Hormone Mechanisms and Endocrine Function

Endocrine System Overview

• The endocrine system is the major regulatory system for maintaining homeostasis and body functions
• Endocrine glands are ductless, secreting hormones directly into the circulatory system
• Hormones include amines (e.g., thyroxine, epinephrine), proteins (e.g., insulin, growth hormone, oxytocin), and steroids (e.g., cortisol, estrogen)

Objectives

• Understand hormone action mechanisms
• Identify major glands and their hormones
• Discuss hormone functions and interactions
• Describe feedback and antagonistic hormone mechanisms

Endocrine Glands and Hormones

• Includes: hypothalamus, pituitary, thyroid, parathyroid, thymus, pancreas, adrenal glands, ovaries, testes.
• Each gland releases specific hormones with particular functions

Pituitary Gland (Hypophysis)

• Located at the base of the brain
• Hangs from hypothalamus via infundibulum
• Two parts: anterior pituitary (adenohypophysis), and posterior pituitary (neurohypophysis)
• anterior pituitary: GH, TSH, ACTH, FSH, LH, Prolactin
• posterior pituitary: ADH, Oxytocin

Posterior Pituitary Gland Hormones

• ADH (Antidiuretic Hormone):

  • Increases water reabsorption in kidneys
  • Decreases urine output
  • Also causes vasoconstriction (in large amounts)

• Oxytocin:

  • Stimulates uterine contractions during labor
  • Promotes milk ejection from mammary glands
  • Also involved in social bonding and other behaviors (not detailed here)

Anterior Pituitary Gland Hormones

• Growth Hormone (GH):
  • Promotes growth and protein repair,
  • Stimulated by GHRH; inhibited by GHIH
• Thyroid-stimulating Hormone (TSH): Stimulates thyroxine and triiodothyronine production in thyroid
• Adrenocorticotropic Hormone (ACTH): Stimulates cortisol release from adrenal cortex
• Prolactin: Initiates and maintains milk production in mammary glands
• Follicle-stimulating Hormone (FSH): Stimulates follicle growth in ovaries and sperm production in testes
• Luteinizing Hormone (LH): Stimulates release of mature ovum and testosterone production in testes

Thyroid Gland

• Located in the neck, anterior to the trachea
• Produces thyroxine (T4) and triiodothyronine (T3)
• Iodine required for hormone synthesis
• Produces calcitonin, which reduces blood calcium levels

Parathyroid Glands

• Embedded in the thyroid gland
• Secrete parathyroid hormone (PTH), which increases blood calcium levels
• Antagonist to calcitonin

Pancreas

• Located in the upper left abdominal quadrant
• Contains Islets of Langerhans (alpha cells produce glucagon, beta cells produce insulin)

Glucagon

• Released in response to hypoglycemia (low blood sugar)
• Stimulates the liver to convert glycogen to glucose (glycogenolysis)
• Increases use of fats & excess amino acids in a process called gluconeogenesis

Insulin

• Released in response to hyperglycemia (high blood sugar)
• Facilitates glucose uptake by cells
• Promotes glycogen storage and decreases blood glucose levels

Adrenal Glands

• Located on top of each kidney
• Two parts: adrenal medulla and adrenal cortex

Adrenal Medulla

• Secretes epinephrine and norepinephrine (catecholamines)
• "Fight or flight" response: increased heart rate, blood pressure, and breathing rate
• Stimulated by sympathetic impulses from the hypothalamus.

Adrenal Cortex

• Secretes steroid hormones (e.g., mineralocorticoids, glucocorticoids, sex hormones)

Mineralocorticoids, specifically Aldosterone:

• Regulates blood volume and pressure
• Increases sodium reabsorption, and potassium excretion in the kidneys, indirectly increasing blood volume and blood pressure
• Stimulated by decreased blood sodium, increased blood potassium, and decreased blood volume/pressure

Glucocorticoids, specifically Cortisol:

• Plays a role in carbohydrate, fat and protein metabolism
• Increases blood glucose and amino acid levels
• Used for energy in stress , inflammation, and other responses

Ovaries

• Produce estrogen (stimulated by FSH) and progesterone (stimulated by LH).
• Estrogen is involved in development of secondary sexual characteristics
• Progesterone plays a major role in preparing the uterus for pregnancy

Testes

• Secrete testosterone (stimulated by LH).
• Supports secondary sexual characteristics in males and sperm production
• Inhibin is used to decrease FSH production by the pituitary

Hormone Action Mechanisms

• Two types of mechanisms are available for hormones to work
• Protein hormones: Bind to membrane receptors (outside the cell) Activating a second messenger (cAMP) causing a biochemical cascade.
• Steroid hormones: Diffuse easily across the cell membrane, & Bind to cytoplasmic receptors. These hormone-receptor complexes move to the nucleus to affect gene expression.

Explore hormone actions, from cAMP's role in two-messenger systems to steroid hormone mechanisms. Questions cover hormone-receptor interactions, protein synthesis effects, and endocrine feedback loops. Includes the impact of hypothalamic damage.