In fact, it is critical to effective cardiac performance.Arteries (one pictured) have a significant layer of smooth muscle composing the tunica media, the middle layer. This muscle, under control of the autonomic nervous system, responds to a number of stimuli, with the "goal" being to maintain sufficient pressure in the arterial circulation such that all tissues maintain adequate blood flow (OXYGEN). If this muscle contracts, the pressure increases; if it relaxes, the pressure goes down. The net effect of this pressure is termed RESISTANCE, or AFTERLOAD, and is a key factor in setting stroke volume, which in turn determines cardiac output. The neuroendocrine system is exquisitely tuned to tissue oxygen status, and directs blood blow to where it is needed, nanosecond by nanosecond. For example, during "flight or fight" responses nervous activity and blood blow are directed away from the gut toward the muscles; this is why you may feel nauseated ("butterflies in the stomach") when you are stressed. On the other hand, after you eat blood flow is redirected to the gut and you often, then, will feel sleepy as blood is redirected away from the brain and muscles. The body wastes no energy--everything that it does is beautifully balanced to maintain your health!
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