Number of places of an algebraic function field

Question

Let $F_q$ be a finite field and $F$ be an algebraic function field with full constant field $F_q$. Let $B_r$ be the number of places of $F$ of degree $r$. Then why $\displaystyle\sum_{r|i} rB_r=N_i$, where $N_i$ is the number of $F_{q^i}$ ratioal places of $F_{q^i} \cdot F.$