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  • 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