Endocrine System Overview

Hormone Secretion Types

• Endocrine Secretions: Hormones released directly into the bloodstream; targets distant organs.
• Exocrine Secretions: Substances secreted to the external environment or into hollow organs; includes enzymes, saliva, mucus, bile, and tears.
• Paracrine Secretions: Hormones that act locally within the same tissue.

Hypothalamus

• Key brain structure linking the endocrine and nervous systems.
• Regulates thirst, hunger, body temperature, and emotions.
• Known as the "master gland" of the endocrine system.
• Produces releasing and inhibiting hormones like Thyroid Releasing Hormone (TRH).

Posterior Pituitary

• Stores and releases hormones synthesized in the hypothalamus (does not produce its own hormones).
• Key Hormones:
  • Anti-Diuretic Hormone (ADH): Increases water reabsorption in kidneys; released when body fluids are concentrated.
  • Oxytocin: Triggers uterine contractions during childbirth, promotes breast milk ejection, and is associated with social bonding.

Anterior Pituitary

• Produces several key hormones:
  • Follicle Stimulating Hormone (FSH): Stimulates sperm production in testes and oocyte maturation in ovaries.
  • Luteinizing Hormone (LH): Surges on Day 14 of the menstrual cycle, initiating ovulation.
  • Adrenocorticotropic Hormone (ACTH): Stimulates adrenal glands to produce cortisol.
  • Thyroid Stimulating Hormone (TSH): Triggers the thyroid to produce thyroid hormones (T3 and T4).
  • Prolactin: Stimulates breast milk production.
  • Growth Hormone (GH): Primarily converted to insulin-like growth factor 1 by the liver to promote growth in various tissues.

Pineal Gland

• Located in the diencephalon of the brain.
• Produces Melatonin, which regulates sleep/wake cycles (circadian rhythm).

Pancreas

• Positioned posterior to the stomach; consists of Islets of Langerhans that produce hormones.
• Key Hormones:
  • Insulin: Lowers blood sugar post-meal by facilitating glucose uptake in cells and promoting glycogenesis and lipogenesis.
  • Glucagon: Raises blood sugar levels during fasting by promoting glycogenolysis, lipolysis, and gluconeogenesis.

Thyroid Gland

• Produces Thyroid Hormones (T3 and T4) that regulate metabolic rate, impacting energy conversion to ATP.
• Calcitonin: Lowers blood calcium by promoting calcium uptake in bones.

Parathyroid Glands

• Four small glands on the posterior side of the thyroid.
• Parathyroid Hormone: Increases blood calcium levels by stimulating bone breakdown.

Adrenal Glands

• Located above the kidneys, divided into medulla and cortex.
• Medulla:
  • Epinephrine: Involved in the "fight or flight" response alongside the sympathetic nervous system.
• Cortex:
  • Glucocorticoids (e.g., Cortisol): Raise blood sugar as a stress response.
  • Mineralcorticoids (e.g., Aldosterone): Regulate sodium reabsorption in kidneys in response to low blood volume/pressure.
  • Androgens (e.g., DHEA): Precursors for the synthesis of sex hormones like testosterone.

Renin-Angiotensin-Aldosterone Cycle

• Essential for increasing blood volume and blood pressure.
• Kidneys secrete renin, converting angiotensinogen into Angiotensin I.
• ACE in the lungs transforms Angiotensin I into Angiotensin II, which:
  • Stimulates Aldosterone and ADH secretion.
  • Causes arteriolar constriction.
  • Induces thirst.

Hormone Secretion Types

• Endocrine Secretions: Hormones released directly into the bloodstream; targets distant organs.
• Exocrine Secretions: Substances secreted to the external environment or into hollow organs; includes enzymes, saliva, mucus, bile, and tears.
• Paracrine Secretions: Hormones that act locally within the same tissue.

Hypothalamus

• Key brain structure linking the endocrine and nervous systems.
• Regulates thirst, hunger, body temperature, and emotions.
• Known as the "master gland" of the endocrine system.
• Produces releasing and inhibiting hormones like Thyroid Releasing Hormone (TRH).

Posterior Pituitary

• Stores and releases hormones synthesized in the hypothalamus (does not produce its own hormones).
• Key Hormones:
  • Anti-Diuretic Hormone (ADH): Increases water reabsorption in kidneys; released when body fluids are concentrated.
  • Oxytocin: Triggers uterine contractions during childbirth, promotes breast milk ejection, and is associated with social bonding.

Anterior Pituitary

• Produces several key hormones:
  • Follicle Stimulating Hormone (FSH): Stimulates sperm production in testes and oocyte maturation in ovaries.
  • Luteinizing Hormone (LH): Surges on Day 14 of the menstrual cycle, initiating ovulation.
  • Adrenocorticotropic Hormone (ACTH): Stimulates adrenal glands to produce cortisol.
  • Thyroid Stimulating Hormone (TSH): Triggers the thyroid to produce thyroid hormones (T3 and T4).
  • Prolactin: Stimulates breast milk production.
  • Growth Hormone (GH): Primarily converted to insulin-like growth factor 1 by the liver to promote growth in various tissues.

Pineal Gland

• Located in the diencephalon of the brain.
• Produces Melatonin, which regulates sleep/wake cycles (circadian rhythm).

Pancreas

• Positioned posterior to the stomach; consists of Islets of Langerhans that produce hormones.
• Key Hormones:
  • Insulin: Lowers blood sugar post-meal by facilitating glucose uptake in cells and promoting glycogenesis and lipogenesis.
  • Glucagon: Raises blood sugar levels during fasting by promoting glycogenolysis, lipolysis, and gluconeogenesis.

Thyroid Gland

• Produces Thyroid Hormones (T3 and T4) that regulate metabolic rate, impacting energy conversion to ATP.
• Calcitonin: Lowers blood calcium by promoting calcium uptake in bones.

Parathyroid Glands

• Four small glands on the posterior side of the thyroid.
• Parathyroid Hormone: Increases blood calcium levels by stimulating bone breakdown.

Adrenal Glands

• Located above the kidneys, divided into medulla and cortex.
• Medulla:
  • Epinephrine: Involved in the "fight or flight" response alongside the sympathetic nervous system.
• Cortex:
  • Glucocorticoids (e.g., Cortisol): Raise blood sugar as a stress response.
  • Mineralcorticoids (e.g., Aldosterone): Regulate sodium reabsorption in kidneys in response to low blood volume/pressure.
  • Androgens (e.g., DHEA): Precursors for the synthesis of sex hormones like testosterone.

Renin-Angiotensin-Aldosterone Cycle

• Essential for increasing blood volume and blood pressure.
• Kidneys secrete renin, converting angiotensinogen into Angiotensin I.
• ACE in the lungs transforms Angiotensin I into Angiotensin II, which:
  • Stimulates Aldosterone and ADH secretion.
  • Causes arteriolar constriction.
  • Induces thirst.