Chapter 13 Pers. Notes

Chapter 13: Endocrine System

13.1 General Characteristics of the Endocrine System

  • Endocrine System: Alongside the nervous system, regulates body functions to maintain homeostasis and coordinates communication across the body.
    • Unique characteristic: Organs are not anatomically connected.
    • Major Endocrine Glands:
    • Pituitary gland
    • Thyroid gland
    • Parathyroid glands
    • Adrenal glands
    • Pancreas
    • Pineal gland
    • Thymus
    • Ovaries (female) and Testes (male)

13.2 Endocrine and Exocrine Glands and Other Chemical Messengers

  • Endocrine Glands:

    • Composed of cells, tissues, and organs making up the endocrine system.
    • Ductless; secrete hormones directly into body fluids.
    • "Endocrine" translates to "internal secretion."
    • Hormones act specifically on target cells that have the appropriate receptors.

  • Exocrine Glands:

    • Secrete substances into ducts or tubes that lead to a body surface.
    • Deliver products externally to specific sites.

  • Local Hormones (not true hormones):

    • Paracrine Secretions: Affect nearby cells.
    • Autocrine Secretions: Affect the cells that secrete them.

Comparison of the Nervous and Endocrine Systems

  • Both systems facilitate communication in the body.
    • Both use chemical signals that bind to receptor molecules.
Key Differences:

  • Nervous System:

    • Releases neurotransmitters into synapses.
    • Faster response time (less than 1 second).
    • Effects are brief unless neuronal activity continues.

  • Endocrine System:

    • Secretes hormones into the bloodstream.
    • Slower response time (seconds to hours).
    • Effects can last longer, from minutes to days.

13.3 Hormone Names and Abbreviations

  • Hypothalamus Hormones:

    • Corticotropin-releasing hormone (CRH)
    • Gonadotropin-releasing hormone (GnRH)
    • Luteinizing hormone-releasing hormone (
      LHRH)
    • Somatostatin (SS)

  • Anterior Pituitary Hormones:

    • Adrenocorticotropic hormone (ACTH)
    • Follicle-stimulating hormone (FSH)
    • Growth hormone (GH)
    • Luteinizing hormone (LH)
    • Prolactin (PRL)
    • Thyroid-stimulating hormone (TSH)

  • Posterior Pituitary Hormones:

    • Antidiuretic hormone (ADH) - also known as vasopressin.
    • Oxytocin (OT).

  • Thyroid Gland Hormones:

    • Calcitonin
    • Thyroxine (T4)
    • Triiodothyronine (T3)

  • Adrenal Gland Hormones:

    • Epinephrine (EPI, adrenaline)
    • Norepinephrine (NE, noradrenaline)
    • Aldosterone
    • Cortisol

  • Pancreatic Hormones:

    • Glucagon
    • Insulin
    • Somatostatin (SS).

13.4 Hormone Action

  • Hormones:
    • Released into extracellular fluid, then diffuse into the blood.
    • Transport method through blood depends on the solubility (lipid-soluble vs. water-soluble).
    • Very potent in low concentrations.
Types of Hormones:

  1. Steroid Hormones:

    • Derive from cholesterol; composed of complex rings of carbon and hydrogen.
    • Examples: testosterone, estrogens, cortisol, and aldosterone.

  2. Nonsteroid Hormones:

    • Amines: Derived from tyrosine (e.g., epinephrine, norepinephrine, thyroxine).
    • Proteins: Long chains of amino acids (e.g., growth hormone).
    • Peptides: Short chains of amino acids (e.g., ADH, oxytocin).
    • Glycoproteins: Combinations of carbohydrates and proteins (e.g., TSH).

Actions of Hormones

  • Hormones exert effects by altering metabolic processes:
    • May affect enzyme activity.
    • May change rate of membrane transport of substances.
    • Deliver messages by binding to receptors on/in target cells.
Receptor Dynamics:
  • Can cause changes in target cells even at extremely low concentrations.
  • Number of receptors can dictate the strength of a response. Changes can occur through:
    • Upregulation: Increase in receptor number due to decreased hormone levels.
    • Downregulation: Decrease in receptor number due to increased hormone levels.

Steroid and Thyroid Hormones

  • Steroid and thyroid hormones (poor water solubility):
    • Transported in blood, usually bound to plasma proteins.
    • Can diffuse through the lipid bilayer of cell membranes.
    • Typically bind to receptors inside cells, often in the nucleus, causing gene transcription changes, leading to specific actions by new proteins.
Sequence of Steroid Hormone Action:
  1. Endocrine gland secretes steroid hormone.
  2. Blood carries hormone (often weakly bound to transport proteins).
  3. Unbound steroid hormone diffuses through the target cell membrane.
  4. Hormone binds with receptor molecule in the cytoplasm/nucleus.
  5. Hormone-receptor complex binds to DNA, promoting transcription of mRNA.
  6. Synthesized proteins produce the hormone's specific effects.

Nonsteroid Hormones

  • Nonsteroid hormones cannot penetrate the lipid bilayer:
    • Bind to receptors on target cell membranes, acting as a first messenger.
    • Induces changes that lead to hormonal effects (second messenger).
Signal Transduction using cyclic AMP (cAMP):
  1. Endocrine gland secretes nonsteroid hormone.
  2. Blood carries hormone throughout the body.
  3. Hormone binds to membrane receptor, activating G protein.
  4. Adenylate cyclase in target cell membrane is activated.
  5. ATP is converted into cyclic AMP by adenylate cyclase.
  6. cAMP activates protein kinases.
  7. Activated protein substrates modify metabolic processes.
  8. Resultant cellular changes produce hormone effects.

