When blood enters an arteriole, fluid leaks out into the tissues. Explain how most of the fluid is returned to the arteriole.

Blood in an arteriole is under high pressure at the start of the vessel, called hydrostatic pressure, and fluid can leak through its permeable membrane into the surrounding tissues. This leaves blood cells in the vessels and fluid outside the vessels. There is another pressure attracting water back into the vessels by osmosis, called oncotic pressure, created by the red blood cells. At the start of the arteriole: hydrostatic pressure > Oncotic pressure, this causes fluid to leak out.

But further along the vessel, with the fluid having left the vessel, and the red blood cells remaining in high concentration: Oncotic pressure > hydrostatic pressure, drawing most of the fluid back into the vessel to keep the circulating volume the same. The remaining fluid that isn't able to get back into the tissues passes into the lymphatic system which moves the fluid from the tissues back into the blood stream.