Final answer:
Vasoconstriction is the narrowing of blood vessels due to smooth muscle contraction, increasing resistance and blood pressure but decreasing flow. Vasodilation is the widening of blood vessels due to smooth muscle relaxation, decreasing resistance and blood pressure but increasing flow. These processes are regulated by the autonomic nervous system and other local factors.
Step-by-step explanation:
The change in blood vessel diameter caused by smooth muscle contraction is known as vasoconstriction when the diameter decreases, and vasodilation when the diameter increases. Contraction of the smooth muscle in the vessel walls leads to vasoconstriction, which increases resistance and blood pressure but decreases blood flow. Conversely, relaxation of these muscles leads to vasodilation, which decreases resistance and blood pressure while increasing blood flow.
The vascular tone of the vessel, determined by the contractile state of the smooth muscle, is the primary determinant of blood vessel diameter, and thus of resistance and flow. Consequently, neural and chemical signals that influence smooth muscle contraction and relaxation can significantly alter blood circulation and pressure within the body.
Factors like the autonomic nervous system and local conditions can affect the vascular tone, leading to either vasoconstriction or vasodilation to meet the body's needs, including temperature regulation, fight-or-flight responses, and blood flow adjustments during exercise or changes in body position.