CHAPTER 13 ENDOCRINE

Endocrine System

13.1 General Characteristics of the Endocrine System

• The endocrine system, along with the nervous system, regulates bodily functions to maintain homeostasis and coordinate communication.

• It is a unique system because its organs are not anatomically connected.

• Major endocrine glands include:

  • Pituitary gland

  • Thyroid gland

  • Parathyroid glands

  • Adrenal glands

  • Pancreas

  • Pineal gland

  • Thymus

  • Ovaries and Testes (reproductive glands)

Characteristics of the Endocrine System

• Endocrine glands:

  • Comprise cells, tissues, and organs making up the endocrine system.

  • Ductless and secrete hormones directly into body fluids.

  • The term "endocrine" means “internal secretion.”

  • Hormones act only on target cells containing receptors for them.

• Exocrine glands:

  • Secrete substances into ducts leading to body surfaces.

  • Deliver their products directly to specific sites.

• Local hormones:

  • Paracrine secretions affect nearby cells.

  • Autocrine secretions affect the very cells that secrete them.

Comparison Between Nervous and Endocrine Systems

• Both systems are critical for communication within the body.

• They use chemicals that bind to receptor molecules:

  • Nervous system: Releases neurotransmitters into synapses.

  • Endocrine system: Secretes hormones directly into the bloodstream.

• Response speed:

  • The nervous system responds faster than the endocrine system.

• Duration of effects:

  • Endocrine effects can outlast those of the nervous system.

Chemical Communication in Nervous and Endocrine Systems

• The endocrine system is highly specific.

  • Only target cells can respond to a specific hormone.

  • Target cells possess specific receptors for the hormone, which are absent in other cells.

Hormone Names and Abbreviations 1

Hormone Names and Abbreviations 2

13.2 Hormone Action

• Hormones are released into extracellular fluid and then diffuse into the blood.

• The method of transport through the blood depends on whether the hormone is:

  • Lipid-soluble

  • Water-soluble

• Hormones are powerful substances active at low concentrations.

Chemistry of Hormones

• Hormones are organic compounds classified into two general types:

  • Steroid or steroid-like hormones:

  • Composed of lipids containing complex rings of carbon and hydrogen atoms.

  • All steroid hormones are derived from cholesterol.

  • Examples include sex hormones (testosterone, estrogens) and adrenal cortex hormones (cortisol, aldosterone).

  • Nonsteroid hormones:

  • Amines: Derived from tyrosine (e.g., epinephrine, norepinephrine, thyroxine).

  • Peptides: Short chains of amino acids (e.g., ADH, oxytocin).

  • Proteins: Long chains of amino acids (e.g., growth hormone).

  • Glycoproteins: Carbohydrates bonded to proteins (e.g., TSH).

Table 13.3 Types of Hormones

Actions of Hormones

• Hormones alter metabolic processes, which can involve:

  • Altering enzyme activity.

  • Changing the rate of membrane transport of substances.

• Hormones bind to receptors on or in target cells, causing changes even at low concentrations.

• Receptor dynamics:

  • Upregulation: An increase in the number of receptors on target cells in response to a decrease in hormone level.

  • Downregulation: A decrease in the number of receptors on target cells due to an increase in hormone level.

Steroid and Thyroid Hormones

• Both types have poor water solubility.

• They are transported in blood bound to plasma proteins.

• Steroid hormones can diffuse through the lipid bilayer of cell membranes, while thyroid hormones enter cells via specific transport methods.

• Both hormone types bind to receptors inside cells, usually in the nucleus, and initiate the transcription of specific DNA genes, leading to protein synthesis that constitutes hormone action.

Nonsteroid Hormones

• Nonsteroid hormones cannot penetrate cell membranes.

• They bind to receptors located on target cell membranes.

• The hormone acts as a first messenger, while the chemical leading to the hormone’s effect is known as the second messenger (e.g., cyclic AMP).

• The entire process of chemical communication from outside cells to inside is termed signal transduction.

13.3 Control of Hormonal Secretions

• Hormonal secretion is largely controlled by negative feedback mechanisms.

  • A few cases involve positive feedback, particularly in the reproductive system.

• Hormone effects may be temporary (lasting minutes) or prolonged (lasting days).

• Hormones can be excreted in urine or broken down by enzymes, primarily in the liver, to halt their effects.

Control Sources

• Negative feedback:

  • Rising hormone levels decrease their secretion.

  • Hormone consumption stops the inhibition, allowing secretion to resume.

