Damn, Summer: How the Human Body Cooks to Death

The human body is remarkably good at dealing with temperature extremes. It might not seem like it on a July afternoon trapped in the blast furnace of a Manhattan sidewalk, but among animal species on Earth, adaptations to extreme temperatures pretty much just get worse. A snake, for example, as an ectotherm ("cold-blooded" creature) has next to no ability to regulate its own body temperature and is thus forced to search for suitable environmental conditions to maintain a metabolically healthy temperature: a warm rock in the sun, a cool underground burrow.

Creatures like this might be able to survive over a wide range of body temperatures, but the costs are evolutionary (such as requiring the development of enzymes that can be effective over wide ranges of temperature) and metabolic: the lower metabolisms of our cold-blooded peers mostly keeps things like flight and big brains out of reach.

Humans, on the other hand, sweat. Sweating is hardly our only on-the-fly adaptation to temperature—like other warm-blooded animals, we do things like send more blood to the outer regions of our bodies and manipulate our body hairs to maximize air flow—but it's the one that really advances humans to the next level of cooling. Sweating, of course, encourages evaporative cooling and we have nearly the entire surface area of our bodies with which to take advantage of the phenomenon. Cats and dogs, by contrast, are just able to sweat from the pads of their feet and are forced to pant as an alternative (panting moves air over the relatively large surface area of the lungs, thus cooling from the inside). Shrewdly adapted or not, human bodies have limits and these limits are not terribly far out of the normal day-to-day range of life on temperate Earth. Humans can go for short periods (up to 15 minutes) at absolutely insane temperatures—well above 240°F, as demonstrated in UCLA's famous "hot box" experiments—but after that, bad things happen. And, really, bad things can happen as soon as you get much past 100 or so degrees, depending on what you're doing and how healthy you are. It's possible to overwhelm the body's cooling systems on any average summer day, and when that happens, heat sickness moves in. At the bottom of the spectrum are things like rashes and muscle cramps (where smooth muscle tissue "short circuits" because of electrolyte shortages), and then there's the full-on passageway to death, heat stroke. You might've even had a small taste of heat stroke in your life, in the form of heat exhaustion, which severity-wise falls somewhere between heat cramps and full-on stroke. Exhaustion is the span between when the body's normal cooling mechanisms stop keeping up and the 105°F body temperature lower-limit for stroke. It's more of an early warning sign than a syndrome in itself. Symptoms include (but are not limited to) a rapid pulse, pouring sweat, nausea, and the first inching upward of the body's internal temperature from normal. If action isn't taken immediately (and even if it is, in some cases), the result is pretty much guaranteed: stroke. Heat stroke is much more clearly defined than exhaustion. It's simply a body temperature above 105.1°F. Your cooling mechanisms at this point are completely overwhelmed and if something isn't done very soon to get cool, the result is probable death or at least permanent damage/disability. It's a medical emergency, e.g. you should be calling an ambulance without even thinking about it. A 1998 study from the University of Chicago found that once a patient has already reached this point, their prognosis goes through the floor. So: don't fuck around. It's worth poking around some in the actual pathophysiology of heat stroke, e.g. what it technically does to your body to make that heat stroke prognosis so poor. First off, the general phenomenon introduced by heat stroke is called hyperthermia, which is just what it sounds. Its generalness comes from the fact that it can occur outside of heat stroke in the form of fevers, which are different in that the increase in body temperature comes strictly from an internal malfunction: something in the body triggers your hypothalamus, the body's thermostat, to increase the temperature set-point of the body. It might be the direct influence of some outside bacteria or it might be the result of the body's immune system response to a pathogen (bacteria, virus, etc.), but it's the body's own systems keeping the fire stoked, rather than the body's environment. Our heat-death guide will be none other than the US Army's own Medical Aspects of Harsh Environments, Vol. I, which points out that tracing the exact mechanisms behind heat stroke is something of a fool's errand. The process is too poorly understood, particularly the relative emphases the body puts on its different cooling mechanisms. We can make some guesses, but, "if there is a hierarchy of homeostatic mechanisms, then which is more important: the need to control elevated body temperature by sweating, or the need to maintain plasma volume by not sweating? Heat alters the physiology of all the systems in the body." What's more, the differences between individuals and their physical reactions to heat are immense. Perhaps the most critical aspect of heat stroke is what happens to the heart. As you pour sweat in an effort to maintain a safe body temperature, your blood loses volume, becomes sludgy. In response, the heart kicks up both its rate and its stroke, or the amount of additional volume being pumped from any one of the heart's chambers. It works in total overdrive, the result of a gnarly positive feedback loop in which the heart tries to make up for smaller and smaller concentrations of water in the blood and smaller volumes of blood generally.

That ratio between water and other stuff is what determines how much water (and the nutrients it might carry) is available for all of the body's other cells and tissues; below a certain threshold, the body is literally drying out. But the heart keeps thinking that pumping more blood is the answer. Meanwhile, in response to the heat (or, rather, heat stress signalling mechanisms, including heat-shock proteins and a variety of more general immune system factors), the body's metabolism remains in overdrive, responding to increased calls for oxygen (as increased temperature has dramatically lowered the O2 concentrations in the blood) and carbohydrates. Those high metabolic rates in turn create even more heat. As you can see, it becomes even more of a downward spiral than a feedback loop. At this point, your body is pumping out its own excess heat to match its environment, as cells' transport power plants work harder and harder to get oxygen to release the energy needed to pump waste and other materials across their internal concentration gradients. Basically, everything in your body is working in overdrive as a reaction to both heat-stress and lack of water. Even as the heart beats faster and harder, it can't provide what the blood doesn't have, and eventually cells run out of energy. When this happens, it's not just a simple matter of chilling out until more fuel and oxygen becomes available. Note that the passive state of cells is actually active, as keeping different sorts materials from building up in a cell needs that transport power plant to do the work of pushing materials up what is basically an intracellular hill, or at least keeping them from falling down it (dispersing into the rest of the cell, in real life). Ironically, the end stage of heat stroke involves cells essentially drowning. With no energy, they're unable to maintain the ion pumps that move water in and out of cells and so water builds up, resulting in cellular swelling and haemorrhaging. Meanwhile, rigor mortis sets in as a result of the cells' energy depletion. Everything that was on the top of the gradient "hill" within the cell tumbles down, including calcium, which is responsible for the actual rigidity. This cellular collapse occurs in a larger, macroscopic context that's begun to look very, very bad. The blood begins filling with endotoxins, released from bacteria normally kept isolated in the intestines, which at this stage are compromised by reduced blood flow. With toxins flowing throughout your bloodstream, the body's immune system goes nuts, with one effect being a cascade of blood coagulation (solidification) taking place all over your circulatory system. The net effect at this closing stage is essentially sepsis, or blood poisoning. (Note that progression to heat stroke seems to be facilitated by pre-existing inflammation in the body.) The problem has become vastly bigger than jumping in an ice bath and the prospect of survival is rather more dim.

Mostly, doctors just say to keep an eye out for heat exhaustion symptoms. At that stage, you should be aware that you've reached a point of no return. Once you've defeated the body's powers of thermoregulation just a little bit, the process becomes a matter of positive feedback loops, as the body works against itself in more and more deadly ways, culminating eventually in a heart attack or some manifestation of the above death-chemistry. Fortunately, not getting heat stroke is pretty easy for young, reasonably healthy humans: stay cool, make sure your water has some salt and potassium in it, and you're golden.

"Damn, Summer" is a semi-regular series exploring the science behind summer's various miseries and pleasures.