Physioex Exercise 5 Activity 2

I am not sure which physioex i have but i am have trouble understanding the answer to this question

Activity 2
qn. when a blood vessel bifurcates (splits) into two smaller vessels, the radii of the two smaller vessels add up to a larger cumulative radius than the radius of the original vessel. however, blood flow is slower in the two vessels than in the original. why?

Apparently it is due to the fact that the system flows in a parallel circuit?? i dont quite understand how this affects the blood flow. An explanation would be much appreciated!!