Can our blood freely enter the brain?

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No, There is a system to prevent harmful substances in the blood from entering the brain, which is called blood-brain barrier.
The brain require a proper environment. It needs certain chemicals to function, but in the correct quantity. If the brain is exposed to changing quantities of such chemicals, or to other foreign substances, it would function abnormally. Your bloodstream does transport many substances that could damage the brain. It is for this reason that the brain has a special protective system to control the flow of blood.
How is this done? In most parts of the bosy, the smallest blood vessels are porous. But the cells of such vessels in the brain are joined to each other tightly, with adjacent cells almost fused together.
Thus, chemicals made of large molecules cannot generally pass from the blood into the brain. At the same time, substances like oxygen, which have small molecules can cross this barrier easily. This is how the brain gets the oxygen it needs.