Blood vessels are connected to nerve fibers which are regulated by the nervous system. When innervated they respond by doing what?

When blood vessels are innervated they respond by contracting and relaxing their muscle wall, as a result of the activity of the autonomic nervous system.

Explanation:

The nerves that are responsible for the innervation of the blood vessels are called nervi vasorum, and are composed of nerve fibers of the sympathetic and parasympathetic nervous systems.

The innervation of the blood vessels specifically acts on the muscular wall of the vessel, so it contracts or relaxes depending on the activity of the nervous system:

The action of the autonomic nervous system has an effect on blood pressure, being a determinant of normal or pathological behavior of the arteries.

i am going to guess that it is co2.