  • Methods of negative feedback control:

  • Hormonal control: Some hormones act on other glands (e.g., hypothalamus controls anterior pituitary hormone release, which then secretes hormones that affect other glands).

  • Nervous control: The nervous system can directly stimulate some glands to secrete hormones.

  • Humoral control: Changes in the internal environment (e.g., fluctuations in blood ion or glucose levels) stimulate or inhibit hormone secretion.

• Positive feedback:

  • Rising hormone levels stimulate increased secretion, observed in certain reproductive control cases.

13.4 Pituitary Gland

• Often referred to as the master gland of the body, it resides at the brain's base in the sella turcica of the sphenoid bone.

• Located below the hypothalamus and linked to it by the pituitary stalk (infundibulum).

• Controlled by the brain, it is considered part of the nervous system.

• Composed of two main parts:

  • Anterior lobe (adenohypophysis): Controlled by hormones from the hypothalamus.

  • Posterior lobe (neurohypophysis): Controlled by nerve impulses from the hypothalamus.

• There is a small intermediate lobe (pars intermedia) that produces melanocyte-stimulating hormone (MSH), which initiates melanin production.

• Major hormones produced include:

  • GH, PRL, TSH, ACTH, FSH, LH/ICSH, MSH, Vasopressin, and Oxytocin.

• The function of these hormones is to regulate the activities of target organs.

13.5 Anterior Pituitary Hormones

• The anterior lobe is made of glandular epithelial tissue, producing several key hormones.

• Anterior lobe regulation occurs via hypothalamic releasing hormones, where each hormone is released in response to a specific releasing hormone from the hypothalamus, while some hormones may also be inhibited by release-inhibiting hormones.

• Major anterior pituitary hormones include:

  • Growth hormone (GH): Stimulates cell growth and division; increases amino acid uptake/protein synthesis; decreases carbohydrate metabolism; increases fat metabolism.

  • Prolactin (PRL): Promotes milk production in females; has an uncertain function in males.

  • Thyroid-stimulating hormone (TSH): Stimulates secretion of thyroid hormones (T3 and T4) from the thyroid gland.

  • Adrenocorticotropic hormone (ACTH): Stimulates secretion of cortisol and other glucocorticoids from the adrenal cortex.

  • Follicle-stimulating hormone (FSH): Promotes the growth and development of ovarian follicles in females; stimulates sperm production in males.

  • Luteinizing hormone (LH): Triggers ovulation in females; stimulates sex hormone production in both genders.

Functions of Anterior Pituitary Hormones

• Growth Hormone (somatotropin):

  • Stimulates cells to enlarge and divide rapidly.

  • Increases amino acid uptake and protein synthesis.

  • Decreases the metabolic rate of carbohydrates.

  • Increases the rate of fat metabolism.

• Prolactin:

  • Primarily promotes milk production in females.

  • Its function in males is less clear.

• Thyroid-stimulating Hormone (TSH):

  • Stimulates the release of thyroid hormones (T3 and T4) from the thyroid.

• Adrenocorticotropic Hormone (ACTH):

  • Stimulates cortisol secretion and other glucocorticoids from the adrenal cortex.

• Follicle-stimulating Hormone (FSH):

  • Stimulates the development of ovarian follicles in females and promotes sperm production in males.

• Luteinizing Hormone (LH):

  • Triggers ovulation and promotes sex hormone production in both males and females.

Posterior Pituitary Hormones

• The posterior lobe consists of nerve fibers from the hypothalamus.

• Unlike the anterior lobe, it does not contain glandular epithelium.

• Regulation:

  • Two hormones are synthesized in the hypothalamus and stored in and released by the posterior pituitary.

• Hormones:

  • Antidiuretic hormone (ADH, vasopressin):

  • Reduces urine production by increasing water reabsorption by the kidneys, thereby decreasing urine volume.

  • Causes vasoconstriction, which increases blood pressure.

  • Oxytocin:

  • Triggers muscle contractions in the uterine wall during childbirth.

  • Stimulates milk ejection during lactation.

  • May assist in sperm movement and sexual response in males, though its role is less clear evidence.

Disorders of the Pituitary Gland

• Hyperfunction:

  • Gigantism: Caused by excessive GH secretion during childhood.

  • Acromegaly: Caused by excessive GH secretion in adulthood after epiphyseal ossification.

• Hypofunction:

  • Dwarfism: Occurs due to a deficiency in GH during childhood.

  • Diabetes insipidus: Results from insufficient production of ADH.

