Ch-17

ANATOMY & PHYSIOLOGY: THE ENDOCRINE SYSTEM

CHAPTER OBJECTIVES

  • After studying this chapter, you will be able to:

    • Identify the contributions of the endocrine system to homeostasis

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

    • Summarize the site of production, regulation, and effects of the hormones of the pituitary, thyroid, parathyroid, adrenal, and pineal glands

    • Discuss the hormonal regulation of the reproductive system

    • Explain the role of the pancreatic endocrine cells in the regulation of blood glucose

    • Identify the hormones released by the heart, kidneys, and other organs with secondary endocrine functions

    • Discuss several common diseases associated with endocrine system dysfunction

    • Discuss the embryonic development of, and the effects of aging on, the endocrine system

ENDOCRINE SYSTEM OVERVIEW

  • Regulates:

    • Growth

    • Development

    • Reproduction

  • Homeostasis Maintenance:

    • Works in conjunction with the nervous system

    • Comprises all endocrine cells and body tissues producing chemical messengers (hormones).

  • Hormones:

    • Chemical messengers produced in one cell and transported in the bloodstream to alter the activities of another tissue.

    • Most hormones travel freely or are bound to transport proteins in general circulation.

PARTS OF THE ENDOCRINE SYSTEM

  1. Endocrine Glands: Ductless glands including:

    • Hypothalamus

    • Pituitary

    • Thyroid

    • Parathyroid

    • Adrenal

    • Pancreas

    • Ovary

    • Testes

    • Thymus

    • Pineal

  2. Target Cells

  3. Receptor Sites

FUNCTIONS OF HORMONES

  1. Stimulate synthesis of enzymes or proteins:

    • Alters rate of synthesis via transcription and translation.

  2. Activate existing enzymes or membrane channels:

    • Turns “on” or “off” metabolic processes.

  3. Increase metabolism

CLASSES OF HORMONES

  1. Amino Acid Derivatives (Amines)

  2. Peptides (Proteins)

  3. Steroids

STRUCTURE OF HORMONES

  • Hormone Class Components:

    • Amino Acid Derivatives:

    • Example: Norepinephrine

    • Peptide Hormones:

    • Example: Oxytocin

    • Protein Hormones:

    • Example: Human Growth Hormone (GH)

    • Steroid Hormones:

    • Derived from lipid cholesterol

AMINO ACID DERIVATIVES (AMINES)

  • Characteristics:

    • Small hormones made from tyrosine and tryptophan (amino acids).

    • Include:

    • Thyroxine (thyroid hormone)

    • Catecholamines such as epinephrine (E) and norepinephrine (NE).

    • Dopamine

PEPTIDE HORMONES (PROTEINS)

  • Definition: Chains of amino acids

  • Secreted by:

    • Hypothalamus

    • Pituitary Gland (hypophysis)

    • Heart

    • Thymus

    • Digestive Tract

    • Pancreas

LIPID DERIVATIVES (STEROIDS)

  • Description: Small hormones derived from cholesterol.

  • Types Include:

    • Eicosanoids (coordinate cellular activities)

    • Steroid hormones produced in reproductive organs (testes and ovaries), adrenal cortex (corticosteroids), and kidneys (calcitriol).

HORMONE MECHANISM OF ACTION

  1. Receptors on Cell Membrane

    • Utilize second messengers such as cyclic-AMP (cAMP)

    • Commonly employed by peptide hormones, allowing for amplification of effects.

  2. Intracellular Receptors

    • Steroid hormones and thyroid hormones directly affect cell activity by entering the cell and binding to receptors.

    • Mechanism:

      • Hormone diffuses through the cell membrane

      • Binds to cytoplasmic receptor; hormone-receptor complex translocates to nucleus

      • Binds to DNA and initiates transcription, creating messenger RNA (mRNA) that is translated into proteins.

SECOND MESSENGER SYSTEM

  • Steps:

    1. Hormone binds to receptor.

    2. Activates G protein in the cell membrane.

    3. G protein activates adenylate cyclase (an enzyme in cell membrane).

    4. Adenylate cyclase converts ATP to cyclic-AMP (cAMP, a second messenger).

    5. cAMP activates kinases:

      • Opens Ca++ channels in membrane

      • Stimulates cellular secretions

      • Activates various enzymes.

