Endocrine System: Principles of Chemical Communication

Endocrine System Principles

• Chemical messengers allow cells to communicate with each other
• Secretion is the controlled release of chemicals from a cell

Classes of Chemical Messengers

• Autocrine: stimulates the cell that originally secreted it (e.g., WBCs during infection)
• Paracrine: acts locally on nearby cells (e.g., WBCs during allergic reactions)
• Neurotransmitter: activates an adjacent cell
• Endocrine: secreted into the bloodstream by certain glands and cells

Functions of the Endocrine System

• Metabolism
• Control of blood glucose and other nutrients
• Control of reproductive functions
• Control of food intake and digestion
• Ion regulation
• Immune system regulation
• Water balance
• Heart rate and blood pressure regulation
• Uterine contraction and milk release
• Tissue development

Characteristics of the Endocrine System

• Composed of endocrine glands and specialized cells
• Hormones are chemical messengers secreted into the blood
• Target tissues are effectors, specific sites of hormone action
• Endocrine glands are ductless, secreting products directly into the bloodstream
• Exocrine glands have ducts that carry their secretions to the outside of the body or into a hollow organ

Hormones

• Chemical messengers secreted directly into the bloodstream by endocrine glands or specialized cells
• Travel through the bloodstream to reach target cells, producing a response
• Major link between the nervous and endocrine systems
• Termed as the endocrine control center of the brain

Anterior Pituitary

• Produces eight hormones:
  • Growth hormone (GH)
  • Thyroid-stimulating hormone (TSH)
  • Adrenocorticotropic hormone (ACTH)
  • Melanocyte-stimulating hormone (MSH)
  • Luteinizing hormone (LH)
  • Follicle-stimulating hormone (FSH)
  • Prolactin
  • Antidiuretic hormone
• GH affects body growth, protein synthesis, lipid breakdown, and blood glucose levels
• TSH promotes synthesis and secretion of thyroid hormone
• ACTH increases secretion of glucocorticoid hormones and skin pigmentation
• MSH increases melanin production in melanocytes
• LH and FSH regulate ovulation, progesterone production, and sperm cell production
• Prolactin promotes breast development, milk production, and prolactin secretion
• Antidiuretic hormone conserves water, constricts blood vessels, and regulates blood pressure

Pituitary Gland

• A small gland about the size of a pea
• Controls functions of many other glands
• Secrets hormones that influence growth, kidney function, birth, and milk production
• Formerly called the "Master Gland"
• 3 portions: anterior, posterior, and intermediate lobes

Thyroid Gland

• A shield-shaped organ located inferior to the larynx
• Typically has two lobes, located laterally to the trachea
• May have a third lobe (pyramidal lobe) in 50% of cases
• Produces:
  • Calcitonin: decreases bone breakdown, prevents large increases in blood Ca2+ levels
  • Thyroid hormones: increase metabolic rates, essential for normal growth and development
    • Thyroxine (T4): contains four iodine atoms, increases basal metabolic rate (BMR), enhances actions of catecholamines, and regulates development of nervous and bone tissues
    • Triiodothyronine (T3): contains three iodine atoms

Hyperthyroidism and Hypothyroidism

• Hyperthyroidism: elevated rate of thyroid hormone secretion
• Hypothyroidism: lack of thyroid hormones
• Symptoms: goiter, cretinism, myxedema, Graves' disease, exophthalmia

Parathyroid Gland

• Abnormally high rate of parathyroid hormone (PTH) secretion: hyperparathyroidism (hypercalcemia)
• Abnormally low rate of PTH secretion: hypoparathyroidism (hypocalcemia)
• Symptoms: muscle weakness, soft bones, prone to kidney stone formation, frequent muscle cramps, tetanus, respiratory arrest

Testes and Ovaries

• Testes: produce testosterone, aid in sperm cell production, maintain functional reproductive organs, and promote secondary sexual characteristics
• Ovaries: produce estrogen and progesterone, aid in uterine and mammary gland development and function, regulate the menstrual cycle, and promote secondary sexual characteristics

Other Hormones

• Erythropoietin: acts on bone marrow to increase red blood cell production
• Human chorionic gonadotropin (HCG): stimulates production of estrogen and progesterone

Adrenal Gland

• Adrenal cortex: produces hormones, including glucocorticoids, mineralocorticoids, and androgens
  • Glucocorticoids: regulate metabolism, resistance to stress, and immune suppression
  • Mineralocorticoids: regulate electrolyte balance and blood pressure
  • Androgens: stimulate axillary and pubic hair formation, female libido, and post-menopausal estrogen production
• Adrenal medulla: produces epinephrine and norepinephrine, regulates sympathetic nervous system activity