing glucose

  • In Liver: Breaks down glycogen reserves, releases glucose into bloodstream

Melanocyte-stimulating Hormone (MSH)

  • Increases activity of melanocytes in skin, increasing melanin production

  • Nonfunctional in adults, secreted during fetal development, early childhood, pregnancy, and certain diseases

The Posterior Lobe of the Pituitary Gland

  • Contains axons from two groups of hypothalamic neurons

  • Produces two hormones:

    1. Antidiuretic hormone (ADH)

    2. Oxytocin (OXT)

  • Hormones are transported within axons to the posterior pituitary lobe where released into capillaries

Antidiuretic Hormone (ADH)

  • Also called vasopressin

  • Released when the body is low on water (stimulated by increased solutes in blood or decreased blood volume/pressure)

  • Primary target: kidneys for decreasing water loss

  • Triggers vasoconstriction, increasing blood pressure

Oxytocin (OXT)

  • In women: Stimulates contraction of uterine muscles during labor

  • Also stimulates contraction of milk secretory cells in mammary glands

  • Plays an unclear role in sexual activity

  • In men: Stimulates smooth muscle in the walls of sperm duct, aiding in emission

The Pituitary Hormones Summary Table

Table 10-1: The Pituitary Hormones

  • Pituitary Lobe/Hormone

    • Anterior Lobe: TSH, ACTH, FSH, LH, PRL, GH, MSH

    • Posterior Lobe: ADH, OXT

  • Target and Hormonal Effects:

    • TSH: Thyroid gland → thyroid hormone secretion

    • ACTH: Adrenal cortex → glucocorticoid secretion

    • FSH: Follicle cells of ovaries → estrogen secretion and follicle development

    • LH: Uterus/mammary glands → ovulation, progesterone secretion

    • PRL: Mammary glands → milk production

    • GH: All cells → growth, protein synthesis

    • MSH: Melanocytes of skin → increased melanin synthesis

    • ADH: Kidneys → water reabsorption

    • OXT: Uterus/mammary glands (females) and sperm duct/prostate (males) → labor contractions, milk ejection

The Thyroid Gland

  • Location: Anterior to trachea, inferior to thyroid cartilage

  • Structure: Two lobes connected by a narrow isthmus, contains many spherical thyroid follicles lined by simple cuboidal epithelium filled with viscous colloid

The Thyroid Follicles

  • Follicular cells produce thyroid hormones stored in colloid

  • TSH stimulates release of thyroid hormones

  • Thyroid hormones are transported bound to plasma proteins, providing large reserves

  • Thyroid hormones derived from amino acid tyrosine and iodine:

    • Thyroxine (T4): Four iodine atoms

    • Triiodothyronine (T3): Three iodine atoms, more potent than T4

The Effects of Thyroid Hormones

  • Affect nearly every cell in the body

  • Increase ATP production rates in mitochondria

  • Activate genes encoding enzymes for glycolysis and energy production

  • Calorigenic Effect: Cells use more energy, producing heat

  • Essential for normal skeletal, muscular, and nervous system development

The C Cells of the Thyroid Gland

  • Also called parafollicular cells that produce calcitonin (CT)

  • Triggered by increases in plasma Ca2+, inhibits osteoclasts, and stimulates calcium excretion by kidneys, essential for normal bone growth

Calcium Balance and Calcitonin

  • Hypercalcemia (high blood calcium) reduces sodium permeability of nerves and muscles, affecting responsiveness

The Parathyroid Glands

  • Two pairs of small glands embedded in the posterior surface of the thyroid gland

  • Contains principal cells producing parathyroid hormone (PTH)

  • Stimulated by decreases in plasma Ca2+:

    • Activates osteoclasts and inhibits osteoblasts in bone

    • Reduces calcium excretion by kidneys

    • Stimulates kidney to secrete calcitriol, enhancing Ca2+ and PO43- absorption in the digestive tract

Calcium Balance and PTH

  • Hypocalcemia (low blood calcium) increases sodium permeability of membranes, causing excitability and spasms

