When the first heat wave of summer arrives, a grim tally will start: the number of people who die as a result.
According to the Centers for Disease Control, we can expect high temperatures to kill nearly 700 people each year in this country alone, most of them over 70 years of age. The vast majority won’t die of heat stroke, heat exhaustion, or dehydration. They will die of heart-related problems, including heart attacks and heart failure.
The reason, says Penn State physiologist Larry Kenney, is that we cool ourselves largely by pumping more blood to the skin, where the heat it carries can be lost to the environment. During hot weather, the increase in blood flow to the skin is huge, up to 20 times as much as in cooler weather. Even at rest, if we’re very hot, we may be pumping nearly two gallons of blood to our skin every minute.
High temperatures kill nearly 700 people each year in this country alone, most of them over 70 years of age.
That creates a strain on the heart that can be a particular problem for older people, as their hearts work harder trying to pump more blood to the skin. To make matters worse, the blood vessels in older skin don’t dilate as well as the vessels in the skin of younger people. They can’t accommodate the greater flow and can’t return as much blood to the heart.
“It’s kind of a double-whammy,” says Kenney. “Older adults don’t pump as much blood to the skin, but the left ventricle is still trying to contract very forcefully to do that. So in some older individuals who have heart failure, who had had a heart attack, or who just have a weak left ventricle, all of a sudden they’re putting much more stress on the heart”—sometimes with disastrous results.
Sweating the details
Kenney has worked on the effects of age on temperature regulation since 1983, when he received his first National Institutes of Health grant for the research. Initially he focused on sweating.
“There was an old notion that as people age, sweat glands actually atrophy and don’t function anymore—that the elderly don’t produce as much sweat. That would lead to less evaporative cooling,” he says.
His lab found that while we do tend to sweat less as we age, that’s only partly due to age-related changes in the glands. It’s also due to changes in our activities.
“Sweating as a means of thermoregulation is much more affected by aerobic fitness level, how acclimated you are to the heat, how well hydrated you are, whether you have a sedentary lifestyle, et cetera,” he says—things that often accompany aging but are not directly caused by aging.
Those early studies showed, however, that the ability to lose heat through our skin is directly related to age.
“While sweating is not always directly related to how old we are, the ability to constrict and dilate the blood vessels in the skin really is,” says Kenney. “So for the past 15 years or so, we’ve focused on that aspect of temperature regulation.”
Human skin responds rapidly and precisely to changes in both heat and cold, with tiny vessels called arterioles dilating or constricting to help dissipate heat or conserve it. The mechanisms that allow humans to achieve this precise control, and the magnitude of changes in skin blood flow, set us apart from our nearest relatives as much as walking upright and having opposable thumbs.
“It’s a uniquely human system,” says Kenney. “There’s not any other animal that regulates their skin blood flow the way humans do.” Some other mammals change the flow of blood to their skin, but by completely different mechanisms and often only in certain parts of the body. Rats control blood flow to their tails; rabbits change blood flow to their ears. But only humans increase skin blood flow over their whole bodies, and by such a large amount.
"There's not any other animal that regulates their skin blood flow the way humans do."
The rise in skin blood flow can be so dramatic that it increases the diameter of our limbs enough that Kenney can measure it with a strain gauge wrapped around a forearm. “It’s very subtle,” he says. “It’s in the fractions of a percent change in circumference of the strain gauge—but the more blood that flows to that forearm, the more the strain gauge stretches.”
In recent years, his lab has turned to laser techniques to more precisely measure blood flow in the skin. With laser Doppler flowmetry, a researcher shines a laser into an area of skin about the diameter of a pencil. Red blood cells flowing through tiny vessels there reflect the light. The amount of change in the reflection indicates how many red blood cells are moving through the area.
Another technique, laser speckle imaging, uses dots, or speckles, of reflected laser light to show relative changes in blood flow across a larger area of skin—an entire hand or foot, for example. “All pictures are made of speckles,” Kenney says—think of pixels on a computer screen. “If it’s a biological tissue, those speckles move and change, and they change with blood flow.” He uses a special camera to track that movement in real time. In the resulting images, the speckles are color-coded to indicate how many of them were moving in each portion of the image. Areas of dilation (more flow) show up bright red and yellow, while areas where the vessels were constricted (less flow) are a calm dark blue.
Because these large changes in skin blood flow and the molecular mechanisms that control them are unique to humans, Kenney can’t use mice or other lab animals for his studies. He relies on human volunteers willing to be heated up or cooled down while exercising or going through other experimental approaches. For his experiments, the “older” category starts at age 65; some of his volunteers are in their 90s.
Volunteers often recline or sit, and can watch movies (comedies, mostly), read, or just hang out—unless they’re in one of the studies looking at how well they dissipate heat generated by exercise. In that case, they sit or recline on a clinical bed outfitted with pedals, something like a recumbent bicycle, or walk or jog on a treadmill. In some experiments, the subjects breathe through an apparatus that allows Kenney’s team to measure cardiac output at the same time they measure skin blood flow.