What is the underlying mechanism by which nicotine affects the cardiovascular system?

Nicotine primarily affects the cardiovascular system by increasing heart rate. This is accomplished through its action as a stimulant on the central nervous system. When nicotine enters the bloodstream, it stimulates the release of neurotransmitters like norepinephrine and epinephrine (adrenaline), which activate the body's "fight or flight" response. This cascade leads to an increase in heart rate as the body prepares to respond to perceived threats.

While other options relate to cardiovascular effects, they do not represent the primary mechanism of action associated with nicotine. For example, lowering blood pressure is generally associated with medications that relax blood vessels, while nicotine tends to elevate blood pressure as well. The enhancement of oxygen flow is somewhat indirect; while nicotine does affect circulation, it primarily acts to increase heart rate rather than improve oxygen delivery directly. Similarly, vasodilation refers to the widening of blood vessels, which contrasts with the constrictive effects that nicotine has on certain blood vessels, ultimately leading to increased blood pressure rather than vasodilation. Thus, the most accurate representation of nicotine's impact on the cardiovascular system is its ability to increase heart rate.