Page 1002 - Fundamentals of anatomy physiology

ATP, but much of it is lost to the environment as heat. The pro-
cess of calorimetry (kal-o. -RIM-e-tre. ) measures the total amount
of energy released when the bonds of organic molecules are
broken. The unit of measurement is the calorie (KAL-o. -re. )
(cal), defined as the amount of energy required to raise the
temperature of 1 g of water 1 degree Celsius. One gram of water
is not a very practical measure when you are interested in the
metabolic processes that keep a 70-kg human alive, so we use
the kilocalorie (KIL-o. -kal-o. -re. ) (kcal), or Calorie (with a capi-
tal C), also known as the “large calorie,” instead. One Calorie
is the amount of energy needed to raise the temperature of
1 kilogram of water 1 degree Celsius. Calorie-counting guides
list the caloric value of foods in Calories, not calories.
The Energy Content of Food
In cells, organic molecules are oxidized to carbon dioxide and
water. Oxidation also takes place when something burns, and
this process can be experimentally controlled. A known amount
of food is placed in a chamber called a calorimeter (kal-o. -RIM-
e-ter), which is filled with oxygen and surrounded by a known
volume of water. Once the food is inside, the chamber is sealed
and the contents are electrically ignited. When the material has
completely oxidized and only ash remains in the chamber, the
number of Calories released can be determined by comparing
the water temperatures before and after the test.
The energy potential of food is usually expressed in Calo-
ries per gram (Cal/g). The catabolism of lipids releases a con-
siderable amount of energy, roughly 9.46 Cal/g. The catabolism
of carbohydrates or proteins is not as productive, because many
of the carbon and hydrogen atoms are already bound to oxy-
gen. Their average yields are comparable: 4.18 Cal/g for carbo-
hydrates and 4.32 Cal/g for proteins. Most foods are mixtures
of fats, proteins, and carbohydrates, so the calculated values
listed in a “calorie counter” vary.
Energy Expenditure: Metabolic Rate
Clinicians can examine your metabolic state and determine
how many Calories you are utilizing. The result can be ex-
pressed as Calories per hour, Calories per day, or Calories per
unit of body weight per day. What is actually measured is the
sum of all the varied anabolic and catabolic processes taking
place in your body—your metabolic rate at that time. The
metabolic rate changes according to the activity under way.
For instance, measurements taken while a person is sprinting
are quite different from those taken while a person is sleep-
ing. Figure 25–13 shows the caloric expenditures for various
common activities. In an attempt to reduce the variations
in studies of energetics and metabolism, physiologists and
clinicians standardize the testing conditions so as to deter-
mine the basal metabolic rate (BMR). Ideally, the BMR is
the minimum resting energy expenditure of an awake, alert
person.
Chapter 25 Metabolism and Energetics 989
A direct method of determining the BMR involves moni-
toring respiratory activity. In resting individuals, energy use is
proportional to oxygen consumption. If we assume that average
amounts of carbohydrates, lipids, and proteins are being catabo-
lized, 4.825 Calories are expended per liter of oxygen consumed.
An average individual has a BMR of 70 Cal per hour, or
about 1680 Cal per day. Although the test conditions are stan-
dardized, many uncontrollable factors can influence the BMR.
These factors include age, gender, physical condition, body
weight, and genetic differences.
Rather than measuring the actual metabolic rate, clini-
cians usually monitor the concentration of thyroid hormones.
Clinicians take this approach because the BMR is technically
difficult to measure, and because circulating thyroid hormone
levels have a profound effect on the BMR. The results are then
compared with normal values to obtain an index of metabolic
activity. One such test, the T4 assay, measures the amount of
thyroxine in the blood.
Obesity is defined as body weight more than 20 percent
above the ideal weight for a given individual. Currently, obesity
is considered an epidemic and is taking its toll on the health of
adults and children. Diseases associated with obesity include
heart disease, cancer, and diabetes. For this reason, Calorie count-
ing and exercise are important in a weight-control program.
The control of appetite is poorly understood. Stretch recep-
tors along the digestive tract, especially in the stomach, play a
role, but other factors are probably more significant. Social fac-
tors, psychological pressures, and dietary habits are important.
Evidence also indicates that complex hormonal stimuli interact
to affect appetite. For example, the hormones cholecystokinin
(CCK) and adrenocorticotropic hormone (ACTH) suppress the
appetite. The hormone leptin, released by adipose tissues, also
plays a role. During the absorptive state, adipose tissues release
leptin into the bloodstream as they synthesize triglycerides.
When leptin binds to CNS neurons that function in emotion
Figure 25–13 Caloric Expenditures for Various Activities. 25
1000 Estimated calories expended
800 by a 154 lb (70 kg) individual
Calories per hour 600
400
200
0 Resting Slow Speed Climbing Jogging Competitive
walking walking stairs swimming