Study guide: Endocrine System and Hormones

📋 Course Outline

1. Endocrine System Fundamentals
2. Hormone Action on Target Cells
3. Feedback and Tropic Hormones
4. Pituitary and Thyroid Hormones
5. Calcium Regulation
6. Adrenal Stress Hormones
7. Pancreatic Glucose Regulation
8. Diabetes and ADH Disorders

📅 Key Dates

1. 1849Berthold castrated roosters and found that replacing their testes restored typical crowing, fighting, mating behavior, body size, and feathers even though the replacement testes were not connected by nerves, suggesting that a substance entered the bloodstream.
2. 1889Removing the pancreas from a dog caused diabetes and established a relationship between the pancreas and diabetes.
3. 1921Banting at the University of Toronto isolated insulin from beta cells and later won the Nobel Prize.

📖 1. Endocrine System Fundamentals

🔑 Key Concepts & Definitions

• Endocrine glands: Secrete hormones directly into the bloodstream, which transports the hormones throughout the body.
• Hormones: Chemical messengers sent to many parts of the body to produce a specific effect on a target cell or organ.

💡 Memory Hook

Endocrine glands release hormones into blood, whereas exocrine glands use ducts.

📖 2. Hormone Action on Target Cells

★ Must-know

📌 Lipid-soluble hormones, including steroid hormones such as testosterone, estrogen, and cortisol, diffuse through the lipid bilayer and bind to receptor proteins inside target cells, activating specific genes.

• Examples of water-soluble hormones include:
  • epinephrine
  • human growth hormone
  • thyroxine
  • insulin

Further detail

• Humans have over 200 hormones or hormone-like chemicals involved in growth, development, metabolism, homeostasis, and immunity.

💡 Memory Hook

Lipid-soluble hormones enter cells; water-soluble hormones signal from the membrane.

📖 3. Feedback and Tropic Hormones

🔑 Key Concepts & Definitions

• Negative feedback: A homeostatic response in which the output of a system suppresses or inhibits activity within that system.

📝 Essential Points

• When blood plasma becomes too concentrated, hypothalamic receptors signal the posterior pituitary to release ADH; ADH makes the kidneys retain water, dilution restores normal plasma concentration, and the hypothalamus then signals the pituitary to stop releasing ADH.

• In a tropic hormone pathway, the hypothalamus releases a releasing hormone into the anterior pituitary, the anterior pituitary releases a tropic hormone into the blood, the tropic hormone stimulates another endocrine gland, and the final hormone inhibits further release of the first two hormones.

💡 Memory Hook

Rising final hormone levels → inhibition of hypothalamus and pituitary signals.

📖 4. Pituitary and Thyroid Hormones

🔑 Key Concepts & Definitions

• Human growth hormone: Regulates growth, development, and metabolism and stimulates muscle, connective-tissue, and long-bone growth, protein synthesis, cell division, and the release of stored fats.
• Thyroxine: A thyroid hormone that increases the rate at which the body metabolizes fats, proteins, and carbohydrates for energy and stimulates cellular respiration in the heart, skeletal muscle, liver, and kidneys.

★ Must-know

• The pituitary gland has anterior and posterior lobes, is attached to the hypothalamus, and releases eight hormones.

• The anterior pituitary releases:

  • human growth hormone
  • prolactin
  • thyroid-stimulating hormone
  • adrenocorticotropic hormone
  • follicle-stimulating hormone
  • luteinizing hormone

📌 Thyroxine secretion is controlled by negative feedback: TSH stimulates the thyroid to release thyroxine, and rising thyroxine suppresses hypothalamic and anterior-pituitary TSH release.

Further detail

📌 Iodine is required to make thyroid hormones, and inadequate iodine prevents thyroxine production and can cause relentless TSH stimulation that enlarges the thyroid into a goitre.

💡 Memory Hook

Hypothalamus → pituitary → thyroid → thyroxine.

