Figure 32.18: Fluid Moves out of Capillaries into the Interstitial Fluids The widths of the thick red and black arrows symbolize relative magnitudes of the forces at work in the mammalian circulatory system. When blood pressure is greater than osmotic pressure, fluid leaves the blood in the capillaries, and when blood pressure falls below osmotic pressure, fluid returns to the blood in the capillaries. The balance of these two forces changes over the length of a capillary bed as blood pressure falls. The phenomena shown here occur in the systemic tissues. They do not normally occur in the lungs (where the alveoli could be flooded) because blood pressure is far lower in the lung circuit than in the systemic circuit.