Prep Guide: Endocrine System

Key Concepts (Recall & Understanding)

Endocrine Glands: Ductless glands that secrete hormones directly into the bloodstream.
Hormones: Chemical messengers that regulate the activity of target cells or organs.
Major Glands and Hormones:
- Adrenal Gland:
  - Cortex: Cortisol (stress response), Aldosterone (salt/water balance).
  - Medulla: Adrenaline (fight or flight).
- Pancreas (Islets of Langerhans):
  - Insulin: Lowers blood glucose.
  - Glucagon: Raises blood glucose.
- Thyroid Gland:
  - Thyroxine: Regulates metabolism.
- Pituitary Gland ("Master Gland"):
  - Anterior: Growth Hormone (GH), Tropic Hormones (TSH, ACTH).
  - Posterior: Antidiuretic Hormone (ADH), Oxytocin.
Hormonal Imbalances:
- Diabetes Mellitus: Caused by insulin deficiency (hyperglycemia).
- Gigantism/Dwarfism: Caused by hyper/hyposecretion of GH in childhood.
- Hyperthyroidism/Hypothyroidism: Caused by hyper/hyposecretion of thyroxine.
Feedback Mechanism: A control system where the output of a system inhibits (negative feedback) or stimulates (positive feedback) its further output. Most hormone regulation is by negative feedback (e.g., TSH and thyroxine).

Application Corner

After a large, carbohydrate-rich meal, which hormone would you expect to be released in significant amounts by the pancreas? What is its function?
- Answer: Insulin. Its function is to lower the high blood glucose levels by promoting the uptake of glucose from the blood into the body's cells (like liver and muscle cells) for use or storage.
A person is feeling constantly tired, has gained weight despite not changing their diet, and feels cold often. A blood test reveals low levels of thyroxine. What condition do they likely have?
- Answer: Hypothyroidism. The low level of thyroxine leads to a slowed metabolic rate, which accounts for the symptoms of fatigue, weight gain, and feeling cold.

Analytical Thinking

Odd One Out: Insulin, Glucagon, Adrenaline, Thyroxine.
- Odd One: Adrenaline.
- Category: While all are hormones, insulin, glucagon, and thyroxine are primarily involved in regulating metabolism under normal conditions. Adrenaline is primarily an emergency hormone for the "fight or flight" response.
Compare and Contrast: Explain the opposing actions of insulin and glucagon in regulating blood sugar.
- Answer: Insulin and glucagon have antagonistic effects. Insulin is released when blood sugar is high; it lowers blood sugar by stimulating cells to take up glucose. Glucagon is released when blood sugar is low; it raises blood sugar by stimulating the liver to break down stored glycogen and release glucose into the bloodstream.

Key Case Study

Type 1 Diabetes: An Autoimmune Disorder

Type 1 diabetes is a chronic condition in which the pancreas produces little or no insulin. It is an autoimmune disease, meaning the body's own immune system mistakenly attacks and destroys the insulin-producing cells (beta cells) in the Islets of Langerhans. Without insulin, glucose cannot enter the cells to be used for energy and instead builds up in the bloodstream, leading to hyperglycemia. Patients with Type 1 diabetes require lifelong insulin therapy, either through injections or an insulin pump, to manage their blood sugar levels. This case study highlights the critical role of a single hormone, insulin, in metabolism and demonstrates the severe consequences of its absence.

Assertion-Reason Practice

Assertion (A): The pituitary gland is often called the "master gland." Reason (R): The pituitary gland secretes tropic hormones, such as TSH and ACTH, which control the function of other endocrine glands.

(a) Both (A) and (R) are true and (R) is the correct explanation of (A). (b) Both (A) and (R) are true but (R) is not the correct explanation of (A). (c) (A) is true but (R) is false. (d) (A) is false but (R) is true.

Answer: (a) Both (A) and (R) are true and (R) is the correct explanation of (A). The pituitary gets its nickname "master gland" precisely because it directs the activity of several other major endocrine glands through its tropic hormones.

HOTS (Higher-Order Thinking Skills) Question

Some athletes illegally use synthetic Growth Hormone (GH) to build muscle mass. Using your knowledge of negative feedback loops, what would be the likely effect of long-term use of synthetic GH on the athlete's own pituitary gland and its natural production of GH?

Answer: The regulation of GH is controlled by a negative feedback loop. High levels of GH (and its products) in the blood signal the hypothalamus and pituitary gland to reduce their own production of GH-releasing hormone and GH, respectively. If an athlete continuously introduces high levels of synthetic GH into their body, this feedback loop will be constantly activated. As a result, the athlete's own pituitary gland will significantly decrease, or even stop, its natural production of GH. The gland may even atrophy (shrink) due to this prolonged lack of stimulation. This can lead to serious health problems if the synthetic GH use is stopped.

Specimen Prep - Endocrine System