By Randy Jackson—

“It was one of those March days when the sun shines hot and the wind blows cold: when it is summer in the light, and winter in the shade.” From Great Expectations by Charles Dickens.

Even that image from Victorian London seems warmer than this. Here, no sun or shade, only grey and flurries, and it’s mid-April already. The steady heat of Huatulco is now a distant memory. Standing at the window watching snow that should have stopped weeks ago, the body does what bodies do in the cold. It complains. Specifically, and elaborately at the cellular level, and more ‘whiny’ at the snowbird level. Why is it such a struggle to stay warm?

HOW DO OUR BODIES CREATE HEAT?

In a nutshell, body heat is a waste byproduct of cellular activity. Those cellular activities are enormously complex and varied, like running our organs, firing our muscles and digesting our food. But they have one thing in common. Like a car engine that heats up trying to turn the wheels, our cells cannot do their job without generating heat.

The energy that drives all our cells to perform their different functions comes from the body’s universal energy packet, the ATP molecule. This molecule is produced by the digestive process, transferring energy from the food we eat into a fuel that every cell in our bodies needs to perform its function. But not all parts of the body generate heat equally. There are four functions that account for most of the heat generation.

MUSCLES: Even sitting idle, muscles have their engines running. They need to be ready for that dash to the bus or to get up to pee. For a muscle to fire, or remain ready to fire, muscle cells hold that ATP energy molecule in a primed state. When our brains send the electronic signal, the muscle-primed ATP molecule splits, releasing energy, and the muscle fires. But that muscle firing is not perfectly efficient, and some of that energy simply escapes as heat. At rest, muscles produce about 25% of our body heat. And WAY more when sprinting from a bear.

THE LIVER: Muscles may get all the headlines in the body gazette, but the liver is relentlessly busy in the background, processing food, filtering toxins and making proteins. Liver cells, like all cells, run on ATP, and all that constant consumption generates significant heat as a byproduct. But unlike the muscle cells, liver cells don’t perform a mechanical function, and so most of the ATP energy ends up as heat, about 20% of our body’s heat overall.

ESSENTIAL SERVICES – HEART, BRAIN, KIDNEYS: These organs run 24/7 and, as a result of their constant consumption of ATP, contribute about 10% each to our body’s heat generation

BROWN FAT: Unlike all those busy organs, doing stuff and accidentally creating heat, brown fat just hangs out and creates heat. Sort of like your nephew who plays computer games in the basement. Except when brown fat isn’t working, everyone notices. Our bodies need a certain amount of heat, and when we don’t have enough, things can go south quickly. Unlike other body cells, where heat is a byproduct of other functions, brown fat cells contain a protein that essentially converts all the ATP energy into heat. This is critical for newborns who are unable to shiver to create heat. And for adults who do not spend their winters in Huatulco, brown fat saves them from having to shovel the whole block just to keep warm.

WHY DO OUR BODIES REGULATE HEAT?

We all know that people can suffer and even die when their bodies get too much or not enough heat. It turns out our Goldilocks body temperature is 37°C (98.6°F), and our bodies have evolved to keep us at or near this temperature. There is a chemical reason for this. Back at the cellular level, the chemical processes cells use to produce proteins and enzymes are highly sensitive to temperature. Nudge that temperature a bit too far in either direction, and the whole system starts to break down.

So the body has a highly sophisticated heating and cooling system to keep us, like Goldilocks’s porridge, ‘just right’. Our body’s thermostat is the hypothalamus, a small region of the brain that constantly monitors our core temperature and triggers responses to keep it within range. Too hot, and it triggers sweating and redirects blood toward the skin to release heat. Too cold and it triggers shivering, ramps up metabolism and redirects blood away from the skin to conserve warmth.

LIVING OUTSIDE THE COMFORT ZONE

All this sophisticated biological body heat regulation can only do so much. Our bodies are happiest when our environment is in the Thermoneutral Zone of 20°C to 25°C (68°F to 77°F ). Moving further from this temperature range in either direction requires action on our part.

TOO COLD: It has been said that one of the most important inventions in human history was the sewing needle. Fifty thousand years ago, needles enabled the making of fur clothing, thus enabling humans to adapt to climates colder than those in tropical Africa. As with most important things, this was both good news and bad news. The good news is that the vast majority of Earth’s landmass lies north of the tropical zone, opening up entire continents for human expansion. The bad news was that humans needed parkas. Yes, chafing was involved, but it also left humans, to this day, spending a great deal of time and energy just trying to stay warm.

Besides clothing and huddling around whatever could be burned, the ‘endurance option’, nature offered warm-blooded creatures just two alternatives: get out or go unconscious. Migrate or hibernate.

The ‘get out’ option came naturally to birds. Birds also have a hypothalamus, which, in migratory species, contains photoreceptors that detect day length. This triggers a hormonal release that drives the migratory instinct. When the days shorten, the hypothalamus sounds the alarm, and the bird heads south. In the highly evolved ‘snowbird’, their computer calendar reminds them to book airfare. Hormones kick in, and they find themselves inexplicably drawn to bathing suits, suitcases, and Huatulco Facebook posts.

Unconsciousness, nature’s only other winter coping option, requires hibernation. Hibernation works reasonably well for bears, chipmunks and squirrels, who don’t have Netflix. Again, the hypothalamus is involved, triggering hormones that dial down metabolism, heart rate and core body temperature to the minimal levels required for survival. During hibernation, an animal’s body is too cold to produce the electrical currents required for dreaming, and that just sucks.

TOO HOT: Options for regulating body heat when temperatures exceed the thermoneutral zone are far less onerous than those on the too-cold end of the spectrum. The body’s cooling mechanism triggers sweat glands on the skin, and blood vessels near the skin surface dilate to bring more warm blood to the surface for cooling. Shade, water and moving air all accelerate exactly these processes.

Water, pool, ocean or shower is the most immediate solution, pulling heat from the body twenty-five times faster than air alone. Moving air, whether breeze or ceiling fan, amplifies the cooling effect by accelerating evaporation from the skin, the same process your sweat glands are already working hard to achieve. Using wind-chill calculations as a rough approximation, a light breeze or a moderate ceiling fan speed in a 30°C (86°F) environment reduces your felt skin temperature to around 28°C (82°F), nudging your body back toward the thermoneutral zone while you possibly enjoy a margarita.

Randy Jackson blends local reporting from the perspective of a seasonal Huatulco resident with explorations of life and change in Huatulco, Oaxaca and Mexico. Email: box95jackson@gmail.com