Lecture on the Endocrine System

Overview

• The endocrine system involves hormones produced by specific organs.
• Overlaps with the nervous system, but focuses on long-term changes, while the nervous system handles short-term communication.

Communication Methods

• Nervous system: Nerve impulses to specific locations.
• Endocrine system: Hormones released into the bloodstream; affect only specific target cells with compatible receptors.

Types of Glands

• Exocrine Glands: Secrete into ducts opening onto epithelial surfaces.
• Endocrine Glands: Ductless; secrete hormones directly into the bloodstream.

Hormones

• Hormones are specific molecules affecting target organs with specific receptors.

Types of Hormones

1. Peptide Hormones:
   • Formed from amino acid chains (protein hormones).
   • Example: Growth hormone.
2. Steroid Hormones:
   • Derived from steroid-based lipids (cholesterol).
   • Example: Testosterone.
   • Pass through cell membranes to activate genes in the nucleus.
3. Biogenic Hormones:
   • Small molecules from altered amino acids.
   • Example: Thyroid hormone.

Feedback Loops

Negative Feedback Loop

• Stimulus starts a process; hormone secretions slow/turn off the process.
• Example: Regulation of blood glucose levels.

Positive Feedback Loop

• Accelerates the original process.
• Example: Milk production in mammals.

Hypothalamus

• Oversees most endocrine activity.
• Secretes releasing and inhibiting hormones.
• Produces and stores hormones in the posterior pituitary (e.g., Oxytocin, ADH).

Pituitary Gland

• Connects to the hypothalamus via the infundibulum.
• Anterior Pituitary:
  • Secretes Growth Hormone, TSH, ACTH, LH, FSH, PRL.
• Posterior Pituitary: Releases hormones produced by the hypothalamus.

Thyroid Gland

• Located inferior to the thyroid cartilage.
• Secretes thyroxin (metabolism) and calcitonin (calcium regulation).

Parathyroid Glands

• Small nodules on the posterior of the thyroid.
• Parathyroid Hormone (PTH): Increases blood calcium levels.

Adrenal Glands

• Located atop the kidneys.
• Medulla: Produces epinephrine and norepinephrine.
• Cortex: Produces corticosteroids like aldosterone and glucocorticoids.

Pancreas

• Has both exocrine and endocrine functions.
• Endocrine Role:
  • Glucagon: Raises blood glucose levels.
  • Insulin: Lowers blood glucose levels.
  • Somatostatin: Inhibits glucagon and insulin.

Pineal Gland

• Secretes melatonin; regulates circadian rhythm.

Thymus

• Decreases in size with age.
• Produces hormones for T-cell maturation.

Other Endocrine Functions

Heart and GI Tract

• Secretes hormones regulating blood volume, pressure, and digestive processes.

Kidneys

• Produce hormones like renin and erythropoietin (EPO).

Reproductive Glands

• Ovaries: Produce estrogen and progesterone.
• Testes: Produce testosterone.

Conclusion

• The endocrine system involves complex interactions and feedback mechanisms to regulate various body functions and maintain homeostasis.