HYPOTHALAMIC HORMONES

  1. Releasing hormones: Stimulate synthesis and secretion of one or more hormones at anterior lobe. Examples:

    • Thyroid-Stimulating Hormone-Releasing Factor (TSH-RF) or Thyrotropin-Releasing Hormone (TRH).

  2. Inhibiting hormones: Prevent synthesis or secretion of hormones.

  3. Regulation: Controlled by negative feedback mechanisms.

HYPOTHALAMUS-PITUITARY COMPLEX

  • Location: Inferior and anterior to the thalamus; connects to the pituitary gland through the infundibulum.

  • Pituitary Gland Structure:

    • Comprises anterior lobe and posterior lobe, each secreting different hormones in response to hypothalamic signals.

POSTERIOR LOBE/NEUROHYPOPHYSIS

  • Contains: Unmyelinated axons of hypothalamic neurons.

  • Hormones:

    • Antidiuretic Hormone (ADH)/Vasopressin:

    • Synthesized in supraoptic nuclei

    • Functions to decrease water loss at kidneys and increase blood pressure.

    • Release inhibited by alcohol.

    • Oxytocin (OT):

    • Synthesized in paraventricular nuclei

    • Functions include:

      • Secretion and ejection of milk by mammary glands

      • Stimulating smooth muscles in the uterus

      • Role in sexual arousal and orgasm in both genders.

ANTERIOR LOBE/ADENOHYPOPHYSIS

  • Structure: Consists of three parts:

    • Pars distalis (largest part)

    • Pars intermedia (slender)

    • Pars tuberalis (wraps around infundibulum)

ADENOHYPOPHYSEAL HORMONES

  • Hormones Released:

    • TSH (Thyroid-Stimulating Hormone)

    • ACTH (Adrenocorticotropic Hormone/Corticotropin):

    • Targets adrenal cortex.

    • FSH (Follicle-Stimulating Hormone) and LH (Luteinizing Hormone):

    • Known as gonadotropins, target gonads (testes and ovaries).

    • Prolactin (PRL):

    • Stimulates mammary gland development and production of milk.

    • Growth Hormone (GH/Somatotropin):

    • Stimulates cell growth and replication.

    • MSH (Melanocyte-Stimulating Hormone):

    • Stimulates melanocytes to produce melanin.

PITUITARY GLAND LOCATION

  • Located: In the sella turcica.

  • Connections: Infundibulum connects it to the hypothalamus.

  • Portal System: Hypothalamic-hypophyseal portal system carries releasing hormones from the hypothalamus to the adenohypophysis.

THYROID & PARATHYROID GLANDS

  • Objectives:

    • Describe the location and function of the thyroid and parathyroid glands.

    • Trace the production of thyroxine and the feedback mechanisms involving calcitonin and parathyroid hormones.

    • Identify thyroid and parathyroid disorders.

THYROID GLAND

  • Location: In the neck, wrapping around the trachea.

  • Glandular Composition: Comprised mainly of thyroid follicles, larger parafollicular cells among follicle cells.

THYROXINE PRODUCTION AND FEEDBACK MECHANISM

  1. When blood levels of metabolic rate and T3/T4 are low:

    • Hypothalamus releases TRH, stimulating TSH release by the pituitary.

  2. When high:

    • Hypothalamus stops TRH release, and anterior pituitary stops TSH release.

  3. Classic Negative Feedback Loop:

    • Elevated T3 and T4 levels inhibit the release of TRH and TSH.

THYROID HORMONES

  • Produced Hormones:

    • Thyroxine (T4): Contains 2 tyrosines & 4 iodide ions.

    • Triiodothyronine (T3): Contains 2 tyrosines & 3 iodide ions.

    • Both T3 and T4 are classified as amines.

THYROXINE FEEDBACK MECHANISM

  • Low thyroxine levels in blood:

    • TRH is released from the hypothalamus.

    • TRH stimulates adenohypophysis to release TSH.

  • TSH Activates:

    • TSH binds to membrane receptors causing release of thyroxine into bloodstream.

    • High thyroxine levels inhibit TRH secretion.

  • **Functions of Thyroxine: **

    • Needs for growth and development

    • Regulates metabolic rate of all cells through:

    • Glycolysis

    • Krebs cycle

    • Protein breakdown (mainly from the liver)

    • Glucose uptake from gastrointestinal tract and aid in mitosis together with GH.

