SENSORS WORKING OVERTIME

Even as he slept, Krakauer’s body was working hard to thermoregulate. Like many physiological processes, thermoregulation is not something that requires conscious thought. It is more like the automated response of a home heating system, triggered when the thermostat is tripped.

The body’s thermostat is the hypothalamus, a grape-size structure that sits at the base of the brain, right above the brain stem. The hypothalamus receives signals from many different sensors, specialized cells in the body that detect changes in both the internal and external environment. For cold, the major sensors are thermoreceptors in the skin and in the hypothalamus itself. Information from various sensors is fed to the hypothalamus, where it is integrated

Acting as a thermostat, the hypothalamus has a specific temperature set point below which a warning message is triggered that body temperature is dropping. When that happens, the hypothalamus tells the body to take corrective action. For example, it can send a signal to blood vessels in the skin, causing them to constrict in peripheral vasoconstriction. It can also send a signal to muscles to start shivering. Both signals are sent from the hypothalamus to their target tissues by way of nerve fibers running from the brain to the rest of the body. The cells, tissues, or organs that respond to such signals are known as effectors: they act to cause a change in the internal environment. Once the effectors have raised the body temperature, the sensors detect the changed conditions and the signals are turned off.

This circuit of sensing, processing, and responding is an example of a homeostatic feedback loop (INFOGRAPHIC 25.4). In this case, the loop is a negative feedback loop because the output of the loop—an increase in body temperature—feeds back on the sensors to decrease the response. As body temperature rises, the responses that would further increase body temperature—shivering and vasoconstriction—are no longer needed and so they are turned off. This loop helps keep the system at the set point.

Not all feedback loops act in a negative fashion. Positive feedback loops occur when the output of a system acts to increase the response of the system. An example is the formation of a blood clot when you cut yourself, a response critical to preventing blood loss. Blood platelets stick to damaged blood vessels and release molecules that attract even more platelets to the area, which in turn attract even more platelets, and eventually a blood clot forms (see Chapter 8). Positive feedback loops are effective at rapidly amplifying a response, but negative feedback loops tend to be more common in physiology because they help return the body to its set point and ensure that homeostasis is maintained.

The hypothalamus does more than regulate body temperature. In fact, it is the body’s main homeostasis control center, regulating many bodily states including hunger, thirst, sexual arousal, and sleep. The hypothalamus is part of what Kenefick calls our “lizard brain”—the evolutionarily ancient parts of the brain, which control our most basic physiological responses through unconscious reflexes.

The hypothalamus is able to play such an important role in homeostasis because it is so well connected to sensors and effectors. The hypothalamus is not only a key part of the nervous system, connected to parts of the body through nerves, it is also intimately associated with the endocrine system, which produces changes in the body through the action of chemical signals called hormones, which travel through the blood. The hypothalamus connects to the endocrine system via a direct circulatory connection to the pituitary gland, a pea-size structure that sits right below the hypothalamus. Hormones released by the hypothalamus travel directly to the pituitary gland, signaling it to release more hormones, which in turn travel through the bloodstream and act on many tissues in the body, including other glands. The endocrine system, with its numerous hormone-secreting glands, is just one of 11 organ systems found in the human body that cooperate to perform the tasks necessary for survival—from food intake and waste removal to self defense and reproduction (see Up Close: Organ Systems and subsequent chapters in this unit).

During the night, Krakauer was awakened by a teammate who gave him grave news: a number of his teammates, including Rob Hall, had not yet returned to Camp IV. They were still out in the blistering subzero cold somewhere above 26,000 feet. Krakauer’s heart sank. He knew the chances of surviving in the cold for that long were slim. By 5 P.M., everyone’s oxygen tank would have been empty. It was now midnight. Krakauer feared for the others’ lives. But he was also dumbfounded. Hall and the rest of his team had not been far behind him on the mountain. What had gone wrong?

The storm he had spotted on the horizon began as a cyclone in the Bay of Bengal. It came in low from the valley and then rose up the mountain, gaining in ferocity and strength as it climbed. “One minute, we could look down and we could see the camp below. And the next minute, you couldn’t see it,” recalled Lou Kasischke, a member of Krakauer’s team, who was one of 11 people trapped on the Col when the storm hit and who recounted his experience in the PBS documentary Storm over Everest. “Within the space of 5 minutes, it changed from really a good day with a little bit of wind to desperate conditions, something I’d never experienced the ferocity of before,” said John Taske, another member of Krakauer’s team, on the same program.

According to Kent Moore, a physics professor at the University of Toronto, the storm that hit Everest that day also caused a particularly severe drop in barometric pressure, greatly reducing the availability of oxygen. “At these altitudes climbers are already at the limits of endurance,” Moore told New Scientist magazine in 2004. “The sudden drop in pressure could have driven some of these climbers into severe physiological distress.” In particular, they would have experienced the mental side effects of extremely low oxygen levels, which include confusion and disorientation.

Unable to tell in which direction they were going, and not wanting to take a wrong turn and step off a cliff, the climbers were forced to hunker down in the hurricane-force winds and wait for the storm to abate. Eventually, after 4 long hours, the clouds parted long enough for one of them to see where they were. Six climbers who were able to walk made it back to camp during this lull. An additional three were brought back safely by the efforts of Anatoli Boukreev, a Russian guide who, having descended to Camp IV, went back to search for them.

But others were not so lucky. Two climbers, too weak to make it back to camp, suffered severe frostbite before being rescued. One lost all his fingers and toes; the other had to have his right hand amputated. Those climbers stuck higher on the mountain—including Hall—could not be rescued. Trapped without shelter in the subzero temperatures all night, their supplemental oxygen and food long gone, the hikers eventually lost their ability to cope with the cold and succumbed to hypothermia, a drop in body temperature below 35°C (95°F). In all, eight climbers died on Everest that day.

This was not the first time that disaster had struck the summit. A 2008 study of all reported Everest deaths from 1921 to 2006, led by researchers at Massachusetts General Hospital and published in the British Medical Journal, found that more than 80% of deaths occurred above 26,000 feet, either during a summit attempt or the day after. While many of these deaths were attributable to traumatic injuries resulting from falls and avalanches, nearly as many were caused by hypoxia and hypothermia.