📖 5. Calcium Regulation

🔑 Key Concepts & Definitions

• Calcitonin: Regulates blood calcium by stimulating calcium uptake into bones when blood calcium concentration is too high.
• Parathyroid hormone: Released when blood calcium falls and raises blood calcium by stimulating bone breakdown, kidney calcium reabsorption, vitamin D activation, and intestinal calcium absorption.

📝 Essential Points

• Four small parathyroid glands are attached to the thyroid gland.

💡 Memory Hook

Calcitonin deposits calcium in bone, whereas PTH releases calcium into blood.

📖 6. Adrenal Stress Hormones

🔑 Key Concepts & Definitions

• Cortisol: The most abundant glucocorticoid and raises blood glucose by promoting muscle-protein and fat breakdown, with the liver using amino acids to make glucose.

★ Must-know

📌 The two adrenal glands lie on top of the kidneys and each contains an inner adrenal medulla and an outer adrenal cortex.

• The adrenal medulla releases epinephrine and norepinephrine when sympathetic neurons signal it directly, producing a rapid fight-or-flight response with increased breathing rate, heart rate, blood pressure, blood flow to the heart and muscles, and liver glycogen breakdown.

Further detail

📌 High cortisol levels provide negative feedback to the hypothalamus and anterior pituitary, suppressing ACTH production and stopping further cortisol release.

• Chronic high cortisol can:
  • impair thinking
  • damage the heart
  • cause high blood pressure and diabetes
  • increase susceptibility to infection
  • cause early death

💡 Memory Hook

Adrenal medulla produces rapid epinephrine responses; adrenal cortex produces longer cortisol responses.

📖 7. Pancreatic Glucose Regulation

🔑 Key Concepts & Definitions

• Insulin: Secreted by beta cells in the islets of Langerhans, makes target cells more permeable to glucose and promotes glucose storage as glycogen in liver cells.
• Glucagon: Released by alpha cells during rigorous exercise or fasting, stimulates the liver to convert glycogen into glucose and release it into the blood.

📝 Essential Points

• The endocrine cells of the pancreas are organized into about 2,000 groups called islets of Langerhans that release hormones directly into the bloodstream.

💡 Memory Hook

Insulin lowers blood glucose, whereas glucagon raises it.

📖 8. Diabetes and ADH Disorders

🔑 Key Concepts & Definitions

• Diabetes mellitus: Occurs when the body does not produce insulin or does not respond to it, causing persistently high blood glucose after meals.
• Antidiuretic hormone: Created in the hypothalamus, stored in the posterior pituitary, and released when blood plasma is highly concentrated to make kidney nephrons absorb more water into the blood.
• Diabetes insipidus: Caused by insufficient ADH production and leads to a large volume of urine and loss of ions from the blood; it can be treated with ADH medication and is unrelated to insulin or glucose.

📝 Essential Points

📌 Type 1 diabetes results from immune destruction of pancreatic beta cells, usually begins in childhood, and requires insulin injections, whereas type 2 diabetes develops gradually as insulin receptors become less responsive and can be controlled through diet and exercise.

💡 Memory Hook

Diabetes mellitus concerns insulin and glucose; diabetes insipidus concerns ADH and water balance.

📊 Synthesis Tables

Lipid-Soluble and Water-Soluble Hormones

Feature	Lipid-soluble hormones	Water-soluble hormones
Membrane passage	Diffuse through lipid bilayer	Cannot cross cell membrane
Receptor location	Inside target cell	On target-cell surface
Cellular effect	Activate specific genes	Start intracellular reaction cascades
Examples	Testosterone, estrogen, cortisol	Epinephrine, hGH, thyroxine, insulin

Pancreatic Hormones

Hormone	Source and trigger	Effect on blood glucose
Insulin	Beta cells; released when glucose is high	Promotes glucose uptake and glycogen storage
Glucagon	Alpha cells; released during fasting or rigorous exercise	Promotes glycogen breakdown and glucose release