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Anatomy & Physiology/Lesson 4 of 7

Endocrine System

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The endocrine system is a network of glands that produce and release hormones, which are chemical messengers that regulate body functions.

Hormones

Hormones are chemical messengers released into the bloodstream that travel to target organs or cells to regulate their activity.

Functions

  • Regulate metabolism
  • Control growth and development
  • Maintain homeostasis
  • Regulate reproduction
  • Control stress responses
  • Maintain blood glucose, blood pressure, and fluid balance

Types of Chemical Signaling

  • Endocrine: Hormones travel through the bloodstream to distant target organs. Examples: insulin, thyroid hormone.
  • Paracrine: Chemical messengers act on nearby cells without traveling through the systemic circulation. Examples include somatostatin signaling between pancreatic islet cells.
  • Autocrine: Hormones act on the same cell that released them. Example: certain immune cell signaling.

Major Endocrine Glands

  • Hypothalamus: Base of the brain, above the pituitary. Produces ADH, oxytocin, and releasing/inhibiting hormones. Links the nervous and endocrine systems; maintains homeostasis and controls the pituitary gland.
  • Pituitary Gland: Below the hypothalamus. Known as the "master gland" because it regulates many other endocrine glands.
    • Anterior pituitary produces:
      • TSH
      • ACTH
      • LH
      • FSH
      • GH
      • Prolactin
    • Posterior pituitary stores and releases:
      • ADH
      • Oxytocin
    • ADH and oxytocin are synthesized in the hypothalamus.
  • Thyroid Gland: Front of the neck, around the trachea. Produces T3 and T4. Regulates metabolism, growth, and energy production.
  • Parathyroid Glands: Four small glands on the back of the thyroid. Produce PTH. Regulate blood calcium levels.
  • Adrenal Glands: On top of each kidney. Produce cortisol, aldosterone, epinephrine, and norepinephrine. Regulate stress response, blood pressure, electrolyte balance, and fight-or-flight response.
  • Pancreas: Upper abdomen, behind the stomach. Produces insulin and glucagon. Regulates blood glucose levels.
  • Ovaries: Female pelvis. Produce estrogen and progesterone. Support female reproduction and secondary sex characteristics.
  • Testes: Scrotum. Produce testosterone. Support male reproduction and secondary sex characteristics.

Important Hormones

  • TSH (Pituitary → Thyroid): Stimulates the thyroid gland to produce T3 and T4.
  • T3 & T4 (Thyroid → Most body cells): Increase metabolism, growth, and energy production.
  • ACTH (Pituitary → Adrenal cortex): Stimulates the adrenal cortex to release cortisol.
  • Cortisol (Adrenal cortex → Most body cells): Regulates stress response, glucose metabolism, and inflammation.
  • Aldosterone (Adrenal cortex → Kidneys): Increases sodium reabsorption and potassium secretion. Water retention can increase as a result, helping support blood volume and blood pressure.
  • Epinephrine (Adrenal medulla → Heart, lungs, liver): Initiates the fight-or-flight response by increasing heart rate and blood glucose.
  • Norepinephrine (Adrenal medulla → Most body cells): Increases blood pressure and supports the fight-or-flight response.
  • Insulin (Pancreas → Muscle, liver, fat): Lowers blood glucose by promoting glucose uptake into cells.
  • Glucagon (Pancreas → Liver): Raises blood glucose by stimulating glucose release from the liver.
  • ADH (Hypothalamus, released by posterior pituitary → Kidneys): Increases water reabsorption by the kidneys.
  • Oxytocin (Hypothalamus, released by posterior pituitary → Uterus, mammary glands): Stimulates uterine contractions and milk ejection.
  • Growth Hormone (GH) (Pituitary → Bones and muscles): Promotes growth of bones and tissues.
  • Prolactin (Pituitary → Mammary glands): Stimulates milk production after childbirth.
  • LH (Pituitary → Ovaries/Testes): Stimulates ovulation in females and testosterone production in males.
  • FSH (Pituitary → Ovaries/Testes): Stimulates egg and sperm development.
  • PTH (Parathyroid → Bone, kidneys, intestines): Raises blood calcium levels through effects on bone and kidneys, and indirectly increases intestinal calcium absorption through vitamin D.

