11.3: Body Temperature
The body's temperature, measured in degrees, is determined by the balance between heat production and dissipation to the surrounding environment. For instance, if exercising vigorously, the body will produce more heat, causing sweat and dissipating that heat. Despite extreme environmental conditions and physical exertion, the human temperature-control system maintains a constant core body temperature (the temperature of deep tissues, which are the tissues located beneath the skin and other superficial tissues). In contrast, the surface temperature varies depending on blood circulation to the skin and the extent of heat dissipation.
Heat is produced by metabolic activities in the body's core tissues, transmitted to the skin's surface by circulating blood, and then released into the environment. Core body temperature typically ranges from 36.0°C (97.0°F) to 37.5°C (99.5°F), which tends to be higher than the surface body temperature. Core temperatures are commonly measured at the tympanic (ear) or rectal locations using non-invasive methods like infrared thermometers, or invasive monitoring devices like thermocouples. These devices can also measure temperatures in the esophagus, pulmonary artery, or bladder. Surface body temperatures are measured at the oral (sublingual), axillary (underarm), and skin surface regions using methods like oral or axillary thermometers or thermal imaging cameras.
Thermoregulation, the delicate balance between heat loss and heat production, is a complex process maintained by physiological and behavioral mechanisms. The hypothalamus, a small but mighty organ located between the cerebral hemispheres, plays a crucial role in regulating body temperature by detecting even minor temperature variations. The anterior hypothalamus, for instance, controls heat loss through sweating, vasodilation (expansion of blood vessels), and inhibition of heat production to facilitate heat dissipation. When internal temperatures rise, it redirects blood flow to surface veins to optimize heat release, showcasing the intricate mechanisms at play.
Conversely, when the posterior hypothalamus detects body temperature below the setpoint, it activates heat conservation mechanisms. For example, the body will try to conserve heat outside in cold weather. Vasoconstriction narrows blood vessels, reducing blood flow to the skin and extremities, thereby minimizing heat loss. If further heat conservation is needed, voluntary muscular contractions and shivering are triggered to generate heat.
Heat Production and Loss:
Heat production is a fascinating byproduct of metabolism, the chemical reactions which occur in all body cells. Several factors can influence these reactions, like activities that increase them, raising metabolic rates, and heat production. Conversely, a slower metabolism produces less heat. Heat is generated during rest, voluntary movements, involuntary shivering, and non-shivering thermogenesis through contraction of the pilomotor muscles, leading to piloerection, or "goosebumps," to minimize heat loss. This diversity of heat production methods showcases the complexity of the human body.
The skin is the primary site of heat loss, where heat from circulating blood transfers to the skin's surface. Here, arteriovenous shunts—connections between arterioles and venules directly under the skin—can open to allow heat to escape or close to retain heat. Continuous heat loss through the skin occurs via radiation, conduction, convection, and evaporation, influenced by the skin's structure and the surrounding environment.
