Ch.16A Endocrine System Notes

Endocrine System Overview

  • The endocrine system works with the nervous system to coordinate and integrate the activity of body cells.
  • It influences metabolic activities via hormones transported in the blood.
  • Endocrine responses are slower but longer-lasting than nervous system responses.
  • Endocrinology is the study of hormones and endocrine organs.
  • Hormones are chemical substances secreted by cells into extracellular fluids that travel through the blood and regulate the metabolic function of other cells in the body.
  • The endocrine system controls and integrates:
    • Reproduction
    • Growth and development
    • Maintenance of electrolyte, water, and nutrient balance of blood
    • Regulation of cellular metabolism and energy balance
  • Hormones circulate systemically, affecting only cells with receptors for that hormone.
  • Target cells are tissues with receptors for a specific hormone.
  • Hormones alter target cell activity.
  • Hormones act in one of two ways, depending on their chemical nature and receptor location:
    • Water-soluble hormones (all amino acid–based hormones except thyroid hormone):
      • Act on plasma membrane receptors
      • Cannot enter the cell
    • Lipid-soluble hormones (steroid and thyroid hormones):
      • Act on intracellular receptors that directly activate genes
      • Can enter the cell

Endocrine Gland Stimuli

  • Blood levels of hormones are controlled by negative feedback systems.
    • Increased hormone effects on target organs can inhibit further hormone release.
  • Endocrine glands are stimulated to synthesize and release hormones in response to one of three stimuli:
    • Humoral stimuli
    • Neural stimuli
    • Hormonal stimuli

Pituitary Gland

  • The pituitary gland is affected by the hypothalamus.
  • Posterior lobe hormones:
    • Oxytocin
    • Antidiuretic hormone (ADH)
  • Anterior lobe hormones: Six hormones will be discussed.

Posterior Pituitary and Hypothalamic Hormones

  • Oxytocin:
    • Strong stimulant of uterine contractions released during childbirth.
    • Acts as a hormonal trigger for milk ejection.
  • Antidiuretic hormone (ADH):
    • Targets kidney tubules to reabsorb more water to inhibit or prevent urine formation.
  • The posterior lobe of the pituitary gland is NOT a true endocrine gland because it is only a hormone storage area that receives hormones from the hypothalamus for release.

Clinical - Homeostatic Imbalance

  • Diabetes insipidus:
    • ADH deficiency due to damage to the hypothalamus or posterior pituitary.
    • Requires staying well-hydrated.

Anterior Pituitary Hormones

  • All six hormones are peptide hormones.
  • All but two are tropic hormones (tropins) that regulate the secretion of other hormones:
    • Growth hormone (GH)
    • Thyroid-stimulating hormone (TSH)
    • Adrenocorticotropic hormone (ACTH)
    • Follicle-stimulating hormone (FSH)
    • Luteinizing hormone (LH)
    • Prolactin (PRL)
    • Growth hormone (GH)
      • Has direct actions on metabolism and indirect growth-promoting actions.
      • Direct actions on metabolism:
        • Glucose-sparing actions decrease the rate of cellular glucose uptake and metabolism (anti-insulin effects).
        • Triggers the liver to break down glycogen into glucose.
        • Increases blood levels of fatty acids for use as fuel and encourages cellular protein synthesis.
      • GH stimulates most cells to enlarge and divide, but major targets are bone and skeletal muscle.
      • Major targets are the liver, bone, and skeletal muscles.
      • Regulation of secretion:
        • GH release or inhibition chiefly regulated by hypothalamic hormones.
        • Growth hormone–releasing hormone (GHRH) stimulates GH release.
          • Triggered by low blood GH or glucose, or high amino acid levels.
        • Growth hormone–inhibiting hormone (GHIH) (somatostatin) inhibits release.
          • Triggered by an increase in GH.

Clinical - Homeostatic Imbalance

  • Hypersecretion of GH is usually caused by an anterior pituitary tumor.
    • In children, it results in gigantism.
      • Can reach heights of 8 feet.
    • In adults, it results in acromegaly.
      • Overgrowth of hands, feet, and face.
  • Hyposecretion of GH:
    • In children, it results in pituitary dwarfism.
      • May reach a height of only 4 feet.
    • In adults, it usually causes no problems.

Anterior Pituitary Hormones

  • Thyroid-stimulating hormone (TSH):
    • Stimulates normal development and secretory activity of the thyroid.
    • Release is triggered by thyrotropin-releasing hormone from the hypothalamus.
    • Inhibited by rising blood levels of thyroid hormones that act on both the pituitary and hypothalamus.
  • Adrenocorticotropic hormone (ACTH):
    • ACTH stimulates the adrenal cortex to release corticosteroids.
    • Regulation of ACTH release:
      • Triggered by hypothalamic corticotropin-releasing hormone (CRH) in a daily rhythm
        • Highest levels in the morning
      • Internal and external factors that alter the release of CRH include fever, hypoglycemia, and stressors.
  • Gonadotropins (FSH and LH):
    • FSH stimulates the production of gametes (egg or sperm).
    • LH promotes the production of gonadal hormones.
      • In females, LH helps mature follicles of the egg, triggers ovulation and the release of estrogen and progesterone.
      • In males, LH stimulates the production of testosterone.
  • Prolactin (PRL):
    • Secreted by prolactin cells of the anterior pituitary.
    • Stimulates milk production in females; the role in males is not well understood.

Clinical - Homeostatic Imbalance

  • Hypersecretion of prolactin is more common than hyposecretion.
    • Hyposecretion is not a problem in anyone except women who choose to nurse.
  • Hyperprolactinemia is the most frequent abnormality of anterior pituitary tumors.
  • Clinical signs include inappropriate lactation, lack of menses, infertility in females, and impotence in males.