Chapter 18 - Endocrine System

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Direct communication

  • Exchange of ions and molecules between adjacent cells across gap junctions
  • Occurs between two cells of the same type
  • Highly specialized and relatively rare

Paracrine communication

  • Chemical signals transfer information from cell to cell within a single tissue

Autocrine communication

  • Messages affect the same cells that secrete them
  • Chemicals involved are autocrines
  • Example: prostaglandins secreted by smooth muscle cells cause the same cells to contract

Endocrine communication

  • Endocrine cells release chemicals (hormones) that are transported in bloodstream
  • Alters metabolic activities of many organs
  • Target cells

–Have receptors needed to bind and “read” hormonal messages

  • Synaptic communication

–Neurons release neurotransmitters at a synapse

–Leads to action potentials that are propagated along axons

–Allows for high-speed “messages” to reach specific destinations

–Ideal for crisis management

  • Endocrine system

–Includes all endocrine cells and tissues that produce hormones or paracrines

–Endocrine cells release secretions into extracellular fluid

  • Unlike exocrine cells

–Endocrine organs are scattered throughout body

  • Classes of hormones

–Amino acid derivatives

–Peptide hormones

–Lipid derivatives

  • Amino acid derivatives (biogenic amines)

–Small molecules structurally related to amino acids

–Derivatives of tyrosine

  • Thyroid hormones
  • Catecholamines (epinephrine, norepinephrine, and dopamine)

–Derivatives of tryptophan

  • Serotonin and melatonin
  • Peptide hormones

–Chains of amino acids

–Most are synthesized as prohormones

  • Inactive molecules converted to active hormones before or after they are secreted


  • Proteins more than 200 amino acids long that have carbohydrate side chains (e.g., TSH, LH, FSH)

–Short polypeptides/small proteins

–Short-chain polypeptides

  • ADH and OXT are each 9 amino acids long

–Small proteins

  • Insulin (51 amino acids)
  • Growth hormone (191 amino acids)
  • Prolactin (198 amino acids)

–Includes all hormones secreted by hypothalamus, heart, thymus, digestive tract, pancreas, posterior lobe of the pituitary gland, etc.

  • Lipid derivatives

–Eicosanoids—derived from arachidonic acid, a 20-carbon fatty acid

  • Paracrines that coordinate cellular activities and affect enzymatic processes (such as blood clotting)
  • Some eicosanoids (such as leukotrienes) have secondary roles as hormones
  • Prostaglandins coordinate local cellular activities

–Converted to thromboxanes and prostacyclins in some tissues

–Steroid hormones—derived from cholesterol

  • Include

–Androgens from testes in males

–Estrogens and progesterone from ovaries in females

–Corticosteroids from adrenal cortex

–Calcitriol from kidneys

  • Bound to specific transport proteins in the plasma

–Remain in circulation longer than peptide hormones