lab 4

Question 1

introduction

Answer 1

As exercise intensity increases, so to does the demand to resynthesize adenosine triphosphate
(ATP). Oxidative phosphorylation is the preferred metabolic pathway by which ATP resynthesis
requirements are met. As a result, with increasing exercise intensity the rate at which our bodies
utilize oxygen (i.e., V ̇ O2) rises – most of which is attributed to V ̇ O2 from active muscle.
The rate at which muscle can utilize oxygen (O2) is dependent on two factors: 1) the rate of O2
delivery to the muscle tissue in arterial blood; and 2) the quantity of O2 extracted from the O2-rich
arterial blood. At the whole-body level, this relationship is described by the Fick equation:
V ̇ O2 = Q ̇ ∙ a-vO2diff
Where V ̇ O2 is the rate of O2 utilization (mL·min-1), Q ̇ is cardiac output (mL·min-1) or the rate of
O2 delivery, and a-vO2diff is the amount of O2 extracted from the blood (i.e., the difference in
arterial (CaO2) and mixed venous muscle O2 content (CvO2) in mL per 100 mL of blood).
At rest, a normal Q ̇ is between 4000-6000 mL·min-1 and a normal a-vO2diff is 4-5 mL per 100 mL
of blood (0.04-0.05 mL per mL of blood). This gives a mean value of resting V ̇ O2 of 225 mL·min-
1 (5000 mL blood per minute x 4.5 mL O2 ÷ 100 mL of blood). During exercise, V ̇ O2 increases
with exercise intensity. This is accomplished by raising cardiac output and a widening the a-vO2diff.
Figure 1 depicts how these variables change in relation to increasing power output.
Figure 1. Schematic representation of changes in V ̇ O2, Q ̇ , and a-vO2diff in relation to increasing power output (W).
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KIN 2230 Lab Protocol #4
Version: MAR-11-24 - 2
The Fick equation can also be rearranged to quantify blood flow (Q ̇ = V ̇ O2 ÷ a-vO2diff) or to
determine O2 extraction (a-vO2diff = V ̇ O2 ÷ Q ̇ ). These manipulations are helpful to understand the
relationship between O2 utilization, delivery, and extraction.
In addition to “whole-body”-type exercise the Fick equation can be applied at the muscle level
(e.g., knee extensors, wrist flexors) where the rate of muscle O2 utilization (V ̇ O2m) is equal to the
product of muscle blood flow (Q ̇ m) and the amount of O2 extracted from the blood as it transits the
muscle vasculature (i.e., the difference in CaO2 and CvO2; V ̇ O2m = Q ̇ m ∙ [CaO2 – CvO2]). This
equation shows that if Q ̇ m falls, a-vO2diff would need to rise to maintain the same V ̇ O2m. In contrast,
if a-vO2diff is reduced, a rise in Q ̇ m would be required to compensate. Thus, oxygen delivery and
extraction need to change dynamically to ensure that the muscle has adequate O2 to satisfy the
utilization demands of physical activity at all times.
Local O2 extraction (i.e., a-vO2diff) represents the balance between O2 utilization and delivery. If
V ̇ O2m goes up and Q ̇ m is slow to respond, a-vO2diff will need to increase faster and to a greater
extent to meet the greater rate of O2 utilization (see Figure 2 at exercise onset). During steady-
state exercise if Q ̇ m is suddenly reduced, a-vO2diff will need to rise (see Figure 2 – blue arrows). In
contrast, if Q ̇ m is suddenly raised, a-vO2diff will fall (see Figure 2 – blue arrows) to maintain the
same V ̇ O2m.
Figure 2. Representation of quadriceps muscle oxygen utilization (V ̇ O2m), quadriceps blood flow (Q ̇ m) and femoral
artery-to-femoral venous O2 difference (a-vO2diff) in response to a step change in dynamic kicking power. The
instantaneous change in kicking power is denoted by the dashed vertical line. Not how at exercise onset, the rate of
increase in V ̇ O2m is much faster than Q ̇ m. As a result, a-vO2diff increases rapidly and overshoots its eventual steady-
state until Q ̇ m meets the new metabolic demand. The blue and yellow arrows indicate an instantaneous reduction and
