Hi Lina,
You're correct: when area decreases, velocity goes up, and pressure drops, according to Bernoulli's equation.
HOWEVER, the form of Bernoulli's equation that we learn does not take into account any additional sources of pressure. In the case of the body, there IS an additional source of pressure: the heart! So, even though everything you said previously takes place (area decreases, velocity goes up, pressure drops), the heart (a pump) COMPENSATES FOR THIS and INCREASES PRESSURE. Since this is against what naturally should happen, it is very dangerous.