Endocrine Glands and Hormone Regulation

Overview of Endocrine Glands

The endocrine system consists of glands that secrete hormones directly into the bloodstream to regulate various physiological processes. Each gland responds to specific stimuli and releases hormones that target organs or tissues to maintain homeostasis.

Pituitary Gland

Anterior Pituitary (Adenohypophysis)

• Stimulus: Controlled by releasing and inhibiting hormones from the hypothalamus.

• Hormones Produced:

  • Growth Hormone (GH): Stimulates growth of tissues and bones.

  • Thyroid-Stimulating Hormone (TSH): Stimulates the thyroid gland to produce thyroid hormones.

  • Adrenocorticotropic Hormone (ACTH): Stimulates the adrenal cortex to release corticosteroids.

  • Follicle-Stimulating Hormone (FSH) & Luteinizing Hormone (LH): Regulate reproductive processes.

  • Prolactin (PRL): Promotes milk production in mammary glands.

• Example: TSH is released in response to thyrotropin-releasing hormone (TRH) from the hypothalamus and stimulates the thyroid gland to secrete thyroid hormones.

Posterior Pituitary (Neurohypophysis)

• Hormones Released:

  • Antidiuretic Hormone (ADH): Increases water reabsorption in the kidneys in response to high blood osmolarity.

  • Oxytocin: Stimulates uterine contractions during labor and milk ejection during breastfeeding.

• Note: These hormones are produced in the hypothalamus and stored/released by the posterior pituitary.

Adrenal Glands

Adrenal Cortex

• Stimulus: ACTH from the anterior pituitary or changes in blood potassium/sodium levels.

• Hormones Produced:

  • Mineralocorticoids (e.g., Aldosterone): Regulate sodium and potassium balance.

  • Glucocorticoids (e.g., Cortisol): Increase blood glucose and help the body respond to stress.

  • Gonadocorticoids (e.g., Androgens): Contribute to secondary sex characteristics.

• Example: Cortisol is released in response to ACTH and helps mobilize energy during stress.

Adrenal Medulla

• Stimulus: Sympathetic nervous system activation ("fight or flight" response).

• Hormones Produced:

  • Epinephrine (Adrenaline) & Norepinephrine: Increase heart rate, blood pressure, and blood glucose.

• Example: Epinephrine is released during acute stress, preparing the body for rapid action.

Thyroid Gland

Follicular Cells

• Stimulus: TSH from the anterior pituitary.

• Hormones Produced:

  • Thyroxine (T4) & Triiodothyronine (T3): Regulate metabolism, growth, and development.

• Example: Increased TSH leads to increased T3 and T4 production, raising metabolic rate.

Parafollicular Cells (C Cells)

• Stimulus: High blood calcium levels.

• Hormone Produced: Calcitonin

• Action: Lowers blood calcium by inhibiting osteoclast activity and increasing calcium excretion by the kidneys.

Parathyroid Glands

• Stimulus: Low blood calcium levels.

• Hormone Produced: Parathyroid Hormone (PTH)

• Action: Increases blood calcium by stimulating osteoclasts to resorb bone, increasing calcium reabsorption in the kidneys, and activating vitamin D to enhance intestinal calcium absorption.

• Example: When blood calcium drops, PTH is released, leading to increased bone resorption and higher blood calcium.

Pancreas (Islets of Langerhans)

• Alpha Cells: Produce Glucagon in response to low blood glucose.

  • Action: Raises blood glucose by stimulating glycogen breakdown and gluconeogenesis in the liver.

• Beta Cells: Produce Insulin in response to high blood glucose.

  • Action: Lowers blood glucose by promoting cellular uptake of glucose and stimulating glycogen synthesis.

• Example: After a meal, increased blood glucose stimulates insulin release, which helps cells absorb glucose and lowers blood sugar levels.

Summary Table: Glands, Hormones, Stimuli, and Actions

Key Equations

• Blood Calcium Regulation:

• Blood Glucose Regulation:

Additional info: The above notes expand on the original points by providing definitions, mechanisms, and examples for each gland and hormone, as well as a summary table and relevant equations for homeostatic regulation.