When it comes to hot weather, the human body acts remarkably like a home's central air conditioning - complete with a thermostat and cooling mechanism.
But extreme conditions can overwhelm that system - resulting in heat exhaustion, heat stroke and death.
"You get hot and the mechanisms you use to regulate that heat go haywire," said Dr. Joshua Sharfstein, Baltimore's health commissioner, who declared a Code Red heat alert yesterday.
To avoid heat-related illness, experts recommend drinking plenty of fluids, finding a cool place indoors and avoiding overexertion. Here's why that's important:
The body's target core temperature is within a degree or so of 98.6 degrees Fahrenheit. Blood temperature is continually monitored by sensors in the spinal cord and the hypothalamus, a part of the brain. When the core blood temperature rises as a result of, say, playing touch football, those sensors tell the body's cooling systems to rev up.
Unlike dogs and cats, which have to pant to cool down, humans have two internal mechanisms that transfer heat from internal organs to the external environment. The first is evaporation, in the form of sweating. The second is convection, through blood flow near the skin.
Both mechanisms have limits - particularly when high heat and humidity conspire to undermine the body's cooling ability.
As sweat evaporates, it cools the skin and the blood underneath - much as an air conditioner cools air by passing it over pipes chilled by the evaporation of a refrigerant.
Heavy sweating can sap the body's fluid reserves, leaving no reserve for cooling. It also depletes electrolytes such as potassium and salt, both crucial for metabolism. High humidity compounds the problem.
"When humidity rises, evaporation doesn't work as well," said Robert Koos, a physiology professor at the University of Maryland School of Medicine.
Increasing blood flow near the skin is another way of cooling the body's core. Blood vessels near the skin can widen, and the heart can pump faster to increase blood flow. That releases heat by convection through the skin.
However, heat exchange becomes inefficient when outside air is warmer than the body. On summer days, heat and humidity can become so intense that the body's internal cooling systems are unable to cope.
"To drive that blood flow to the skin, cardiovascular output can almost double," said Koos. "In an elderly person whose ability to increase cardiac output is diminished, it can be devastating."