  • PTH prevents hypocalcemia

Table 10-2: Hormones of the Thyroid Gland and Parathyroid Glands

Summary:

Gland/Cells

Hormone(s)

Targets

Effects

Thyroid (Follicular epithelium)

Thyroxine (T4), Triiodothyronine (T3)

Most cells

Increased energy use, growth

Thyroid (C cells)

Calcitonin (CT)

Bone, kidneys

Decreased calcium in body fluids

Parathyroids (Principal cells)

PTH

Bone, kidneys

Increased calcium in body fluids

The Adrenal Gland

  • Also known as the suprarenal gland, located on each kidney's superior border

  • Composed of two portions: adrenal cortex (outer) and adrenal medulla (inner)

The Adrenal Cortex

  • Contains high levels of cholesterol and fatty acids

  • Produces over 24 steroid hormones called corticosteroids, essential for metabolic functions and life

  • Divided into three zones:

    • Zona glomerulosa: Mineralocorticoids

    • Zona fasciculata: Glucocorticoids

    • Zona reticularis: Androgens

Mineralocorticoids

  • Main mineralocorticoid is aldosterone

  • Secreted in response to low Na+, low BP, high K+, or angiotensin II presence

  • Triggers sodium reabsorption and potassium elimination, prevents sodium loss in urine and promotes water reabsorption

Glucocorticoids

  • Main glucocorticoids include cortisol, corticosterone, and cortisone

  • Secreted in response to ACTH

  • Increase rates of glucose synthesis, glycogen formation, blood glucose levels

  • Act as anti-inflammatory agents by inhibiting components of the immune system

The Androgens

  • Produced in small quantities in both sexes; some convert to estrogens

  • Stimulate development of pubic hair before puberty

  • In adult women: Contribute to muscle mass, blood cell formation, and sex drive

The Adrenal Medulla

  • Vascular, includes cells similar to sympathetic ganglia

  • Secretory cells produce epinephrine (75-80%) and norepinephrine (20-25%)

  • Hormones trigger metabolic changes for increased energy availability, supporting the sympathetic response

The Pineal Gland

  • Located on the third ventricle's posterior roof

  • Produces melatonin, secretion influenced by light/dark cycles

  • May influence sexual maturation timing and protect CNS neurons with antioxidant activity

The Endocrine Pancreas

  • Lies between the stomach and proximal small intestine

  • Contains both exocrine (digestive enzymes) and endocrine cells

Endocrine Cells in Pancreas

  • Located in islets of Langerhans, accounting for 1% of pancreatic cells

  • Alpha cells: Secrete glucagon

  • Beta cells: Secrete insulin, regulating blood glucose levels

Blood Glucose and Insulin

  • Increases in blood glucose stimulate beta cells to release more insulin

  • Insulin triggers glucose uptake by almost all cells (exceptions: neurons, RBCs, intestinal epithelium)

  • Increases protein synthesis and fat storage, leading to lower blood glucose after meals

Blood Glucose and Glucagon

  • Decreases in blood glucose activate alpha cells to release glucagon

  • Mobilizes energy reserves by breaking down glycogen and releasing fatty acids

Pancreatic Regulation of Blood Glucose

  • Secretion of insulin and glucagon not directly stimulated by nerves

  • Affected by hormones influencing blood glucose e.g., cortisol

  • ANS activity: Parasympathetic stimulates insulin release, sympathetic inhibits it

Diabetes Mellitus

  • Faulty glucose metabolism causing hyperglycemia

  • Type 1 Diabetes: Inadequate insulin production

  • Type 2 Diabetes: Insulin resistance

  • Symptoms include hyperglycemia, glycosuria, polyuria

Complications of Diabetes Mellitus

  • Retinal Damage: Diabetic retinopathy can lead to blindness

  • Kidney Degeneration: Diabetic nephropathy can cause kidney failure

  • Cardiac Issues: Early heart attacks are more likely

  • Peripheral Nerve Problems: Abnormal blood flow causing various neural issues

  • Tissue Damage: Reduced blood flow leading to ulceration and infection

Secondary Endocrine Organs

  • Various organs produce hormones but primarily function in other systems

  • Examples:

