Give scientific reason: $(1)$ Smoking is a cause of hypertension.

(N/A) During smoking,nicotine enters the bloodstream and induces the contraction of arteries (vasoconstriction). As a result,blood pressure increases. Moreover,the $O_2$ carrying capacity of the lungs decreases. Carbon monoxide $(CO)$ produced during smoking also binds to hemoglobin more strongly than $O_2$,thereby reducing the $O_2$ carrying capacity of the blood. All these factors contribute to the development of hypertension.