Many people ask why their legs ache more in the summer than at other times of the year. The answer involves many different factors—some apparent, some beneath the surface, just like your veins.
The first reason, I tell my patients, is daylight. The longest days in the summer (in 2015) have 15 hours and 15 minutes of sunlight. In contrast, the shortest days in the winter have only 9 hours and 6 minutes of sun.1 In combination with peoples’ more active summer lifestyles—think gardening, lawn mowing, cooking, socializing at barbecue—people are simply on their feet for longer amounts of time before the sun sets. To be exact, 60% more time is spent on their feet during the summer. By the time those glorious 8 pm summer sunsets arrive, gravity has caused blood to pool in the lower limbs, the sensation of achiness following.
The more overlooked reason, I tell my patients, is a fascinating phenomenon that takes place just beneath the surface, a process called vasodilation (“vaso” referring to blood vessels, and “dilation” meaning to enlarge). Vasodilation is when the size of blood vessels will increase in response to certain environmental situations, such as the hot temperature of the summer. You can see vasodilation when someone returns from summer yard work with flushed cheeks and skin.
The veins enlarge, which helps the body quite ingeniously cool itself by increasing blood flow towards the skin. Unfortunately, it may also worsen symptoms of venous insufficiency by increasing the volume of blood held in the vein—a process not fully understood by most people who are unaware of the changes their body undergoes in adapting to heat.
To fully understand vasodilation, consider the veins themselves. Veins have a thin smooth muscle layer that facilitates change of its diameter constantly. They can either relax/widen or constrict/narrow. Consider also the surprising expansiveness of the circulatory system, which is an estimated 60,000 miles long if every vein, artery and capillary were measured end-to-end.2 Both elastic and expansive, our veins have an incredible ability to change. Case in point: in normal temperatures, the amount of blood flow to the skin is roughly 0.25 Liters/minute, whereas in high temperatures, the blood flow increases up to 8 Liters/minute—meaning over 30 times more blood flow.3
Can Heat Cause Pain in Legs?
Consider this capacity for blood volume expansion with another interesting fact. What is the largest “organ” in the body? Not the lungs, not liver, not even the bones. The largest organ in the body is the skin. The skin on our body and all its associated structures like the hair follicles, sweat glands, nails, etc., are called “the Integument.” The integumentary system in our body is the largest organ in our body. The medical student learns this in their introductory course in their first year. It’s a good thing. It’s because living organisms have learned to use their “skin” as an important means to regulate their body temperature.
So, after lying on the beach, we become red, and flushed. Our bodies circulate much more blood toward the skin. How does this change help our bodies cool off?
Much like a convection oven, there is a flow of heat within the body. In a convection oven, hot air circulates from the outside inwards—from hot metal coils of the oven inwards, towards the pasta casserole or glazed salmon. In the body, there is a similar flow of heat, only in the opposite direction. The body circulates heat away from the central organs outwards towards the skin, where sweat and a summer’s breeze allow heat to dissipate off of the body. The heart works as the fan propelling this convection current that circulates heat away from the inside of our body.
This process is significant. In hot temperatures, the body increases the amount of blood being pumped (the heart pumps more) and directs more of its blood outwards towards the skin. In fact, roughly 60% of the circulating blood travels to the skin—meaning, 60% of the heart’s work is devoted to cooling the body.4 Interestingly, the U.S. Department of Energy reports that heating and cooling—think air conditioner in the summer—accounts for more than half of the electric bill in the typical home. So whether for your home or your body, temperature regulation proves a priority.
While the body is built to balance and will adapt itself to maintain a perfect internal temperature, vasodilation presents problems for individuals with venous insufficiency. The body already has trouble sending blood back from the legs upwards to the heart because of inefficient ‘one-way check valves’ inside the veins. When the heart begins to pump faster to circulate 30 times more blood towards the skin, more and more blood tends to accumulate in the legs, producing mild, sometimes intense discomfort and swelling.
How to Prevent Leg Pain in the Heat
For this reason, it is best for individuals with venous insufficiency to be mindful of the effect heat has on their body:
- Keep cool.
- Avoid just standing or sitting around for long periods.
- Wear compression stockings (I know, neither fun nor feasible in hot weather).
- Elevate the legs and allow the legs to rest at points throughout the day.
One more thing: Don’t forget the sunscreen as you enjoy the sun and enjoy those beach moments free from leg pain.
[3 & 4] Nisha Charkoudian MD. Skin Blood Flow in Adult Human Thermoregulation:
How It Works, When It Does Not, and Why. Mayo Clinic Proceedings. 2003;78:603-612.