USMLE

Capillaries

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Cardiovascular
  1. Cardiovascular Overview
  2. CO2 Transport
  3. Heart
  4. Cardiac Impulse
  5. Blood Pressure
  6. Arteries and Arterioles
  7. Veins and Venules
  8. Capillaries
  9. Circulation
  10. Blood
  11. Hemoglobin
  12. Blood Clotting
  • Topic Anchor: Capillaries
    • Background:
      • Smallest blood vessels in the body
      • Between arterioles and venules
      • Arranged in parallel
        • Largest total cross-sectional area of any vessel type (all capillaries added together)
          • decreased total peripheral resistance and lower velocity of blood
        • High surface area = optimized gas exchange
    • Walls of capillaries act as semipermeable membrane
      • Permits fluid, gas, and nutrient exchange
        • Hydrostatic pressure
          • Pressure exerted by blood inside the vessel
          • Pushes fluid out of capillary into interstitium
          • Highest at arteriole end
            • Decreases across the capillary bed (from arteriole to venule)
        • Osmotic pressure
          • Pressure exerted by high solutes concentrations to pull fluid across a semipermeable membrane
            • As a result of cells and proteins within the capillary bed
          • Pulls fluid into capillary from interstitium
          • Constant across capillary bed
      • At the arteriole side of capillary, hydrostatic > osmotic pressure, so fluid flows out.
      • At the venule side of capillary, hydrostatic < osmotic pressure, so fluid flows back in.
        • Net 10% loss of fluid from capillary to lymph, which eventually empties back into blood