13.5 Control of Hormonal Secretions

  • Hormone secretion is tightly controlled, primarily through negative feedback mechanisms:
    • Secretion can be short-lived (minutes) or prolonged (days).
    • Can be excreted in urine post-function or dismantled by enzymes, primarily the liver.
Control Mechanisms:

  • Negative Feedback:

    • Rising hormone levels decrease further hormone secretion.
    • As hormone is utilized, inhibition ceases, and secretion restarts.

  • Positive Feedback:

    • Rising hormone levels lead to increased secretion; seen in limited cases, primarily reproductive functions.

13.6 Pituitary Gland

  • The pituitary gland lies at the brain's base inside the sella turcica of the sphenoid bone.

    • Considered part of the nervous system due to its control by the brain.
    • Connected to the hypothalamus via the pituitary stalk (infundibulum).
    • Divided into two main lobes:
    • Anterior lobe (adenohypophysis)
    • Posterior lobe (neurohypophysis)

  • Hypothalamic Control:

    • Anterior Lobe: Receives releasing hormones via the hypothalamic-hypophyseal portal system, stimulating or inhibiting hormone secretion.
    • Posterior Lobe: Nerve impulses from the hypothalamus stimulate the release of hormones stored in this lobe.

Anterior Pituitary Hormones and Control

  • Contains glandular epithelial tissue.
  • Produces hormones released in response to hypothalamic hormones:
    • Growth Hormone (GH): Promotes cell growth and division, affects metabolism.
    • Prolactin (PRL): Promotes milk production post-birth.
    • Thyroid-stimulating Hormone (TSH): Stimulates thyroid hormone secretion.
    • Adrenocorticotropic Hormone (ACTH): Promotes cortisol secretion from adrenal cortex.
    • Follicle-stimulating Hormone (FSH): Stimulates egg and sperm production.
    • Luteinizing Hormone (LH): Triggers ovulation and hormone production in both sexes.

Regulation:

  • Each hormone has specific releasing/inhibiting hormones from the hypothalamus governing its secretion.

13.7 Disorders of the Pituitary Gland

  • Hypopituitary Dwarfism: Growth hormone deficiency results in normal proportions but short stature.
  • Gigantism: Caused by excess growth hormone; leads to abnormally tall stature due to excess secretion during childhood.
  • Acromegaly: Occurs due to excess growth hormone in adulthood, leading to thickening of bones and tissues, but no increase in height.

13.8 Thyroid Gland

  • The thyroid gland is located below the larynx, has two lobes connected by an isthmus, and is responsible for removing iodine from the blood.
  • Hormones produced:
    • Thyroxine (T4): Increases energy release, promotes growth, vital for nervous system maturation.
    • Triiodothyronine (T3): More potent than T4, with similar actions.
    • Calcitonin: Reduces blood calcium levels by inhibiting release from bones and promoting excretion.
Disorders:
  • Hypothyroidism: Low metabolic rate, sensitivity to cold, weight gain.
    • Congenital causes can lead to mental retardation and growth failure.
  • Hyperthyroidism: Increased metabolic rate, anxiety, weight loss, restlessness.

13.9 Parathyroid Glands

  • Typically, four located on the posterior surface of the thyroid gland.
  • Action: Secretion of parathyroid hormone (PTH), regulates calcium levels in the blood through stimulating bone resorption, calcium absorption, and conservation in kidneys.
Disorders:
  • Hyperparathyroidism: High blood calcium levels, leading to fatigue and bone weakening.
  • Hypoparathyroidism: Leads to low calcium levels and potential seizures.

13.10 Adrenal Glands

  • Located atop each kidney, also referred to as suprarenal glands.
  • Cortex: Produces steroid hormones: aldosterone (mineralocorticoids), cortisol (glucocorticoids).
  • Medulla: Produces epinephrine (adrenaline) and norepinephrine (noradrenaline).
Hormonal Functions:
  • Medullary hormones manage stress responses, increasing heart rate and blood pressure, among others.
  • Cortical hormones regulate metabolism, electrolyte levels, and response to stress.

13.11 Pancreas

  • An organ that exhibits dual function: endocrine and exocrine.
  • Hormones Released:
    • Glucagon: Increases blood sugar levels by breaking down glycogen.
    • Insulin: Decreases blood sugar levels and promotes glucose use in cells.
    • Somatostatin: Inhibits insulin and glucagon secretion.
Disorders:
  • Diabetes Mellitus: A metabolic disorder characterized by issues with insulin production or action, leading to elevated blood glucose.
    • Type 1 (Insulin-Dependent): Autoimmune destruction of insulin-producing beta cells.
    • Type 2 (Non-Insulin-Dependent): Body's cells fail to respond adequately to insulin.

13.12 Pineal, Thymus, and Other Glands

  • Pineal Gland: Regulates circadian rhythms through melatonin secretion.
  • Thymus Gland: Promotes T-cell development via thymosins, crucial for the immune function.
  • Reproductive Organs: Sex hormones produced by ovaries (estrogens, progesterone) and testes (testosterone); placenta also produces sex hormones during pregnancy.

13.13 Stress and Its Effects

  • Maintaining homeostasis is vital for survival; various stressors can disrupt it (e.g., psychological, physical).
  • General Adaptation Syndrome (GAS): Describes the physiological response to stress, comprising three stages:
    • Alarm Stage: Immediate fight or flight response.
    • Resistance Stage: Longer-lasting response using cortisol to mobilize energy.
    • Exhaustion Stage: Nutrient depletion, leading to potential health decline or death.

Life-Span Changes

  • With aging, endocrine glands may decrease in size, influencing metabolism, hormone levels (especially GH, ADH, calcitonin), and susceptibility to diseases.
  • Potential increases in insulin resistance and changes in melatonin secretion affecting circadian rhythms.