Basics of the Endocrine System

Definition of Endocrine System

• Endocrine glands secrete hormones directly into the bloodstream.
• Examples include the pituitary gland, thyroid gland, and adrenal gland.

Nervous vs. Hormonal Stimulation

• Nervous Stimulation:
  • Fast and direct, e.g., neuron to muscle.
  • Can be somatic (voluntary) or autonomic (involuntary).
  • Always active.
• Hormonal Stimulation:
  • Slower, involves secretion into the bloodstream.
  • Always involuntary.
  • Secreted on demand in small amounts (micrograms or nanograms).

Hormones

• Types: Steroidal (lipid-soluble) or non-steroidal (water-soluble).
• Act via messenger systems:
  • Primary messenger: The hormone itself.
  • Secondary messenger: e.g., G-protein.

Types of Secretions

• Autocrine: Acts on the same cell that secretes it.
• Paracrine: Acts on neighboring cells.
• Endocrine: Travels through the bloodstream to distant locations.

Endocrine Glands

• Ductless: Secrete hormones directly into the bloodstream.
• Hierarchy:
  • Hypothalamus (CEO)
  • Pituitary gland (General Manager)
  • Target glands (Employees)

Hormones and the Pituitary Gland

• Anterior Pituitary Hormones: Mostly from here except ADH and oxytocin (from the posterior pituitary).
• Posterior Pituitary: Stores hormones from the hypothalamus (ADH and oxytocin).

Hormones and Their Functions

• ADH: Regulates blood pressure by preventing drop during hypovolemia.
• Oxytocin: Bonds mother and baby, associated with love and memory suppression.

Thyroid Gland

• Secretes T3 (triiodothyronine) and T4 (thyroxine).
• Stimulated by:
  • Hypothalamus: TRH (Thyroid Releasing Hormone).
  • Anterior Pituitary: TSH (Thyroid Stimulating Hormone).

Types of Hypothyroidism

• Primary: Problem with the thyroid gland.
• Secondary: Problem with TSH production by the pituitary.
• Tertiary: Problem with TRH production by the hypothalamus.

Key Hormonal Pathways

• Growth Hormone: Hypothalamus secretes GHRH → Anterior pituitary releases GH → Liver releases IGF-1.
• Gonadotropins: Hypothalamus secretes GnRH → Anterior pituitary releases LH and FSH → Gonads produce sex hormones.
• Adrenal Hormones: Hypothalamus secretes CRH → Anterior pituitary releases ACTH → Adrenal cortex releases cortisol and androgens.

Hormone Solubility

• Lipid-Soluble:
  • Needs plasma protein carriers in blood.
  • Receptors are internal (cytoplasmic or nuclear).
• Water-Soluble:
  • Does not need carriers in blood.
  • Receptors are external (on cell surface).

Hormone Activity

• Bound hormones are inactive; free hormones are active.

Additional Resources

• Suggests checking the endocrinology playlist and premium courses for more detailed studies.