What causes blood fluid to leak across capillary walls?

High blood pressure from arteries is the primary factor that causes blood fluid to leak across capillary walls. This phenomenon occurs when the hydrostatic pressure in the capillaries exceeds the osmotic pressure, leading to the movement of fluid out of the blood vessels and into the surrounding tissues, a process known as filtration. Capillary walls are permeable, allowing for the passage of fluids and small solutes, but when the pressure is elevated, it promotes an increased rate of fluid leakage.

While other factors listed may have indirect effects on capillary dynamics, they do not directly cause fluid to leak across the capillary walls in the same way elevated blood pressure does. For instance, low oxygen levels and high carbon dioxide levels in the blood can influence the dilation of blood vessels and the overall function of the cardiovascular system, but they are not direct causes of fluid leakage. Excessive physical activity could lead to temporary changes in blood flow and pressure, but the fundamental process driving leakage is primarily tied to high blood pressure in the arteries.