Editor’s note: Physicians have a special place among the thinkers who have elaborated the argument for intelligent design. Perhaps that’s because, more than evolutionary biologists, they are familiar with the challenges of maintaining a functioning complex system, the human body. With that in mind, Evolution News is delighted to offer this series, “The Designed Body.” For the complete series, see here. Dr. Glicksman practices palliative medicine for a hospice organization.

We live in a world made up of matter. Matter consists of atoms and molecules that follow the laws of nature. Organic life is made up of atoms and molecules that are organized into cells. Our body has trillions of them. Heat and temperature are physical phenomena and, although related to each other, they are not the same thing.

Heat is the transfer of energy from one object to another. When a machine uses energy, it naturally gives off heat. This applies to the body as well. When our cells use oxygen to release energy from glucose, they give off heat. The laws of nature not only cause the release of heat when energy is used, they also cause the transfer of heat from a warmer object to a cooler object when they come in contact with each other. When you touch a hot stove, the transfer of heat from it to your fingers will burn them. Grab an ice cube and the transfer of heat from your hand to it will cause it to melt.

In contrast, temperature is a measure of an object’s internal energy, reflected in its amount of random molecular motion. This energy is often derived from heat but can come from other sources, like electrical and nuclear energy. The higher an object’s temperature, the more random motion there is among its molecules. Conversely the lower an object’s temperature, the less random motion there is among its molecules.

For some molecules, like H₂O, the amount of random motion can affect its physical state. If the temperature of H₂O is below 32^oF (0^oC), it is a solid — ice. And when its temperature is between 32^oF-212^oF (0^oC-100^oC), H₂O is liquid water. Finally, when the temperature of H₂O is greater than 212^oF (100^oC) it is a gas called water vapor or steam. The effects of heat on an object’s temperature, physical state, and functional capacity apply not only to working machines but to the cells of the body as well.

Everybody knows that going outside in the sun during the summer will make you feel hot. And going outside without a coat in the winter will make you feel cold. And most people know that the temperature inside the body (core temperature) is normally higher than on the skin (surface temperature). All you have to do is blow on your hands and feel the heat to figure that out. As humans, we are warm-blooded, while most reptiles, amphibians, fish, and insects, are cold-blooded. But most people do not understand why and how the body follows the rules and keeps its core temperature within a certain range to stay alive. That’s what the next few articles in this series will explain.

Just as a machine can malfunction if it is too hot or too cold, so too, the cells that make up the organs of the body can malfunction if the core temperature is too high or too low. The core temperature of the body is a reflection of the amount of random molecular motion within its cells. Most of the enzymes the body uses for its metabolic processes work best within an ideal temperature range. For the human body the normal range for the core temperature is 97^o-99^oF (36^o-37^oC).

If the core temperature rises too high or drops too low, it may affect not only the function of the enzymes but also the integrity of the proteins and the plasma membrane. A core temperature greater than 107^oF (42^oC) usually causes structural and enzymatic protein breakdown, causing impairment of cellular respiration and destabilization of the plasma membrane. This ultimately results in brain malfunction, loss of temperature control, muscle breakdown, and multi-system organ failure. A core temperature below 91^oF (33^oC) usually causes a significant reduction in enzyme activity and metabolic function, resulting in a marked decrease in energy production. This too leads to brain malfunction, loss of temperature control, impaired muscle function, and multi-system organ failure.

Clearly, it is important for the body to control its core temperature. To understand how thermoregulation is accomplished, you must first understand how the laws of nature affect the body with respect to heat and temperature.

The core temperature of the body is affected mainly by two processes: how much heat the body produces from the energy its cells use to function and how much heat the body gains from, or loses to, its surroundings.

The chemical reactions in the body can either release or use up energy. The sum total of all these chemical reactions is called the metabolism. Chemical reactions that release energy while breaking down complicated molecules, like glucose (C₆H₁₂O₆), into simpler ones, like carbon dioxide (CO₂) and water (H2O), are called catabolic reactions. Chemical reactions that use energy to build more complex molecules, like proteins, from simpler ones, like amino acids, are called anabolic reactions. Both catabolic and anabolic reactions take place side by side in the cell.

The cell is only able to harness about one quarter of the energy that is released from the breakdown of complex molecules like carbohydrates, fats, and proteins. It places this energy in special energy-storage molecules (e.g., ATP). The remaining three-quarters of the energy is released into the body as heat. The energy-storage molecules, like ATP, then transfer their energy within the cell so it can be used for anabolic processes and functional activities. These include things like the synthesis of proteins for cell structure and enzymes that promote vital chemical reactions, ion pumps (like the sodium-potassium pump) for cellular integrity and function, muscle contraction, gland and nerve cell function, and gastrointestinal absorption. All of these processes ultimately result in the release of heat. So most of the energy the body uses eventually results in the release of heat.

When the body hasn’t eaten for a while and is at total rest, the amount of energy it requires to maintain its cellular integrity and total organ function is called its basal metabolic rate (BMR). Think of the BMR as being like the amount of energy a car uses while idling in traffic. It needs a minimum amount of energy just to keep the engine running before the driver steps on the accelerator. So too, the BMR is a measure of the amount of energy the body uses just to maintain its cellular and organ function while it waits to be put into action. And just like a car, the faster the body moves and the more work it does, the more energy it needs, the more heat it releases, and the higher its internal energy and temperature. So the laws of nature regarding the release of heat when energy is used to do work affects the body’s core temperature, not only when it is at complete rest (BMR) but with any level of activity.

Since the body is surrounded by air (or sometimes water) it is always losing heat to, or gaining heat from, its environment. Since most people prefer to stay in surroundings where their core temperature (97^o-99^oF, 36^o-37^oC) is higher than the ambient temperature, the body is usually constantly losing heat to its surroundings. In the same way that heat radiates from the sun, much of the heat produced by the body’s metabolism is lost through the skin into the surroundings. This accounts for about one-half of the body’s heat loss.

Conduction involves the transfer of heat from one object to another by direct contact. If the body comes in contact with something cooler or warmer than itself, as when swimming in a cold river or sitting in a hot sauna, then heat is transferred to or from the body by conduction. Heat loss by conduction usually takes place between the skin and the air surrounding the body and is often aided by convection. Convection is the phenomenon where heated air at the surface of the skin moves away from the body and is replaced by cooler air, which is more effective in taking away heat. This is why a cool breeze against the skin causes more heat loss. Conduction, aided by convection, generally accounts for about one-quarter of the body’s heat loss.

Finally, evaporation takes place when water on a surface absorbs heat from it and is released into the air as water vapor. Heat loss by evaporation takes place from the lungs, the mouth, and, most importantly, from perspiration on the surface of the skin. Evaporation accounts for about one-quarter of the total heat lost from the body.

In summary, the laws of nature demand that heat be released when energy is used to do work. The body invariably produces heat from its metabolism, which allows it to live and function normally within its environment. The laws of nature also demand that a warmer object transfer heat energy to a cooler one when they come in contact with each other. Since the body is surrounded by air that is usually cooler than its core temperature, this means that it is usually losing heat to its environment. The body’s core temperature is therefore determined by its total production of heat through its metabolism and how much heat it loses to, or gains from, its surroundings.

The molecules that make up the cells and perform the functions of the body work best within a given temperature range. To control its core temperature and stay healthy, the body must take into account these two laws of nature that naturally cause internal heat production and the transfer of heat to, or from, the environment. Next time, we’ll look at how the body does it and whether, given this understanding of how life works, the explanations of evolutionary biology are satisfying.

Image: Honeywell thermostat, by midnightcomm [CC BY 2.0], via Wikimedia Commons.