Endocrine System: Hormones, Glands & Homeostasis

What is the Endocrine System?

The endocrine system is a chemical communication system that uses hormones (chemical messengers) to regulate body functions. Unlike the nervous system (which uses electrical signals), hormones travel through the bloodstream to target organs.

Key Features:

• Slow but long-lasting effects (minutes to hours/days).
• Works with the nervous system to maintain homeostasis (balance).
• Ductless glands (unlike exocrine glands like sweat glands).

Major Endocrine Glands & Their Hormones

Here’s a table of the main glands, their hormones, and functions:

How Hormones Work: Mechanism of Action

Hormones follow a 3-step process:

1. Synthesis & Release: Glands produce hormones (e.g., thyroid releases T₄).
2. Transport: Hormones travel via bloodstream to target cells.
3. Action: Hormones bind to receptors on target cells, triggering a response.

Types of Hormone Receptors:

• Water-soluble hormones (e.g., insulin, adrenaline) → Bind to cell membrane receptors → Trigger second messengers (like cAMP).
• Lipid-soluble hormones (e.g., thyroid hormones, steroids) → Pass through cell membrane → Bind to nuclear receptors → Affect DNA transcription.

Feedback Mechanisms: How the Body Regulates Hormones

The endocrine system uses negative feedback to maintain balance.

Example 1: Blood Glucose Regulation (Insulin & Glucagon)

flowchart TD
    A["High Blood Glucose"] --> B["Pancreas releases Insulin"]
    B --> C["Cells absorb glucose"]
    C --> D["Blood glucose decreases"]
    D --> E["Pancreas stops insulin release"]
    E --> A

    F["Low Blood Glucose"] --> G["Pancreas releases Glucagon"]
    G --> H["Liver breaks glycogen into glucose"]
    H --> I["Blood glucose increases"]
    I --> F

Example 2: Thyroid Hormone Regulation (TSH & T₄)

flowchart TD
    A["Hypothalamus releases TRH"] --> B["Pituitary releases TSH"]
    B --> C["Thyroid releases T₄"]
    C --> D["T₄ inhibits TSH release"]
    D --> B

Disorders of the Endocrine System

Comparison: Endocrine vs. Nervous System

Solved Example: Blood Sugar Regulation

Question: Why does a diabetic patient feel weak and hungry even after eating? Answer:

• In Type 1 diabetes, the pancreas does not produce insulin.
• Without insulin, glucose cannot enter cells → cells starve (even though blood glucose is high).
• The body breaks down fat and protein for energy → weight loss, weakness, and hunger.

NEB-Style Questions & Answers

Short Answer Questions

1. What is the role of ADH (Antidiuretic Hormone)?

   • Answer: ADH is released by the posterior pituitary when the body is dehydrated. It increases water reabsorption in the kidneys → concentrated urine and prevents dehydration.

2. Differentiate between Type 1 and Type 2 diabetes.

   • Answer:

     Type 1	Type 2
     Autoimmune destruction of β-cells	Insulin resistance
     Requires insulin injections	Managed with diet, exercise, oral meds
     Usually diagnosed in childhood	Often in adults (obesity-related)

Long Answer Question

Question: Describe the structure and functions of the thyroid gland. How does hypothyroidism affect the body? Answer:

• Structure:

  • Located in the neck, just below the larynx.
  • Consists of two lobes connected by the isthmus.
  • Made of follicles that store colloid (thyroglobulin + iodine).

• Functions:

  • Produces thyroxine (T₄) and triiodothyronine (T₃) → regulate metabolism.
  • Secretes calcitonin → lowers blood calcium (opposes PTH).

• Hypothyroidism Effects:

  • Low metabolism → weight gain, fatigue, cold intolerance.
  • Puffiness (myxedema) due to fluid retention.
  • Slowed reflexes, depression, hair loss.
  • Caused by: iodine deficiency, autoimmune disease (Hashimoto’s), or pituitary disorders.
  • Treatment: Thyroid hormone replacement (levothyroxine).

Exam Tip: How to Score Full Marks

✅ Diagrams are key! Draw and label:

• Feedback loops (e.g., insulin-glucagon cycle).
• Gland locations (e.g., pituitary, thyroid, adrenal).
• Hormone pathways (e.g., TSH → T₄ regulation).

✅ Compare & contrast (e.g., endocrine vs. nervous system, Type 1 vs. Type 2 diabetes).

✅ Explain disorders with:

• Cause (e.g., autoimmune, tumor, deficiency).
• Symptoms (e.g., weight gain, fatigue).
• Treatment (e.g., hormone replacement, surgery).

✅ Use real-life examples (e.g., "Why do athletes use steroids?" → artificial testosterone disrupts natural hormone balance).

✅ Memorize key hormones & their targets (e.g., ADH → kidneys, oxytocin → uterus/breasts).

Labeled illustration of major endocrine glands (pituitary, thyroid, adrenal, pancreas, etc.) (Image: OpenStax & Tomáš Kebert & umimeto.org, CC BY-SA 4.0, via Wikimedia Commons) Negative feedback mechanism for blood glucose regulation (Image: GliderMaven, CC0, via Wikimedia Commons) Microscopic structure of thyroid follicles storing colloid (Image: Bhavya akula, CC BY-SA 4.0, via Wikimedia Commons)