Page 1006 - Fundamentals of anatomy physiology
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takes place across a body surface. As an object (or person) gets Chapter 25 Metabolism and Energetics 993
larger, its surface area increases at a much slower rate than does
its total volume. This relationship affects thermoregulation, tissue between the shoulder blades, around the neck, and pos-
because heat generated by the “volume” (that is, by internal tis- sibly elsewhere in the upper body, is histologically and func-
sues) is lost at the body surface. For this reason, small individu- tionally different from most of the adipose tissue in adults. The
als lose heat more readily than do large individuals. tissue is highly vascularized, and individual adipocytes contain
numerous mitochondria. Together, these characteristics give
Thermoregulatory Problems in Infants. During embryonic the tissue a deep, rich color that is the source of the name
development, the mother’s body is at normal body tempera- brown fat. p. 155
ture. At birth, the infant’s temperature-regulating mechanisms
are not fully functional. Infants also lose heat quickly as a re- Individual adipocytes in brown fat are innervated by sympa-
sult of their small size. Consequently, newborns must be dried thetic fibers. When these nerves are stimulated, lipolysis speeds
promptly and kept bundled up. Those born prematurely need up in the adipocytes. The cells do not capture the energy released
a thermally regulated incubator. Infants’ body temperatures are through fatty acid catabolism. Instead, the energy radiates into
also less stable than those of adults. the surrounding tissues as heat. This heat quickly warms the
blood passing through the surrounding network of vessels, and it
Infants cannot shiver, but they have a different mechanism is then distributed throughout the body. In this way, an infant can
for raising body temperature rapidly. In infants, the adipose very quickly speed up metabolic heat generation by 100 percent.
In contrast, nonshivering thermogenesis in an adult raises heat
production by only 10–15 percent after a period of weeks.
Clinical Note response to external stimuli—even painful ones. The body 25
temperature continues to decline, and the skin turns blue or
Thermoregulatory Disorders Heat exhaustion and heat pale and cold.
stroke are malfunctions of thermoregulatory mechanisms.
With heat exhaustion, also known as heat prostration, exces- At this point, people commonly assume that the person
sive fluids are lost in perspiration, and the individual has has died. But because metabolic activities have decreased
difficulty maintaining blood volume. The heat-loss center system-wide, the victim may still be saved, even after several
stimulates sweat glands, whose secretions moisten the sur- hours. Treatment consists of cardiopulmonary support and
face of the skin to provide evaporative cooling. As fluid losses gradual rewarming, both external and internal. The skin can
mount, blood volume decreases. The resulting decline in be warmed up to 45°C (113°F) without damage. Warm baths
blood pressure is not countered by peripheral vasoconstric- or blankets can be used. One effective method of raising in-
tion, because the heat-loss center is actively stimulating ternal temperatures involves the introduction of warm saline
peripheral vasodilation. As blood flow to the brain declines, solution into the peritoneal cavity.
headache, nausea, and eventual collapse follow. Treat-
ment is straightforward: Provide fluids, salts, and a cooler Hypothermia is a significant risk for people engaged in
environment. water sports. It may complicate the treatment of a drowning
victim. Water absorbs heat about 27 times as fast as air does,
In heat stroke, body temperature rises uncontrollably and the body’s heat-gain mechanisms are unable to keep
because the thermoregulatory center stops functioning. The pace over long periods or when faced with a large tempera-
sweat glands are inactive, and the skin becomes hot and dry. ture gradient. But hypothermia in cold water does have a
Heat stroke is more serious and can follow an untreated case positive side. On several occasions, small children who have
of heat exhaustion. Predisposing factors include any preexist- “drowned “ in cold water have been successfully revived after
ing condition that affects peripheral circulation, such as heart periods of up to four hours. Children lose body heat quickly,
disease or diabetes. Unless the problem is recognized in time, and their systems soon stop functioning as their body tem-
body temperature may climb to 41°C–45°C (106°F–113°F). perature declines. This rapid drop in temperature prevents the
Temperatures in this range quickly disrupt a variety of vital oxygen starvation and tissue damage that would otherwise
physiological systems and destroy brain, liver, skeletal muscle, take place when breathing stops.
and kidney cells. Effective treatment involves lowering the
body temperature as rapidly as possible. Resuscitation is not attempted if the person has actu-
ally frozen. Water expands roughly 7 percent during ordinary
Hypothermia is below-normal body temperature. If body freezing. The process destroys plasma membranes through-
temperature drops significantly below normal levels, thermo- out the body. Very small organisms can be frozen and subse-
regulatory mechanisms become less sensitive and less effec- quently thawed without ill effects, because their surface-to-
tive. Cardiac output and respiratory rate decrease. If the core volume ratio is enormous, and the freezing process occurs so
temperature falls below 28°C (82°F), cardiac arrest is likely. The rapidly that ice crystals never form.
individual then has no heartbeat, no respiratory rate, and no
