EXERCISE PHYSIOLOGY LABORATORY #4

Measurement of Oxygen Consumption and Measurement of Respiratory Exchange Ratio

Purpose: The purpose of this lab is to measure oxygen consumption and ventilation at rest and during exercise and to demonstrate how the respiratory exchange ratio can identify what fuels are being used for energy. A second purpose is to demonstrate the difference between the respiratory quotient and the respiratory exchange ratio.

Introduction: During rest and submaximal exercise, nearly all of the ATP needed to maintain the bodily functions is produced by aerobic metabolism. Because aerobic metabolism depends on the utilization of oxygen and there is a standard ratio between the amount of oxygen consumed and the amount of ATP produced, it is possible to accurately measure the rate of aerobic expenditure by measuring the rate of oxygen consumption. It should be remembered from that approximately 5 kcal of energy are expended for each liter of oxygen consumed.

Since aerobic metabolism takes place in the skeletal muscles, oxygen must be transported from the atmosphere to the muscle cells in order to replenish ATP. It is the responsibility of the cardiorespiratory system (i.e., heart, lungs, blood vessels, blood) to transport oxygen. Because the rate of oxygen delivery must increase during exercise, the functions of the heart and lungs must also increase during exercise.

Procedures: The subject will cycle on a Cybex ergometer at four separate work rates for 5 minutes each while breathing through a two-way valve. Expired air will be analyzed for the volume of CO2 produced and the volume of O2 consumed. RER can be calculated from these values. The Sensorimedics cart performs these calculations automatically as long as it is calibrated properly prior to the start of the test. The subject's heart rate will also be recorded at each stage. By examining the heart rate vs. VO2 graph, one can predict VO2 max quite accurately.

ATP provides the immediate energy for cellular metabolism. When ATP is split to ADP and Pi, some of the energy released is linked to the energy needs of the cell and used to do biological work. During muscular exercise, the metabolic rate increases as does the breakdown of ATP. For cellular metabolism to continue, ATP must be replenished. Creatine phosphate (CrP) provides a short term energy source for replenishment of ATP. However, during bouts of exercise lasting longer than 10 seconds, our food stuffs provide the primary fuels for replenishment of ATP.

Carbohydrates and fats are the primary fuels metabolized during prolonged exercise. When carbohydrate and fat are aerobically metabolized (broken down with oxygen), they are transferred through a series of enzymatically catalyzed reactions that eventually reduce the original molecules to carbon dioxide and water. During the process, oxygen is consumed. Therefore, during aerobic metabolism, oxygen is consumed and carbon dioxide is produced.

Because carbohydrate and fat have different chemical structures, they are metabolized differently. Carbohydrate is initially catabolized in the cytoplasm of the cell from a 6-carbon sugar to a 3-carbon sugar, called pyruvate, via a process called glycolysis. Pyruvate is then transferred into the mitochondria where it is further broken down in a process called the Kreb's cycle (also called the TCA cycle). Fatty acids are composed of long hydrocarbon chains. During the catabolism of fatty acids, 2-carbon fragments are cleaved off during a process called beta oxidation. The 2-carbon fragments then enter the Kreb's cycle. Because carbohydrates and fats are metabolized differently, they differ in the amount of carbon dioxide produced relative to the oxygen consumed.

The ratio of carbon dioxide produced to oxygen consumed is called the respiratory exchange ratio (RER) and is measured by the gases exchanged at the mouth. If similar measurements are made at the cellular level, the ratio of CO2 produced/O2 consumed is called the respiratory quotient (RQ). The RER is a useful indicator of the type of fuel (fat vs. carbohydrate) that is being metabolized. During carbohydrate metabolism, there is an equal amount of CO2 produced for O2 consumed (RER = 1.0). During fat metabolism, there is less CO2 produced for O2 consumed. To illustrate, the metabolic equation for the catabolism of palmitate, a typical fatty acid, is the following:

 

C16H32O2 + 23 O2 ------> 16 CO2 + 16 H2O. The RER = 16 CO2 / 23 O2 = 0.70

 

 

Procedures: Due to a minimum level of ventilation necessary for our "Sensormedics" cart to analyze oxygen and carbon dioxide concentration, we will not be able to measure the RER at complete rest. Historically, during the last two minutes of each stage, the subject's expired air would be collected in a "Douglas bag" (i.e., a weather balloon) for future analysis. However, the "Sensorimedics" cart in the Exercise Testing and Prescription room will calculate the RER value for us. We will simply record the RER from each of the 4 stages of the oxygen this lab. If the expired air had been collected in Douglas bags, subsequent analysis of the air sample would include volume, % CO2 and % O2 content.

 

Lab Report Requirements:

1. For the four exercise stages, log heart rate as well as the volume (V) of oxygen consumed and carbon dioxide produced in liters per minute and milliliters per kg per minute. (1/2 pt)

2. For the four exercise stages, log the corresponding RER value. From this value, estimate the RQ value and log this value as well. (1/2 pt)

3. From Table 8-1 (p. 147) in your class text, determine the percentage kcal derived from fat and from carbohydrate for each sample. In addition, record the grams of lipid and CHO per liter of oxygen consumed. (1 pt)

4. Create a graph of VO2 (ml/kg/min) vs. HR (heart rate). Knowing that max HR = 220 - subject's age, you should be able to extrapolate to predict the subject's VO2 max. (2 pts)

 

 

 

Stage

Heart Rate

VO2 (l/min)

VCO2 (l/min)

VO2 (ml/kg/min)

1

2

3

4

Stage

RER

RQ

g lipid/ liter oxygen

g CHO / liter oxygen

% kcal from fat

% kcal from CHO

1

2

3

4

 

5) Answer the questions (6 points).

Questions

1) How many kcal would be expended by our subject if they exercised at each work rate for one hour? (1pt.)

 

2) If our subject completed a distance of five miles by running on the track or by riding their bicycle on the road (at the same rates of oxygen consumption), how would the kcal expenditure compare for each workout? (1pt.)

 

3) From the data collected and graph drawn, what would you predict for the VO2 max of our subject? (1pt)

4) In the metabolic pathway for the breakdown of carbohydrate, where, specifically, is oxygen consumed and where is CO2 produced? (see Fig. 6-11, p. 112 for reference). (1pt)

5) During the catabolism of carbohydrates, an equal number of CO2 molecules are produced for every molecule of O2 consumed (i.e., RQ = 1.0). During the catabolism of fatty acids, why are fewer CO2 molecules produced relative to O2 consumed? (see Fig. 6-11, Fig. 6-12, Fig. 6-13, Fig. 6-14 p. 112-116 for reference). (1pt)

6) Give three physiological reasons for the difference between RER and RQ. Which of these reasons generally occurs with exercise? (see p. 147-148) (1pt)

 

Text References