CHAPTER 73 Body Temperature Regulation, and Fever
The temperature of the deep tissues of the body (core temperature) remains constant within ±1° F (±0.6° C) despite large fluctuations in the environmental temperature. The average normal body temperature is generally thought to be between 98.0° F and 98.6° F when measured orally and about 1° F higher rectally.
Heat production is a byproduct of metabolism. Extra heat can be generated by muscle contraction (shivering) in the short term or by an increase in thyroxine in the long term. Most of the heat produced in the body is generated in the deep tissues. The rate of heat loss is determined by the rate of heat conduction to the skin and the rate of heat conduction from the skin to the surroundings.
Blood vessels are distributed profusely immediately underneath the skin. An increase in blood flow to these vessels causes more heat loss, and a decrease in blood flow to these vessels causes less heat loss. The rate of flow to these vessels can vary from 0% to 30% of the cardiac output. The skin is a highly effective “heat radiator” system for transferring heat from the body core to the skin.
Heat loss from the skin to the surroundings occurs by radiation, conduction, convection, and evaporation.
All objects above absolute zero radiate infrared waves in all directions. If the body temperature is greater than that of the surroundings, the body radiates heat to the surroundings. Conversely, if the body temperature is lower than that of the surroundings, the surroundings radiate heat to the body. About 60% of body heat is normally lost through radiation.
The body usually loses about 3% of its heat by conduction to objects. An additional 15% of body heat is lost by conduction to air; the air in contact with the surface of the skin warms to near body temperature. This warm air has a tendency to rise away from the skin.
The air next to the skin surface is warmed by conduction. When this warm air is removed, the skin conducts heat to the “new” layer of unwarmed air.
Convective heat loss is the mechanism for the cooling effect of wind. The cooling effect of water is similar. Because water has such a high specific heat, however, the skin cannot warm a thin layer of water next to the body. As a consequence, heat is continuously removed from the body if the water is below body temperature.
As water evaporates, 0.58 calorie of heat is lost for each gram of water converted to the gaseous state. The energy to change water from a liquid to a gas is derived from the body temperature.
Evaporation usually accounts for 22% of the heat lost by the body; evaporation of water through the skin (insensible water loss) accounts for about 16 to 19 calories of heat loss per hour.
Evaporative heat loss is very important when the environmental temperatures are at or near body temperature. Under these conditions, heat loss by radiation diminishes greatly. Evaporative heat loss becomes the only way to cool the body when environmental temperatures are high.
Air movement across the skin increases the rate of evaporation and as a result increases the effectiveness of evaporative heat loss (e.g., the cooling effect of a fan).
Sweat glands contain a deep, coiled glandular portion and a straight ductal portion that exits on the surface of the skin. A primary secretion similar to plasma but without plasma proteins is formed by the glandular portion of the sweat gland. As the solution moves up the duct toward the surface of the skin, most of the electrolytes are reabsorbed, leaving a dilute, watery secretion.
Sweat glands are innervated by sympathetic cholinergic fibers. When sweat glands are stimulated, the rate of precursor solution secretion is increased. The reabsorption of electrolytes occurs at a constant rate. If large volumes of precursor solution are secreted and at the same time electrolyte reabsorption remains constant, more electrolytes (primarily sodium chloride) are lost in the sweat.
Exposure to a hot climate causes an increase in the maximum rate of sweat production from about 1 L/hr in the nonacclimatized person to as much as 2 to 3 L/hr in the acclimatized individual. This larger amount of sweat increases the rate of evaporative heat loss and helps maintain normal body temperature. Associated with an increase in the rate of sweat production is a decrease in the sodium chloride content of the sweat; this allows better conservation of body salt. The decline in the sodium chloride content of the sweat is primarily the result of increased secretion of aldosterone, which enhances sodium reabsorption from the ductal portion of the sweat gland.
The anterior hypothalamic-preoptic area contains large numbers of heat-sensitive neurons; the septum and reticular substance of the midbrain contain large numbers of cold-sensitive neurons. When the temperature centers detect that the body is either too hot or too cold, these areas institute appropriate and familiar temperature-increasing or temperature-decreasing procedures.
Three important mechanisms are used to cool the body:
When the body is too cold, the temperature control systems initiate mechanisms to reduce heat loss and increase heat production:
The body maintains a critical core temperature of 37.1° C. When body temperature increases above this level, heat-losing mechanisms are initiated. When body temperature falls below this level, heat-generating mechanisms are initiated. This critical temperature is called the set point of the temperature control system. All temperature control mechanisms continually attempt to bring the body temperature back to this level.
The body has another temperature-control mechanism—behavioral control of temperature, which can be explained as follows: Whenever the internal body temperature becomes too high, the temperature-controlling areas in the brain give the person a psychic sensation of being overheated. Conversely, whenever the body becomes too cold, signals from the skin and from some deep body receptors elicit the feeling of cold discomfort. Therefore the person makes appropriate environmental adjustments to re-establish comfort, such as moving into a heated room or wearing well-insulated clothing in freezing weather. This is a powerful system of body temperature control and is the only really effective mechanism to maintain body heat control in severely cold environments.
An elevation in body temperature may be caused by an abnormality in the brain itself or by toxic substances that affect the temperature-regulating centers. Fever results from a resetting of the set point for temperature control; this resetting can be the result of proteins, protein breakdown products, or bacterial toxins (lipopolysaccharides), collectively called pyrogens. Some pyrogens act directly on the temperature control center, but most act indirectly.
When bacterial or viral particles are present in the body, they are phagocytized by leukocytes, tissue macrophages, and large granular killer lymphocytes. In response to the phagocytized particles, these cells release cytokines, a diverse group of peptide signaling molecules involved in the innate and adaptive immune responses. One of the most important of these cytokines in causing fever is interleukin-1. Interleukin-1 induces the formation of prostaglandin E2, which acts on the hypothalamus to elicit the fever reaction. When prostaglandin formation is blocked by drugs, the fever is completely abrogated or at least reduced. This is the proposed mechanism of action for aspirin and other antipyretics to reduce the level of fever, and it explains why these compounds do not lower the body temperature in a normal, healthy person (who does not have elevated levels of interleukin-1).
When the interleukin-1 mechanism resets the set point for temperature control, body temperature is maintained at a higher level. Raising the set point of body temperature induces the subjective sensations of being cold, and nervous mechanisms initiate shivering and piloerection. Once the body temperature has reached the new set point, the individual no longer has the subjective sensation of being cold, and body temperature is elevated above normal. When the pyrogens have been cleared from the body, the set point for temperature control returns to normal levels. At this point, the body temperature is too warm, which induces the subjective sensations of being too hot, and nervous mechanisms initiate vasodilatation of the skin blood vessels and sweating. This sudden change of events in a febrile state is known as the “crisis” or, more appropriately, the “flush” and typically signals that the temperature will soon be decreasing.