PARAFOLLICULAR CELLS (C CELLS) IN THYROID GLAND

  • Function: Produce calcitonin (CT) or thyroid calcitonin (TCT):

    • Regulates calcium (Ca2+) in body fluids.

    • Stimulated by serum calcium levels >20% over normal level.

    • Normal blood calcium: 8.5-11 mg/dL; inhibits serum calcium levels.

THYROID DISORDERS

  • Thyrotoxicosis:

    • Overactive thyroid leading to excessive hormone production.

    • Graves Disease:

    • Autoimmune disorder with exophthalmos (bulging eyes).

    • Goiter:

    • Enlargement of the thyroid gland due to iodine deficiency.

    • Congenital Hypothyroidism:

    • Cretinism in children;

    • Myxedema in adults.

PARATHYROID GLANDS

  • Location: Embedded in the posterior surface of the thyroid gland.

  • Hormone Produced: Parathyroid Hormone (PTH)

    • Stimulus for release: Low serum calcium levels.

    • Function: Raise blood calcium levels, antagonist to calcitonin, primary hormone regulator in calcium homeostasis.

PTH TARGETS/FUNCTIONS

  • Bone: Stimulates osteoclasts (breakdown of bone); inhibits osteoblasts.

  • Kidneys: Reabsorbs Ca++ from kidney tubules into blood; phosphate is excreted in urine.

  • Intestines: Influences calcium absorption and activates cholecalciferol (pre-vitamin D) to stimulate calcitriol formation at the kidneys.

CALCITRIOL TO ENHANCE PTH

  • Function: Enhances the absorption of Ca2+ and PO43- by the digestive tract.

  • Dependency: Vitamin D must be present to metabolize calcium (Ca2+).

HYPERPARATHYROIDISM

  • Definition: Excessive parathyroid gland activity leads to increased PTH levels.

  • Consequences:

    • Excessive calcium reabsorption, decreasing bone density, increasing fracture risks.

    • Calcium deposits may form in other areas of the body, impacting nervous system responsiveness.

ADRENAL (SUPRARENAL) GLAND

  • Location: Sits atop the kidneys.

  • Structure: Composed of two main parts:

    • Adrenal Cortex (3 zones): Secretes steroid hormones.

    1. Zona glomerulosa

    2. Zona fasciculata

    3. Zona reticularis

    • Adrenal Medulla: Secretes catecholamines (amines).

ZONA GLOMERULOSA

  • Composition: 15% of adrenal cortex.

  • Hormones Produced: Mineralocorticoids, mainly aldosterone.

  • Effects of Aldosterone:

    • Sodium(Na+) retention at kidneys leading to water reabsorption.

    • Eliminates potassium(K+).

    • Short-term effects.

ALDOSTERONE SECRETION INFLUENCES

  • Factors Leading to Secretion:

    1. Renin-angiotensin response due to low blood pressure – kidneys release renin triggering angiotensin II formation.

    2. Increased plasma concentration of K+ influences zona glomerulosa.

    3. ACTH – leads to small aldosterone increases during stress.

    4. Atrial natriuretic peptide (ANP) – blocks renin and aldosterone release, decreasing blood pressure.

ALDOSTERONE IMBALANCE

  • Condition: Aldosteronism (hypersecretion); adrenal tumors leading to hypertension and edema.

  • Consequence: Hypokalemia due to excessive Na+ excretion leading to abnormal neuron and muscle function.

ZONA FASCICULATA

  • Secretes: Glucocorticoids, primarily cortisol (hydrocortisone) and corticosterone.

  • Regulation: Stimulated by ACTH from adenohypophysis, under negative feedback control.

FUNCTIONS OF GLUCOCORTICOIDS

  • Maintain normal blood sugar levels and affect all cells by:

    • Accelerating gluconeogenesis (formation of glucose from lipids and proteins).

    • Glucose-sparing effects.

    • Enhancing blood pressure through vasoconstriction.

    • Exhibiting anti-inflammatory properties.

  • Released in response to stress for muscle energy availability.

GLUCOCORTICOIDS IMBALANCE

  • Conditions:

    • Hypersecretion -> Cushing’s syndrome: inhibits cartilage and bone formation, depresses immune response.

    • Hyposecretion -> Addison’s disease: glucose and sodium level deficits, including weight loss and hypotension.

