lab 3

Question 1

introduction

Answer 1

Introduction
In exercise physiology, oxygen (O2) uptake (or V ̇ O2) is a very important measure that is used to
quantify the amount of energy that an individual is using during physical activity. All tissues of
the human body require a constant supply of O2 to support a biochemical “energy-producing”
process called oxidative phosphorylation. O2 is not a store of energy but is consumed as part of the
biochemical process that releases the energy stored in food (i.e., carbohydrate or fat) to
resynthesize adenosine triphosphate (ATP). Therefore, the volume and rate at which O2 is
consumed can be used as a measure of how much stored energy is used. Six moles (or molecules)
of O2 are required to completely breakdown (by “oxidation”) one molecule of glucose producing
6 moles of water and 6 moles of carbon dioxide.
C6H12O6 + 6O2 → 6CO2 + 6H2O + 32 ATP
The oxidation of glucose yields energy equivalent to 756 kcal per mole of glucose. Equation 2
shows the hydrolysis of ATP.
ATP ↔ ADP + Pi + Energy (7.3 kcal/mol)
Since only about 38% of the energy that is released from glucose oxidation is used in phosphate
bond formation (ADP to ATP), 32 moles of ATP are formed from 1 mole of glucose. Looking at
this from the point of view of O2, 6 molecules of O2 are required to produce 32 moles of ATP, or
~5 ATP will be resynthesized per O2 molecule consumed.
This relationship is very useful because we can measure O2 consumption with relative ease. We
only need to know how much O2 we inhale in a breath and how much we exhale in a breath. The
difference between the amount inhaled and exhaled will tell us how much O2 is being consumed.
Once the amount of O2 being consumed is known we can easily convert that value to calories
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(kcal) or Joules. Oxygen is being used as an energy equivalent. For example, when carbohydrate
predominates as the fuel source, the energy yield is 5.05 kcal/L of O2 consumed. To be able to
determine the amount of O2 being used either at rest or during exercise, a number of variables ar
required. As can be seen below, some of the variables must be calculated, others we assume, and
some we have to measure in the lab. However, even with the variables measured in the lab some
must be converted into useful units or corrected.
Table 1: List of short forms used in calculating oxygen consumption
Short Form Explanation Source Units
VO2I Volume of inspired O2 calculate L
VO2E Volume of expired O2 measure L (ATPS)
FEO2 Fraction of expired O2 measure decimal fraction
FECO2 Fraction of expired CO2 measure decimal fraction
FEN2 Fraction of expired N2 calculate decimal fraction
FIO2 Fraction of inspired O2 (0.2093) assume decimal fraction
FICO2 Fraction of inspired CO2 (0.004) assume decimal fraction
FIN2 Fraction of inspired N2 (0.7904) assume decimal fraction
RER Respiratory exchange ratio calculate
VI volume of inspired gas calculate L/min
VE volume of expired gas calculate L/min (STPD)
V ̇ O2 Minute rate of O2 uptake calculate L/min
V ̇ CO2 Minute rate of CO2 output calculate L/min
V ̇ E Minute ventilation calculate L/min (BTPS)
tcollection Total gas collection time measure minutes
Working with Gas Volumes
Air is composed of nitrogen (79.04%), oxygen (20.93%) carbon dioxide (0.04%) and water
vapour. Gases vary in volume according to temperature, pressure, and water content of the gas.
This makes it difficult to compare gas samples on different days or from different locations because
factors such as temperature, barometric pressure, and the water content of the air may change. As
a result, anytime one is working with gases the volumes need to be standardized. A sample taken
from the air on a given day needs to be corrected or standardized using the STPD (Standard
Temperature Pressure Dry) correction.
STPD factor = [(PBaro – PH2O) / 760] x [273 / (273 + TRoom)] (1)
In equation 1, the TRoom is the temperature of the gas in degrees Celsius (273 is 0 degrees Celsius
in Kelvin, i.e. the room temperature), PBaro is the barometric pressure in millimetres of Mercury
(mmHg) and PH2O is the partial pressure of water vapour in the air which varies with TRoom (760 is
the average air pressure at sea level). The partial pressure of water can be found in table 1. Note
that the volumes that you will be collecting and measuring are in ATPS (Atmospheric Temperature
Pressure Saturated). These will need to be converted to STPD.