Adrenal Gland Anatomy

  • Adrenal Cortex: Produces cortisol and aldosterone.
  • Adrenal Medulla: Produces epinephrine and norepinephrine.

Hormone Pathways

The endocrine system uses organized signaling cascades called hormone pathways. These pathways show how glands communicate to regulate metabolism, stress responses, water balance, and blood glucose.

1. Hypothalamic-Pituitary-Thyroid (HPT) Axis

Purpose: Regulates metabolism and energy production.

  • Hypothalamus releases TRH (Thyrotropin-Releasing Hormone)
    • TRH stimulates the pituitary gland to release TSH (Thyroid-Stimulating Hormone)
    • TSH stimulates the thyroid gland to release T3 (Triiodothyronine) and T4 (Thyroxine)
  • Effects of T3 and T4:
    • Increase metabolism
    • Increase heart rate
    • Increase energy production
    • Increase heat production
  • Negative feedback:
    • When enough T3 and T4 are circulating, TRH and TSH decrease to maintain balance

2. Hypothalamic-Pituitary-Adrenal (HPA) Axis

Purpose: Controls the body's response to stress.

  • Hypothalamus releases CRH (Corticotropin-Releasing Hormone)
    • CRH stimulates the pituitary gland to release ACTH (Adrenocorticotropic Hormone)
    • ACTH stimulates the adrenal cortex to release Cortisol
  • Effects of cortisol:
    • Increases blood glucose
    • Helps respond to stress
    • Reduces inflammation
    • Helps maintain blood pressure
  • Negative feedback:
    • Increasing cortisol suppresses CRH and ACTH release.

3. ADH Pathway

  • The hypothalamus produces ADH
  • The posterior pituitary stores and releases ADH into the bloodstream
  • ADH travels to the kidneys
  • Effects in the kidneys:
    • Increases water reabsorption
    • Produces concentrated urine
    • Maintains blood pressure
    • Prevents dehydration

4. Pancreatic Hormone Pathways

Insulin Pathway (After a meal)

  • Blood glucose rises
  • Pancreas detects high blood glucose via beta (β) cells
  • The beta (β) cells, which reside in the islets of Langerhans, release insulin
  • Insulin binds to receptors on:
    • Muscle cells
    • Fat cells
    • Liver cells

Effects of Insulin

  • Lowers blood glucose
  • Increases glucose uptake, particularly in skeletal muscle and adipose tissue
  • Promotes glucose storage as glycogen in the liver and skeletal muscle
  • Promotes energy storage and decreases hepatic glucose production

Glucagon Pathway (During Fasting)

  • Blood glucose decreases (during fasting)
  • Pancreas detects low blood glucose through alpha (α) cells
  • Alpha (α) cells release glucagon
  • Glucagon binds to receptors on liver cells

Effects of Glucagon

  • Stimulates glycogenolysis — breakdown of hepatic glycogen
  • Stimulates gluconeogenesis — production of new glucose
  • Increases hepatic glucose output
  • Raises blood glucose

Clinical Applications

  • Hyperthyroidism: Increased metabolism, weight loss, heat intolerance, rapid heart rate.
  • Hypothyroidism: Fatigue, weight gain, cold intolerance, slowed metabolism.
  • Diabetes Mellitus: Insulin deficiency or resistance causing elevated blood glucose.
  • Addison Disease: Low cortisol and aldosterone production.
  • Cushing Syndrome: Clinical state caused by chronic excess cortisol exposure.
  • Hyperparathyroidism: Elevated blood calcium levels.
  • Diabetes Insipidus: ADH deficiency or resistance leading to excessive dilute urine.

Key Points

  • The endocrine system regulates body functions using hormones
  • Hormones travel through the bloodstream to target cells with specific receptors
  • The hypothalamus links the nervous and endocrine systems
  • The pituitary gland is the 'master gland' because it regulates many other endocrine glands
  • Hormone secretion is primarily regulated through negative feedback to maintain homeostasis
  • Insulin lowers blood glucose; glucagon raises blood glucose
  • Aldosterone increases sodium and water reabsorption, raising blood pressure
  • ADH increases water reabsorption by the kidneys
Renal SystemBlood & Hemostasis