increase in Q ̇ m, respectively. Note the magnitude and direction by which a-vO2diff changes to ensure V ̇ O2m is
unaffected.
Unfortunately, measurement of a-vO2diff during exercise in humans is difficult as it necessitates
the use of invasive blood sampling techniques from the artery feeding a muscle (to establish CaO2)
and the vein draining that muscle (to establish CvO2). However, there are non-invasive tools that
allow us to gain insight into the dynamic balance between O2 extraction, delivery, and utilization.
Near-infrared spectroscopy (NIRS)
Near infrared spectroscopy allows for the measurement of tissue oxygen saturation of hemoglobin
(Hb) and myoglobin (Mb) in muscle. This is referred to as “SmO2”. As discussed below, SmO2 is
calculated using both oxygenated and de-oxygenated Hb and Mb, as well as the total amount of
hemoglobin (tHb) present within a region of muscle using near infrared light.
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KIN 2230 Lab Protocol #4
Version: MAR-11-24 - 3
Oxy-Hb+Mb Amount of oxygenated hemoglobin and
myoglobin in the tissue.
• Represents the form of Hb and Mb that
is oxygen-loaded
Deoxy-Hb+Mb Amount of de-oxygenated hemoglobin and
myoglobin in the tissue.
• Represents the form of Hb and Mb that
has released its bound oxygen (i.e.,
oxygen-unloaded)
tHb = Oxy-Hb+Mb + Deoxy-Hb+Mb The total amount of hemoglobin and
myoglobin (tHb) in the tissue.
SmO2 = Oxy-Hb+Mb / tHb Tissue oxygen saturation of hemoglobin (Hb)
and myoglobin (Mb) in muscle.
• Represents the percentage (%) of total
Hb and Mb that has bound oxygen (i.e.,
oxygen-loaded).
• The inverse of SmO2 represents the %
of total Hb and Mb that is oxygen-
unloaded.
Table 1. Key terms and equations for understanding NIRS
Therefore, the SmO2 measurement can be used as an indirect proxy for changes in a-vO2diff (i.e.,
O2 extraction) to understand the relationship between local muscle O2 delivery and utilization and
tHb can be used reflect changes in capillary hematocrit. Typically, these variables are acquired
using a small probe placed over the skin of a select muscle that emits and detects near-infrared
light (see Figure 3). The probe is secured to the skin with adhesive tape such that the device can
be worn during dynamic exercise.
Near-infrared spectroscopy relies on the fact that molecules (called ‘chromophores’) absorb light.
Moreover, each chromophore will absorb light at different wavelengths. By using a NIRS probe
that emits 4 wavelengths of near infrared light (between 650 to 850 nM), researchers can
differentiate chromophores and tissue type. For example, oxygenated hemoglobin and myoglobin
absorb light at wavelengths more than 790 nm, but de-oxygenated hemoglobin absorbs light at
wavelengths less than 790 nm. By using a variety of wavelengths, the NIRS device detects the
amount of light that was absorbed by each tissue, which permits the calculation of the relative
quantity of oxygen present in the underlying tissue (i.e., the muscle and microvasculature within
the region of interrogation by the probe). Importantly, this light can penetrate tissue (e.g., skin,
fat) into the underlying blood vessels and muscle (Figure 3).
KIN 2230 Lab Protocol #4
Version: MAR-11-24 - 4
Figure 3. A typical NIRS set-up. See text for details.
To review, the absorbative properties of Hb and Mb are different depending on whether they are
bound or not bound to O2. The difference in emitted versus detected light at different wavelengths
gives the relative proportions of oxygenated Hb and Mb (oxy-Hb+Mb) and deoxygenated Hb and
Mb (deoxy-Hb+Mb) from which tHb (oxy-Hb+Mb + deoxy-Hb+Mb) and SmO2 (oxy-Hb+Mb ÷
tHb) can be calculated (Table 1). The SmO2 value reflects, in real time, the balance between O2
delivery (from hemoglobin) and oxygen demand (from the oxidative system in the muscle).
Therefore, when O2 delivery and utilization change (e.g., during exercise) the NIRS measurements
should reflect these changes.