Table 2. Vapor Pressure of Wet Gas at Temperatures in the Laboratory
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Temperature (°C) Partial pressure of water vapour (mmHg)
19 16.5
20 17.5
21 18.7
22 19.8
23 21.1
24 22.4
25 23.8
For example, a subject exhales 90 L of air in 1 minute (usually all measures of V ̇ O2 are per minute).
If the air temperature is 23°C and the barometric pressure is 740 mmHg what is the STPD corrected
value? (Answer: 78.5 L. See if you can get the same answer.)
All exhaled volumes of air should be corrected to STPD!
Calculation of oxygen uptake (V ̇ O2)
As noted in table 1 some values are assumed and some are measured in the lab. Calculation of
oxygen consumption is as follows. What we are trying to find is the difference between inspired
oxygen and expired oxygen (equation 2).
VO2 = VO2Inspired - VO2Expired (2)
Oxygen inspired is the volume of air inspired multiplied by the fraction of oxygen in that air
(0.2093, equation 3).
VO2Inspired = VI x FIO2 (3)
The amount of oxygen expired is the volume of air expired multiplied by the fraction of oxygen in
the expired air (equation 4). This would be measured using the gas analyzer.
VO2Expired = VE x FEO2 (4)
Normally, we only measure the volume expired (VE). As a result we have to calculate the volume
inspired (VI). We make use of the fact that nitrogen from the air we inspire is not used in the body.
That means that the volume of nitrogen inspired is the same as the volume of nitrogen expired
(equation 5).
VI x FIN2 = VE x FEN2 (5)
This can be rearranged as:
VI = VE x (FEN2/FIN2) (6)
where FIN2 = 0.7904 and FEN2 is the fraction of nitrogen in the air expired. Remember that then we
need to know what the amount of oxygen and carbon dioxide is in the expired air (FEO2, FECO2
respectively).
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FEN2 = (1.0 - FEO2 - FECO2) (7)
Thus, if we substitute equation 7 into equation 6 we have all the information required to solve
equation 2. Equation 8 provides the “volume” of oxygen consumed within the period of gas
collection. Do not forget to correct VE with the STPD correction first.
VO2 = (VE x ((1 - FEO2 - FECO2)/.7904) x .2093) - (VE x FEO2) (8)
Finally, to determine the absolute “rate of oxygen uptake” (i.e., V ̇ O2 in L/min), the “volume” from
equation 8 is divided by the total time during which the gas was collected (tcollection, in minutes):
V ̇ O2 = VO2 ÷ tcollection (9)
To compare between different sizes of individuals, it is useful to express V ̇ O2 relative to body
mass (in kg). To determine the relative V ̇ O2:
V ̇ O2 (mL/kg/min) = [V ̇ O2 (L/min) x 1000 mL/L] ÷ body mass (kg) (10)
Calculation of carbon dioxide output (V ̇ CO2)
Calculating the amount of carbon dioxide produced also gives valuable information. For example,
it can be used to calculate the respiratory exchange ratio (RER) which indicates whether fat or
carbohydrate is the source of the ATP and, as you will see in subsequent labs, it can tell you
whether lactate concentration is increasing in the muscle and blood.
Volume of CO2 produced = Volume of exhaled CO2 – Volume of inhaled CO2 (11)
Since the amount of CO2 in ambient air is taken to be zero (approximately true: even with
current climate crisis!):
VCO2 =VE x FECO2 (12)
(*remember to use the STPD value for the volume of expired gas)
To determine the absolute “rate of carbon dioxide output” (i.e., V ̇ CO2 in L/min), the “volume”
from equation 12 is divided by the total time during which the gas was collected (tcollection, in
minutes):
V ̇ CO2 = VCO2 / tcollection (13)
Respiratory Exchange Ratio (RER)
The respiratory exchange ratio is defined as the ratio of carbon dioxide production (V ̇ CO2) to
oxygen uptake (V ̇ O2).