13.6 Thyroid Gland

• The thyroid gland consists of two lateral lobes connected by an isthmus and is located just below the larynx and anterior to the trachea.

• The gland has a unique ability to extract iodine from the blood.

• Produces three hormones:

  • T4 (thyroxine): Produced by follicular cells.

  • T3 (triiodothyronine): Also produced by follicular cells.

  • Calcitonin: Produced by extrafollicular cells.

• The thyroid is structured into follicles, each surrounded by follicular cells and filled with viscous colloid.

Hormones of the Thyroid Gland

Disorders of the Thyroid Gland

13.7 Parathyroid Glands

• The parathyroid glands are located on the posterior surface of the thyroid, typically numbering four.

• They secrete Parathyroid hormone (PTH), which plays a crucial role in regulating calcium and phosphate ion concentrations in the blood.

• Actions of PTH:

  • Raises blood calcium levels while lowering phosphate levels.

  • Stimulates bone resorption and conserves calcium while excreting phosphate through the kidneys.

  • It also facilitates the final step of active vitamin D production for calcium absorption in intestines.

• Antagonistic relationship: Parathormone (PTH) and Calcitonin perform opposing actions on blood calcium levels.

Table 13.9 Disorders of the Parathyroid Gland

13.8 Adrenal Glands

• The adrenal glands (suprarenal glands) are positioned atop each kidney and produce a variety of hormones that modulate blood sodium levels, stress responses, and sex hormones.

• Composed of two main sections:

  • Adrenal cortex: Produces steroid hormones including aldosterone, cortisol, and sex hormones (androgens).

  • Adrenal medulla: Secretes amine hormones, primarily epinephrine (80%) and norepinephrine (20%), mirroring the effects of sympathetic nervous stimulation like increased heart rate and heightened alertness.

• Hormonal effect duration is significantly longer than neurotransmitters.

Hormones of the Adrenal Cortex

Comparative Effects of Epinephrine and Norepinephrine

ADRENAL DISORDERS

• Hyperfunction:

  • Cushing’s syndrome: Caused by cortisol hypersecretion; signs include high blood pressure, elevated blood glucose levels, muscular weakness, slow wound healing, excess hair growth, and characteristic body changes (moon face, buffalo hump, stretch marks).

• Hypofunction:

  • Addison's syndrome: Results from inadequate aldosterone and cortisol; symptoms comprise bronze pigmentation, low blood glucose, low blood pressure, decreased appetite, and dehydration.

13.9 Pancreas

• The pancreas serves dual functions as an endocrine and exocrine gland, secreting hormones into body fluids and digestive enzymes through ducts.

• Its primary role is to regulate blood sugar levels.

• Structure:

  • A soft tissue, feather-shaped organ located behind the stomach, stretching from the duodenum to the spleen.

• Hormones secreted by islet cells:

  • Alpha cells: Secretes glucagon.

  • Beta cells: Secretes insulin.

  • Delta cells: Secretes somatostatin.

  • Insulin levels are regulated through a negative feedback loop.

Regulation of Blood Glucose

• Insulin and glucagon maintain blood glucose levels through antagonistic effects, governed by negative feedback mechanisms.

Table 13.12 Hormones of the Pancreatic Islets

PANCREATIC DISORDERS

• Diabetes Mellitus:

  • A metabolic disorder stemming from insulin deficiency or impaired tissue response to insulin.

  • Symptoms include elevated blood glucose resulting in potential damage to eyes, heart, kidneys, and nerves.

  • Leads to metabolic disruptions in carbohydrate, protein, and fat metabolism.

  • Insulin normally enables glucose uptake by cells; in diabetes, this leads to hyperglycemia, where cells utilize alternate energy sources, resulting in tissue wasting, weight loss, increased hunger, fatigue, poor wound healing, and growth issues in children.

  • Excess glucose in urine (glycosuria) leads to dehydration and thirst due to osmotic shifts.

• Type 1 Diabetes:

  • Often starting before the age of 20, this autoimmune condition causes destruction of insulin-secreting islet cells.

  • Affects 5-10% of cases and requires insulin.

• Type 2 Diabetes:

  • Related to insulin resistance, primarily affects adults over 45 and represents 90-95% of cases.

13.9 Pineal and Thymus

• Pineal Gland:

  • Located in the brain between cerebral hemispheres, atop the 3rd ventricle.

  • Secretes melatonin, regulating circadian rhythms (day/night cycles).

• Thymus Gland:

  • Functions as an endocrine/lymphatic organ located in the mediastinum between the lungs.