ZONA RETICULARIS

  • Produces: Androgens (male sex hormones), converted to testosterone or estrogens by target cells.

  • Functions: May facilitate the onset of puberty and development of secondary sex characteristics as well as sex drive.

ADRENAL MEDULLA

  • Composition: Contains chromaffin cells, part of the sympathetic division of the ANS.

  • Hormones Secreted: Catecholamines:

    • Epinephrine (80%)

    • Norepinephrine (20%)

FUNCTIONS OF EPINEPHRINE & NOREPINEPHRINE

  • Epinephrine Functions:

    • Stimulates metabolic activities (increasing glucose levels).

    • Bronchial dilation.

    • Increases blood flow to skeletal muscles and heart.

  • Norepinephrine Functions:

    • Influences peripheral vasoconstriction and increases blood pressure.

FUNCTIONS ACROSS TISSUES

  • Skeletal Muscle: Mobilizes glycogen reserves, increases ATP from breakdown of glucose, enhancing muscle strength and endurance.

  • Adipose Tissue: Breaks down stored fats into fatty acids for ATP production data.

  • Liver: Glycogen breakdown increases blood glucose levels.

  • Heart: Enhances heart rate and force of contraction; redistributes blood flow.

PINEAL GLAND

  • Role: Produces melatonin, regulating sleep-wake cycles.

PANCREAS STRUCTURE

  • Location: Positioned in the crook of the duodenum.

  • Cell Types:

    • Exocrine Cells (Acini): Secrete alkaline digestive enzymes into ducts.

    • Endocrine Cells (Islets of Langerhans): Secrete hormones.

ENDOCRINE CELLS IN PANCREATIC ISLETS

  • Alpha Cells: Produce glucagon, targeting the liver to raise blood sugar levels.

  • Beta Cells: Produce insulin to lower blood sugar levels by facilitating glucose transport into cells.

  • Delta Cells: Various functions, including somatostatin secretion.

INSULIN FUNCTION

  • Release Trigger: Detected by glucose, amino acid, and fatty acid concentrations in blood (70-110 mg/dL).

  • Effects of Insulin:

    1. Increases glucose uptake, utilization, and ATP production, decreasing blood glucose level.

    2. Stimulates glycogenesis (formation of glycogen from glucose).

    3. Promotes amino acid absorption and protein synthesis.

    4. Enhances glucose transport into fat and muscle cells.

    5. Participates in neuron development, learning, and memory.

GLUCAGON FUNCTION

  • Produced by: Alpha cells when blood glucose levels decrease.

  • Stimulates:

    1. Glycogen breakdown (glycogenolysis) in liver and muscles.

    2. Breakdown of triglycerides in adipose tissue for energy.

    3. Glucose production through gluconeogenesis from protein breakdown in the liver.

HOMEOSTATIC REGULATION OF BLOOD GLUCOSE LEVELS

  • Blood glucose maintained tightly between 70 mg/dL to 110 mg/dL:

    • If levels rise above, insulin is released to facilitate cellular glucose uptake.

    • If levels drop, glucagon is released, prompting glucose release into the bloodstream.

DIABETES MELLITUS

  • Definition: Condition characterized by elevated glucose concentration and impaired cellular glucose entry.

  • **Causes: **

    • Genetic factors

    • Viral infections

    • Poor nutrition/diet

    • Autoimmune disorders

    • Environmental factors (mutagens)

TYPES OF DIABETES

  1. Type I (IDDM/Juvenile Onset):

    • Beta cells destruction leads to insulin deficiency; typically autoimmune; treated with insulin administration.

  2. Type II (NIDDM):

    • Usually appears after age 40, often in overweight individuals with decreased insulin output.

    • Typically managed with oral medications and dietary changes.

RELATED DIABETIC HEALTH ISSUES

  • Complications Include:

    • Diabetic retinopathy

    • Cataracts

    • Diabetic nephropathy

    • Diabetic neuropathies

    • Increased risk of heart attacks

    • Circulatory problems leading to tissue death, ulcers, slow healing.

ORGANS WITH SECONDARY ENDOCRINE FUNCTIONS

  • Included Organs:

    • Heart

    • Gastrointestinal tract

    • Kidneys

    • Skeleton

    • Adipose tissue

    • Skin

    • Thymus