Found an interesting document describing the endothelium.
As I think I have noticed, the smooth muscle in blood vessels is a major user of energy in the human body...
"Thus, endothelial cells line the entire vascular system, from the heart to the smallest capillary, and control the passage of materials—and the transit of white blood cells—into and out of the bloodstream. A study of the embryo reveals, moreover, that arteries and veins develop from small vessels constructed solely of endothelial cells and a basal lamina: pericytes, connective tissue and smooth muscle are added later where required, under the influence of signals from the endothelial cells. The recruitment of pericytes in particular depends on PDGF-B secreted by the endothelial cells, and in mutants lacking this signal protein or its receptor, pericytes in many regions are missing. As a result, the embryonic blood vessels develop microaneurysms—microscopic pathological dilatations—that eventually rupture, as well as other abnormalities, reflecting the importance of signals exchanged in both directions between the pericytes and the endothelial cells.
Once a vessel has matured, signals from the endothelial cells to the surrounding connective tissue and smooth muscle continue to play a crucial part in regulating the vessel's function and structure. For example, the endothelial cells have mechanoreceptors that allow them to sense the shear stress due to flow of blood over their surface; by signaling this information to the surrounding cells, they enable the blood vessel to adapt its diameter and wall thickness to suit the blood flow. Endothelial cells also mediate rapid responses to neural signals for blood vessel dilation, by releasing the gas NO to make smooth muscle relax in the vessel wall..."**
That means that the sympathetic nervous system, while it controls directly some of the cardiovascular functions, especially in the arteries, does not have the same influence over the bloodflow when it comes to the smooth muscle and the endothelium. Control is automatic, both chemistry and physics-based, occurring, as described, as a result of on the one hand, shear stresses, and the other, neural signalling, chemicals which originate from the CNS.
The smooth muscle in blood vessels is directly under the control of the endothelium, which must be in good health for it to work. If it is not, the main background "tonic" use of energy by the smooth muscle all over the body can be in jeopardy. In the case of arteries this job is much easier, since the supply of oxygen is plentiful. However, the closer the blood is to the point of needing recharging with oxygen, the harder this job becomes. That is why the cerebro-spinal veins sometimes do not get enough oxygen. Their turn sometimes comes last.
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http://www.ncbi.nlm.nih.gov/books/NBK26848/