CAPILLARY EXCHANGE: BULK FLOW ACROSS CAPILLARIES DEPENDS ON THE BALANCE BETWEEN FORCES. OUTWARD FLOW, FILTRATION , IS DRIVEN BY HYDROSTATIC PRESSURE: Pcap > Pif For a given hydrostatic pressure the flow is determined by the filtration coefficient, K, a measure of the size and number of the pores in the capillary walls. So, from capillary to interstitial fluid: Outward flow = K(Pcap – Pif) INWARD FLOW, ABSORPTION , IS DRIVEN BY OSMOTIC PRESSURE: πcap > πif The osmotic pressure is determined by the concentration of impermeable molecules. Proteins are normally impermeable across capillaries. The measure of impermeability is the “reflection coefficient” denoted by the Greek symbol σ. When σ = 1, the molecules are impermeant and contribute fully to the osmotic pressure. When σ = 0, the molecules are permeable and do not contribute to the osmotic pressure. The value of σ is normally = 1 for proteins, but can decrease when capillary pore size increases and proteins can leak out. The value of
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