  • It is notably larger during childhood and shrinks in adults.

  • Secretes thymosin, crucial for T-lymphocytes development—vital for immunity.

Reproductive Organs (Gonads)

• Female Gonads (Ovaries):

  • Located within the pelvic region, they produce estrogens and progesterone.

• Male Gonads (Testes):

  • Positioned outside the lower genital region, they produce testosterone and inhibin.

• Functions of the Gonads:

  • Maturation of sex organs, hormone production, and participation in reproduction.

• Placenta:

  • During pregnancy, the placenta produces estrogens, progesterone, and human chorionic gonadotropin (HCG).

Gonad Hormones

• Female Hormones:

  • Estrogen: Promotes ovum maturation, stimulates endometrial blood vessel growth, and drives secondary sexual characteristics (breasts, pubic hair).

  • Progesterone: Secreted by corpus luteum post-ovulation; sustains uterine lining during pregnancy and initiates menstruation if fertilization does not occur.

• Male Hormones:

  • Testosterone: Encourages sperm production; fosters development and maintenance of secondary sexual characteristics (facial hair, voice, muscle development).

  • Inhibin: Works to regulate testosterone levels and maintain constant spermatogenesis rates.

13.9 Other Hormone-Secreting Tissues

• Digestive tissues: Produce hormones regulating digestion.

  • Gastric mucosa: Produces gastrin.

  • Small intestine mucosa: Produces secretin and cholecystokinin.

• Heart: Produces natriuretic peptides to promote sodium secretion in urine.

• Kidney: Produces erythropoietin, stimulating red blood cell formation.

13.10 Stress and Its Effects

• Maintaining homeostasis is vital for survival and is threatened by internal/external environmental changes.

• Stressors (factors triggering loss of homeostasis) signal sensory receptors to dispatch impulses to the hypothalamus.

• Types of Stress:

  • Psychological: Encountered in dangerous situations or personal losses, anger, fear, guilt.

  • Physical: Includes temperature extremes, infections, injuries, or oxygen deficiencies.

Responses to Stress 1

• The hypothalamus activates the stress response termed the General Adaptation Syndrome (GAS).

• Stages of General Stress Syndrome:

  • Alarm Stage:

  • The immediate fight or flight response; heightened sympathetic impulses increase blood glucose and fatty acids while raising heart/breathing rates and blood pressure.

  • Epinephrine secretion intensifies and prolongs these effects.

  • Resistance Stage:

  • A slower, prolonged stage where cortisol secretion via the CRH-ACTH pathway saves glucose.

  • Mobilizes energy sources for other tissues via cortisol, glucagon, and GH.

  • ADH and renin cause water retention.

Responses to Stress 2

• Exhaustion Stage:

  • Occurs after prolonged resistance to stress.

  • Characterized by nutrient depletion, electrolyte imbalances, immune system suppression, and potential death due to long-term cortisol oversecretion.

Major Events in the General Stress Syndrome

1. Stress triggers nerve impulses to the hypothalamus.

2. Impulses raise blood glucose, glycerol, and fatty acid levels, heart rate, and blood pressure, directing blood to skeletal muscles and elevating epinephrine secretion.

3. Epinephrine amplifies sympathetic reactions.

4. Hypothalamus secretes CRH, prompting ACTH release from the anterior pituitary.

5. ACTH stimulates cortisol release from the adrenal cortex.

6. Cortisol raises amino acid concentrations, releases fatty acids, and promotes glucose formation from non-carb sources.

7. Glucagon and GH secretion increases, aiding energy source mobilization while stimulating cell amino acid uptake.

8. ADH secretion rises, promoting kidney water retention and blood volume increase.

9. Renin elevates angiotensin II levels, affecting vasoconstriction and glucose release from adrenal cortex.

10. Aldosterone fosters sodium retention in kidneys.

11. Long-term nutrient mobilization depletes fat stores and leads to protein breakdown, causing wasting.

12. Prolonged sodium retention can impact potassium levels and acid-base balance.

13.11 Life-Span Changes

• Endocrine glands generally decrease in size with age.

• Muscular and skeletal strength decline alongside growth hormone (GH) levels.

• ADH levels rise as liver and kidney functionality diminishes, leading to slower hormonal elimination.

• Calcitonin levels decrease, increasing osteoporosis risk.

• Variations in PTH levels can also elevate osteoporosis risk, notably in females.

• Insulin resistance may develop with age.

• Changes in melatonin secretion can disrupt circadian rhythms.

• Thymosin production declines, heightening infection susceptibility.