Nutrition, Metabolism, and Body Temperature Regulation
Blood serves as the major heat-exchange agent between the core
and the shell. When skin capillaries are ﬂushed with blood and
the skin is warmer than the environment, the body loses heat.
When blood is withdrawn to deep organs, heat loss from the shell
Heat-exchange mechanisms include radiation, conduction,
convection, and evaporation. Evaporation, the conversion of
water to water vapor, requires the absorption of heat. For each
gram of water vaporized, about 0.6 kcal of heat is absorbed.
Te hypothalamus acts as the body’s thermostat. Its heat-
promotion and heat-loss centers receive inputs from peripheral
and central thermoreceptors, integrate these inputs, and initiate
responses leading to heat loss or heat promotion.
Heat-promoting mechanisms include constriction of skin
vasculature, and shivering. If environmental cold is prolonged,
the thyroid gland may be stimulated to release more thyroxine.
When heat must be removed from the body, dermal blood
vessels dilate, allowing heat loss through radiation, conduction,
and convection. When greater heat loss is mandated (or the
environmental temperature is so high that radiation and
conduction are ineﬀective), sweating is initiated. Evaporation
of perspiration is an eﬃcient means of heat loss as long as the
humidity is low.
Profuse sweating can lead to heat exhaustion, indicated by a rise
in temperature, a drop in blood pressure, and collapse. When
the body cannot rid itself of surplus heat, body temperature rises
to the point where all thermoregulatory mechanisms become
ineﬀective—a potentially lethal condition called heat stroke.
Fever is controlled hyperthermia, which follows thermostat
resetting to higher levels by prostaglandins and initiation of
heat-promotion mechanisms, evidenced by the chills. When the
disease process is reversed, heat-loss mechanisms are initiated.
Developmental Aspects of Nutrition
Good nutrition is essential for normal fetal development and
normal growth during childhood.
Inborn errors of metabolism include cystic ﬁbrosis, PKU,
glycogen storage disease, galactosemia, and many others.
Hormonal disorders, such as lack of insulin or thyroid hormone,
may also lead to metabolic abnormalities. Diabetes mellitus is
the most signiﬁcant metabolic disorder in middle-aged and older
In old age, metabolic rate declines as enzyme and endocrine
systems become less eﬃcient and skeletal muscles atrophy.
Reduced caloric needs make it diﬃcult to obtain adequate
nutrition without becoming overweight.
Elderly individuals ingest more medications than any other age
group, and many of these drugs negatively aﬀect their nutrition.
LDLs transport triglycerides and cholesterol from the liver to the
tissues, whereas HDLs transport cholesterol from the tissues to
the liver (for catabolism and elimination).
Excessively high LDL levels are implicated in atherosclerosis,
cardiovascular disease, and stroke.
Body energy intake (derived from food oxidation) is precisely
balanced by energy output (heat, work, and energy storage).
Eventually, all of the energy intake is converted to heat.
When energy balance is maintained, weight remains stable.
When excess amounts of energy are stored, the result is obesity
(condition of excessive fat storage resulting in a BMI above 30).
Regulation of Food Intake
Te hypothalamus (particularly its arcuate nucleus) and other
brain centers are involved in regulating eating behavior.
Factors thought to be involved in regulating food intake include
(a) neural signals from the gut to the brain; (b) nutrient signals
related to total energy storage; (c) plasma concentrations
of hormones that control events of the absorptive and
postabsorptive states, and hormones that provide feedback
signals to brain feeding centers (leptin appears to exert the main
long-term controls of appetite and energy metabolism); (d) body
temperature, psychological factors, and others.
Metabolic Rate and Heat Production
Energy used by the body per hour is the metabolic rate.
Basal metabolic rate (BMR), reported in kcal/m
/h, is the
measurement obtained when the person is at comfortable room
temperature, supine, relaxed, and in the postabsorptive state.
BMR indicates energy needed to drive only the resting body
Factors inﬂuencing metabolic rate include body surface area,
age and gender, body temperature, stress, and thyroxine; also the
speciﬁc dynamic action of foods, and muscular activity.
Regulation of Body Temperature
Body temperature reﬂects the balance between heat production
and heat loss and is normally 37°C (
0.5°C), which is optimal
for physiological activities.
At rest, most body heat is produced by the liver, heart, brain,
kidneys, and endocrine organs. Activation of skeletal muscles
causes dramatic increases in body heat production.
Te body core (organs within the skull and the ventral body
cavity) generally has the highest temperature. Te shell (the skin)
is the heat-exchange surface, and is usually coolest.
(Some questions have more than one correct answer. Select the best
answer or answers from the choices given.)
Which of the following reactions would liberate the most energy?
complete oxidation of a molecule of sucrose to CO
conversion of a molecule of ADP to A±P,
respiration of a
molecule of glucose to lactic acid,
conversion of a molecule of
glucose to carbon dioxide and water.
Te formation of glucose from glycogen is
Te net gain of A±P from the complete metabolism (aerobic) of
glucose is closest to