Introduction Microcirculation capillaries, the smallest blood vessels, are the site of exchange of nutrients, waste products, and fluids capillaries are thin-walled (a single layer of endothelial cells) and allow for the exchange of solutes and gases O2 and CO2 simple diffusion through endothelial cells water-soluble substances e.g., water, glucose, and amino acids diffusion through aqueous clefts between cells e.g., water, glucose, and amino acids diffusion through aqueous clefts between cells e.g., water, glucose, and amino acids diffusion through aqueous clefts between cells e.g., water, glucose, and amino acids diffusion through aqueous clefts between cells e.g., water, glucose, and amino acids diffusion through aqueous clefts between cells Exchange of Fluids Osmosis is driven by Starling forces (hydrostatic and osmotic pressures) protein contributes to osmotic pressure (noted as oncotic pressure) filtration net fluid out of capillaries into the interstitium absorption net fluid into capillaries from the interstitium Starling equation Jv = Kf [(Pc - Pi) - (πc - πi)] fluid movement is determined by sum of hydrostatic and oncotic pressures variables Kf "hydraulic conductance" or water permeability of the capillary wall determines the magnitude of fluid movement Pc capillary hydrostatic pressure favors filtration out of the capillary Pi interstitial hydrostatic pressure opposes filtration πc capillary oncotic pressure opposes filtration osmotic pressure of capillary blood due to plasma proteins πi interstitial oncotic pressure favors filtration the magnitude of fluid movement for a given pressure difference determined by hydraulic conductance (Kf, or water permeability) of the capillary wall Edema Edema, or swelling ↑ interstitial fluid volume occurs when interstitial volume > ability of lymphatics to drain it back into circulation from ↑ filtration causes of edema ↑Pc heart failure ↓ πc (↓ plasma proteins) severe liver failure failure to synthesize proteins nephrotic syndrome loss of proteins in urine ↑ Kf (↑ capillary permeability) burn, infection, and toxins a release of histamine and cytokines ↑ πi lymphatic blockage filtration out of capillaries exceed the ability of lymphatics to return fluid to the circulation