    • Intestines: Release local hormones coordinating digestive activities

    • Kidneys:

    • Calcitriol: Stimulated by PTH, increases calcium absorption

    • Erythropoietin (EPO): Stimulated by low oxygen levels, increases RBC production

    • Renin: Triggers hormonal reactions to increase blood pressure

    • Heart: Atrial natriuretic peptide (ANP) released in response to increased blood volume

    • Thymus: Releases thymosins for immune defense development

    • Gonads and Adipose Tissue: Produce hormones impacting reproductive functions

The Intestines

  • Release hormones to coordinate digestive activities, major control over digestion rate influenced by ANS

Hormones Released by the Kidneys

  • Calcitriol: Increases calcium absorption

  • Erythropoietin: Increases RBC production

  • Renin: Triggers the RAAS for blood pressure regulation

The Heart

  • Endocrine cells in right atrium release ANP in response to increased blood volume, promoting sodium and water loss

The Thymus

  • Deep to sternum, active in childhood, atrophies in adulthood

  • Releases thymosins to support immune defenses

The Gonads: The Testes

  • Produce androgens (mainly testosterone)

  • Promote sperm production and maintain male reproductive structures

  • Inhibin secreted by nurse cells regulates FSH production

The Gonads: The Ovaries

  • Follicular cells produce estrogens and inhibins

  • Estrogens support egg maturation and uterine lining growth

  • After ovulation, corpus luteum secretes estrogen and progesterone preparing for embryo implantation

Hormones of the Reproductive System

Table 10-3: Summary of Reproductive Hormones

Hormone

Primary Target

Effects

Structure/Cells

Androgens (testes)

Most cells

Supports sperm maturation, secondary sex characteristics

Interstitial cells

Inhibin (testes)

Anterior pituitary

Inhibits FSH secretion

Nurse cells

Estrogens (ovaries)

Most cells

Supports follicle maturation, secondary sex characteristics

Follicular cells

Progesterone (ovaries)

Uterus, mammary glands

Prepares for embryo implantation, milk production

Corpus luteum

Adipose Tissue

  • Produces leptin, regulating appetite

  • Triggers satiation, suppressing appetite

  • Vital for normal GnRH and gonadotropin synthesis

  • Low body fat can delay puberty; increased fat may enhance fertility

Hormonal Interactions

  • Hormones in extracellular fluid can produce various effects:

    1. Antagonistic Effects: Opposing (e.g., PTH and calcitonin)

    2. Synergistic Effects: Greater combined effect

    3. Permissive Effects: One hormone's presence is necessary for another's function

    4. Integrative Effects: Different but complementary effects

Hormones and Growth

  • Six hormones essential for normal growth:

    1. Growth Hormone: Supports muscle and skeletal growth

    2. Thyroid Hormones: Required for nervous system development

    3. Insulin: Supplies energy to growing cells

    4. PTH: Promotes calcium availability for bone growth

    5. Calcitriol: Calcium absorption support

    6. Reproductive Hormones: Affect osteoblast activity and secondary characteristics

Hormones and Stress

  • Stress triggers:

    • Physical (injury/disease), emotional (anxiety), environmental (extremes), metabolic (starvation)

  • Triggers the general adaptation syndrome (GAS) which has three phases:

    1. Alarm Phase: Immediate fight or flight response, dominated by epinephrine

    2. Resistance Phase: Lasts several hours, characterized by glucocorticoids

    3. Exhaustion Phase: Homeostatic breakdown leads to organ failure possibility

Hormones and Behavior

  • Regulated by the hypothalamus

  • Hormonal imbalances can alter behavior (e.g., precocious puberty) affecting aggression and assertiveness

Hormones and Aging

  • Usually most hormones remain the same throughout adulthood, except reproductive hormones.


Endocrine Disorders