Let $A_1$ be the area of region I and $v_1$ be the velocity of blood in this region.

Similarly, $A_2$ and $v_2$ be the area and velocity in region $II$.

Using equation of continuity,

$A_1 v_1=A_2 v_2$

As, $A_1 > A_2$, so $v_2 > v_1$

Now, using Bernoulli's theorem, $p+\frac{1}{2} \rho v^2=$ constant

$\text { As, }v_2 > v_1$

$\therefore p_2 < p_1$

Hence, the pressure is lower in region $II$, when platelets enters a constriction.