First, we must understand what blood pressure is, and what influences it to be high or low. Blood pressure is the pressure exerted by the blood and blood products as they move throughout the circulatory system on the vessel walls via the heart's cycle of contraction and relaxation. The two main mechanical factors influencing blood pressure are the cardiac output (how much blood is being pumped and at what rate?) and vascular resistance (how patent/open are blood vessels to allow for blood flow?). If the vessels are more open (vasodilation) and cardiac output is unchanged, blood pressure will be lower. If the vessels are closed or restricted (vasoconstriction) and cardiac output is unchanged, blood pressure will be higher. If vessels are closed and cardiac output is increased, blood pressure will be much higher.
Norepinephrine, also known by brand name Levophed, is a medication indicated for severe hypotensive events and shock. This medication's mechanism of action is to stimulate alpha-adrenergic receptors within blood vessels, causing them to constrict, and somewhat stimulate beta-adrenergic activity in the heart. The result of administration of this medication is then an increase in cardiac output and smaller vessels through which the blood is to be pumped; therefore, the patient experiences an increase in blood pressure. Think about a water hose -- if you leave the end of it untouched, water flows just fine; however, if you decrease the size of the hose, this water becomes a spray that leaves the end of the hose with much more velocity.
Zane was likely given this particular medication because norepinephrine is specifically indicated for increasing blood pressure, whereas other vasopressors like epinephrine are used as antiasthmatics, allergic reaction alleviation, bronchodilators, etc. Also, norepinephrine has an immediate onset with rapid peak when administered intravenously (IV) as opposed to epinephrine with a rapid onset but a 20-minute peak -- if the patient is having a severe circulation issue, the ABCs (airway, breathing circulation) of medicine dictate that this needs to be addressed, treated, and alleviated as soon as possible!