chapter 19 summary

Introduction to the Endocrine System

• Collaboration with Nervous System: The nervous and endocrine systems work in harmony to regulate homeostasis.

  • Nervous System: Short-term crisis management.

  • Endocrine System: Functions on an ongoing basis: longer-term (especially for developmental processes) metabolic regulation via hormone release.

    • Hormones leave a gland or land-like structure

    • Enter into the bloodstream

    • Travel to its target organ

    • Cause the target organ to respond by changing the types, activities, or quantities of key cytoplasmic enzymes

19.1 Overview of the Endocrine System

• Components: All endocrine cells and tissues that release hormones into lymphatic or blood systems.

  • All endocrine structures develop from epithelial tissue

• Hormone Categories:

  1. Amino Acid Derivatives: Derived from amino acids, structurally similar to amino acids.

     • ie thyroid hormones and suprarenal medulla hormones

  2. Peptide Hormones: Chains of amino acids produced via transcription & translation in the endocrine cells

     • ie all hormones from the pituitary gland

  3. Steroid Hormones: Derived from cholesterol, nonpolar lipid horome (can passively diffuse through cell membrane)

     • ie reproductive hormones & suprarenal cortex hormones

  4. Eicosanoids: Derived from arachidonic acid (fatty acid of cell membranes)

     • Lipid Derivatives: Including eicosanoids and steroid hormones.

• Mechanism of Action: Hormones modify the activities of target cells (cells sensitive to specific hormones).

• Endocrine activity can be regulated through:

  1. Humoral stimuli: changes in the composition of the extracellular fluid

  2. Hormonal stimuli: Specific hormone binds to a receptor, triggering a response that alters the cell's function

  3. Negative feedback: Very common, involves complex interactions among hormones and their target organs, helping to maintain homeostasis by reversing deviations from normal ranges. The end product terminates the pathway

  4. Positive feedback: Less common, amplifies a response until a desired outcome is achieved, often leading to a specific event or process being completed, such as childbirth or blood clotting.

19.2 Hypothalamus and the Pituitary Gland

• Hypothalamus Functions:

  • Regulates endocrine and neural activities, has direct control over epinephrine & norepinephrine (fight or flight)

  • Controls adrenal medulla output (sympathetic ANS component).

  • Produces hormones:

    • ADH (Antidiuretic Hormone): Promotes water conservation by targeting nephrons of the kidneys.

    • OT (Oxytocin): Stimulates uterine contractions during labor and milk ejection during lactation.

  • Produces regulatory hormones that control hormone release from the anterior pituitary.

• Pituitary Gland:

  • Posterior Lobe (Neurohypophysis):

    • Stores and releases the two hormones produced by the hypothalamus (ADH & Oxytocin)

  • Anterior Lobe (Adenohypophysis)

    • Highly vascularized; utilizes the hypophyseal portal system for blood flow to target cells.

    • Key Hormones:

      1. TSH (Thyroid-Stimulating Hormone): Triggers thyroid hormone release (T4 & T3).

      2. ACTH (Adrenocorticotropic Hormone): Stimulates glucocorticoid (cortisol) release from adrenal glands.

      3. FSH (Follicle-Stimulating Hormone): Triggers maturation of ovarian follicles, regulates estrogen in females and sperm in males.

      4. LH (Luteinizing Hormone): Induces ovulation and regulates sex hormones.

      5. PRL (Prolactin): Stimulates mammary gland development and milk production.

      6. GH (Somatotropin): Supports cell growth and replication in all cells within the body.

      7. Melanocyte-stimulating hormone (MSH): Stimulates melanin production.

19.3 The Thyroid Gland

• Location: Positioned anteriorly to the trachea, below the larynx, consists of two lobes joined by isthmus.

• Follicular Structure: Contains follicles that produce thyroglobulin and store it in colloid.

  • Hormone Synthesis: Stimulation by TSH leads to the release of thyroxine (T4 most common) and triiodothyronine (T3 less common).

    • Both stimulate metabolic activity in cells by increasing the basal metabolic rate, enhancing protein synthesis, and regulating the metabolism of carbohydrates and fats.

  • C Thyrocytes: Produce calcitonin (CT) to lower blood calcium levels by inhibiting osteoclasts and promoting kidney excretion.

19.4 The Parathyroid Glands

• Location: Embedded in the posterior thyroid.

• Hormone Production:

  • Principal cells produce Parathyroid Hormone (PTH) in response to low blood calcium.

  • Reduces urinary excretion of calcium and stimulates the production of calcitriol (kidney hormone that promotes intestinal absorption of calcium) thereby increasing blood calcium levels.

  • PTH Functions:

    1. Increased osteoclast activity.

    2. Enhanced osteoblast activity.

    3. Reduced urinary calcium excretion.

    4. Stimulates intestinal calcium absorption via calcitriol.

19.5 The Thymus

• Location: Thoracic cavity, involved in immune system development.

• Hormone Production: Thymosins promote lymphocyte development and maturation.

19.6 The Adrenal Glands

• Structure: Each adrenal gland has a cortex and medulla, found superior to each kidney.

  • Adrenal Cortex

  • Hormones:

    1. Mineralocorticoids (Aldosterone): Regulates sodium and water retention; responds to angiotensin II (controls blood volume).

    2. Glucocorticoids (Cortisone, Cortisol): Increases glucose production/storage in response to stress.

    3. Androgens: Sec secreted in lesser amounts; role still being studied.

  • Adrenal Medulla: Directly controlled by the hypothalamus

  • Hormones:

    • Produces epinephrine (80%) and norepinephrine (20%) that activate energy release and mobilize reserves.

19.7 Endocrine Functions of the Kidneys and Heart

• Kidneys: Produce erythropoietin (EPO); which will increase the production of erythrocytes (RBCs)

  • Renin: Activates angiotensin II for aldosterone production.

• Heart: Produces ANP & BNP to reduce excessive blood pressure/volume by promoting sodium and water excretion.

19.8 The Pancreas and Other Endocrine Tissues of the Digestive System

• Structure: Pancreas has endocrine (islets) and exocrine functions (digestive enzymes).

• The pancreas primarily secretes digestive enzymes to aid in digestion

  • Islet Cells: Specilaized cells that produce either insulin or glucagon, which regulate blood glucose levels and maintain metabolic homeostasis.

    • Alpha cells: Produce glucagon (increases blood glucose).

    • Beta cells: Produce insulin (decreases blood glucose).

    • Delta cells: Secrete somatostatin (inhibitor of glucagon/insulin).

    • F cells: Secrete pancreatic polypeptide.

19.9 Endocrine Tissues of the Reproductive System

Testes

• Hormones:

  • Androgens (Testosterone): Promote sperm production and secondary sexual characteristics; regulated by inhibition.

Ovaries

• Hormones:

  • Estrogens (Estradiol): Developed in follicles, maintain uterine lining; regulated by inhibin post-ovulation.

  • Progesterone: Prepares uterus for implantation post-ovulation.

19.10 The Pineal Gland

• Function: Synthesizes melatonin to regulate reproductive organ maturation and circadian rhythms.

19.11 Hormones and Aging

• The endocrine system experiences minor functional changes with age; notable shifts are observed at puberty and menopause.