The fluids leave the capillaries at the arterial end because

The fluids leave the capillaries at the arterial end because of the difference in blood pressure.

Blood pressure or hydrostatic pressure is the primary force that transports blood between the arterial capillaries and tissues. Capillary blood pressure or capillary hydrostatic pressure-CHP of arteries is the force that drives blood out of capillaries and into the tissues. Hydrostatic pressure of interstitial fluid is called the interstitial fluid hydrostatic pressure - IFHP. CHP is higher than IFHP, so the fluid moves from the capillary end into the interstitial fluid via process called filtration.