Capillaries

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Cardiovascular

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