The Kedem Katchalski equation has been the standard formula describing fluid balance and exchange in the capillary. The osmotic component in this formula is suggested to be the difference in osmolalities between the plasma and interstitium corrected by a factor c described as the osmotic reflection coefficient. Our in vitro studies show that the membrane osmotic pressure (MOP) of plasma components are curvelinear with concentration (MOP= ac2 + bc +i where c = concentration) and inversely proportional to membrane pore size. Small increases in membrane pore size can greatly reduce plasma MOP even where there is no change in concentration of plasma. As the pore size in the capillary is highly dynamic, and as pore size is the major factor controlling plasma osmotic pressure, we suggest a better model of fluid balance and exchange is required to describe fluid balance and exchange.