RER = V ̇ CO2 ÷ V ̇ O2 (14)
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Under resting conditions, the RER value will approach 0.70 which will reflects a low production
of CO2 relative to O2 consumption. This low ratio indicates use of fat as the main fuel for
metabolism (e.g., for fat: C16H32O2 + 23O2 → 16CO2 + 16H2O = VCO2/VO2 = 16/23 = 0.70). By
contrast, a RER of near 1.0 indicates that carbohydrate is the fuel (e.g., for glucose C6H12O6 + 6O2
→ 6CO2 + 6H2O = VCO2/VO2 = 6/6 = 1.0). The relative contributions of fats and carbohydrates to
the fuel energy source can be interpolated for RER values between 0.71 and 1.00 (Lusk, 1924).
Table 3. Respiratory Exchange Ratio (RER) and Proportions of Energy Sources
RER Energy (kcal/L of O2) % Energy from carbohydrate %Energy from Fat
0.7070 4.686 0.0 100.0
0.710 4.690 1.1 98.9
0.720 4.702 4.8 95.2
0.730 4.714 8.4 91.6
0.740 4.727 12.0 88.0
0.750 4.739 15.6 84.4
0.760 4.751 19.2 80.8
0.770 4.764 22.8 77.2
0.780 4.778 26.3 73.7
0.790 4.788 39.9 70.1
0.800 4.801 33.4 66.6
0.810 4.813 36.9 63.1
0.820 4.825 40.3 59.7
0.830 4.838 43.8 56.2
0.840 4.850 47.2 52.8
0.850 4.862 50.7 49.3
0.860 4.875 54.1 45.9
0.870 4.887 57.5 42.5
0.880 4.899 60.8 39.2
0.890 4.911 64.2 35.8
0.900 4.924 67.5 32.5
0.910 4.936 70.8 29.2
0.920 4.948 74.1 25.9
0.930 4.961 77.4 22.6
0.940 4.973 80.7 19.3
0.950 4.985 84.0 16.0
0.960 4.998 87.2 12.8
0.970 5.010 90.4 9.6
0.980 5.022 93.6 6.4
0.990 5.035 96.8 3.2
1.000 5.047 100.0 0.0
Such information is helpful in determining exercise energy expenditure:
For example, consider someone who has exercised for 20 min. If their RER is 0.90, the energy
expended is 4.924 kcal/ L of O2 consumed. If their V ̇ O2 is 2.5 L/min (i.e., 2.5 L of O2 consumed
every minute), then we can calculate that they expended 246 kcal during that activity:
2.5 L of O2/min x 20 min x 4.924 kcal/L of O2 = 246 kcal
Note: there are 3500 kcal in one pound of fat, thus, this person would have to perform this 20 min
exercise bout ~14 more times to lose the energy equivalent of 1 pound of fat!
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Minute Ventilation (V ̇ E)
Pulmonary ventilation (V ̇ E) is another important variable in the study of physiological responses
to exercise. Its increase is necessary to match the rates of oxygen uptake or CO2 removal required
by exercise and it is also the body’s first line of defense to maintain acid-base balance at normal
levels.
When calculating expired V ̇ E, it is necessary to convert volume measured in ambient conditions
(cooler) to the conditions within the lungs (warmer). Expiratory volume is water vapor saturated
and almost at body temperature. The small temperature difference between the lung and the gas
within the bag must be corrected to BTPS or Body Temperature, Pressure, Water Vapor Saturated.
BTPS factor = [(PBaro – PH2O) / (PBaro - 47)] x [(273 + 37) / (273 + TRoom)] (15)
where TRoom is the temperature of the room in degrees Celsius, PBaro is the barometric pressure in
millimetres of Mercury (mmHg) and PH2O is the partial pressure of water vapour in the air which
varies with Tgas (760 is the average air pressure at sea level). The partial pressure of water (PH2O)
can be found in table 2.
Once the BTPS factor is known, V ̇ E may be calculated as follows
V ̇ E (L/min) = [VE (L) x BTPS factor] ÷ tcollection (16)