The lining of the arteries have very delicate cells that produce NOH
Endothelial NOS is a nitric oxide synthase that generates NO in blood vessels and is involved with regulating vascular tone by inhibiting smooth muscle contraction and platelet aggregation. A constitutive Ca2+ dependent NOS provides a basal release of NO. eNOS is associated with plasma membranes surrounding cells and the membranes of Golgi bodies within cells. (wikipedia)
Basically these cells allow the arteries to be more flexible.
The data indicate that both saturated and unsaturated high-fat feeding may result in an increase in blood pressure owing to reduced endothelium-dependent vasorelaxation in the arterial system. The impaired endothelium-dependent vasorelaxation induced by saturated and unsaturated fatty acids may involve different mechanisms.