Regulation of Heat Loss
- The hypothalamus is the thermostat of the body
and regulates body temperature by balancing heat
production and heat loss. - The hypothalamus receives information from its
own neurons (blood temperature) and from the
temperature receptors in the dermis. - Mechanisms to increase heat loss are vasodilation
in the dermis and increased sweating. Decreased
muscle tone will decrease heat production. - Mechanisms to conserve heat are vasoconstriction
in the dermis and decreased sweating. Increased
muscle tone (shivering) will increase heat produc-
tion.
Fever—an abnormally elevated body temper-
ature
- Pyrogens are substances that cause a fever: bacteria,
foreign proteins, or chemicals released during
inflammation (endogenous pyrogens). - Pyrogens raise the setting of the hypothalamic
thermostat; the person feels cold and begins to
shiver to produce heat. - When the pyrogen has been eliminated, the hypo-
thalamic setting returns to normal; the person feels
warm, and sweating begins to lose heat to lower the
body temperature. - A low fever may be beneficial because it increases
the activity of WBCs and inhibits the activity of
some pathogens. - A high fever may be detrimental because enzymes
are denatured at high temperatures. This is most
critical in the brain, where cells that die cannot be
replaced.
Metabolism—all the reactions within the
body
- Anabolism—synthesis reactions that usually
require energy in the form of ATP. - Catabolism—decomposition reactions that often
release energy in the form of ATP. - Enzymes catalyze most anabolic and catabolic reac-
tions.
Cell Respiration—the breakdown of food
molecules to release their potential energy
and synthesize ATP (Fig. 17–3)
- Glucose oxygen yields CO 2 H 2 O ATP
heat. - The breakdown of glucose involves three stages:
glycolysis, the Krebs cycle, and the cytochrome
(electron) transport system (see also Table 17–3).- The oxygen necessary comes from breathing.
- The water formed becomes part of intracellular
fluid; CO 2 is exhaled; ATP is used for energy-
requiring reactions; heat provides a body tempera-
ture.
Proteins and Fats—as energy sources (see
Table 17–4 for hormonal regulation)- Excess amino acids are deaminated in the liver and
converted to pyruvic acid or acetyl groups to enter
the Krebs cycle. Amino acids may also be converted
to glucose to supply the brain (Fig. 17–3). - Glycerol is converted to pyruvic acid to enter the
Krebs cycle. - Fatty acids, in the process of beta-oxidation in the
liver, are split into acetyl groups to enter the Krebs
cycle; ketones are formed for transport to other
cells (see Fig. 17–3).
Energy Available from Food- Energy is measured in kilocalories (Calories):
kcal. - There are 4 kcal per gram of carbohydrate, 4 kcal
per gram of protein, 9 kcal per gram of fat. With
reference to food, kilocalories may be called calo-
ries.
Synthesis Uses of Foods (Fig. 17–4)- Glucose—used to synthesize the pentose sugars for
DNA and RNA; used to synthesize glycogen to
store energy in liver and muscles. - Amino acids—used to synthesize new proteins and
the non-essential amino acids; essential amino
acids must be obtained in the diet. - Fatty acids and glycerol—used to synthesize phos-
pholipids for cell membranes, triglycerides for fat
storage in adipose tissue, and cholesterol and other
steroids; essential fatty acids must be obtained in
the diet. - Any food eaten in excess will be changed to fat and
stored. - Vitamins and minerals—see Tables 17–5 and 17–6.
Metabolic Rate—heat production by the
body; measured in kcal- Basal metabolic rate (BMR) is the energy required
to maintain life (see Box 17–4); several factors
influence the metabolic rate of an active person.
414 Body Temperature and Metabolism