The “fight or flight” mechanism is one of the best-known physiological responses. It increases our ability to respond to stressful situations. One way to look at exercise – physiologically -- is as a non-life-threatening example of a stressful situation.
Think about it: When we exercise, our heart and breathing rates rise rapidly and blood vessels in our limbs dilate in order to deliver more oxygenated blood to our working muscle cells. The rapid rise in blood flow to the periphery -- especially to the legs -- can create a “heady” feeling, which reflects a temporary drop in blood pressure. This stimulates sympathetic nerves throughout the body to release substances that constrict blood vessels, even those vessels that feed working muscles.
Blood vessels in our leg muscles must respond to both dilating and constricting substances to meet the competing demands of muscle oxygen delivery and maintenance of bodily blood pressure, so we don’t faint! It’s a balancing act that could change with advancing age.
David Proctor and Urs Leuenberger and physiology student Dennis Koch, researchers at Pennsylvania State University and Hershey Medical Center, studied this possibility by manipulating the activity of the sympathetic nervous system in healthy older and younger men undergoing a moderate level of leg (cycling) exercise. They used a simple, but powerful sympathetic stimulus -- plunging a hand into a bucket of ice while still cycling -- and compared the blood vessel reactions in the legs of both age groups.
Results of the Proctor-Leuenberger-Koch study suggest that the mechanisms controlling blood vessel “tone” in exercising muscles may be altered by age in humans. “The augmented constrictor response seen in legs of older men could be a compensatory mechanism for their reduced level of cardiac output during exercise,” they reported. In other words, older adults may require a higher level of vascular “restraint” to preserve systemic blood pressure during exercise, Proctor said.