ance in the blood of lactic acid, as related to the
anaerobic metabolism.
For the athlete performing aerobic exercise
under most conditions, it is considered that the
individual’s capacity for ventilation is adequate
to provide O 2 from the atmosphere to the alveoli
and to carry CO 2 from the alveoli to the atmos-
phere. In elite endurance athletes who are highly
conditioned for aerobic metabolism and are per-
forming near their capacities for aerobic power
production, it can be frequently observed that
blood leaving the lungs via the pulmonary veins
is not as saturated with oxygen as it is under the
conditions of rest and submaximal aerobic exer-
cise. It can thus be concluded that, under the very
special conditions where a very highly condi-
tioned athlete is performing high-intensity
aerobic exercise, pulmonary ventilation serves as
a limiting factor for external respiration and
therefore oxygen uptake.
Circulation
For the delivery of oxygen, the removal of carbon
dioxide and the transport of anabolites and
catabolites to and from the body cells, the organ-
ism is dependent upon the circulation of the
blood. With regard to the aerobic metabolism
related to exercise and recovery, the most impor-
tant factors are: the oxygen-carrying capacity of
the blood, the blood volume available, the ability
of the heart to pump blood (cardiac output) and
the capillarization of the skeletal muscles.
The term cardiac outputcan actually refer to the
amount of blood ejected through the aorta or the
pulmonary arteries per minute (‘minute volume’
orQ.
) or the amount of blood ejected per sys-
tole (‘stroke volume’ or SV). The relationship
between minute volume and stroke volume
includes the contraction frequency of the heart
(fH) as follows: Q.
=fH·SV.
The relationship of these variables with
oxygen uptake includes the unloading factor of
oxygen in the tissues as determined from the
content of oxygen in systemic arterial blood
(Cao 2 ) and the content in systemic mixed venous
blood (Cvo 2 ). It is:
V.
o 2 =fH·SV·a-vo 2 diff.
Representative values for an 80-kg athlete are
presented in Table 1.3. It can be observed that the
relationship between aerobic power (oxygen
uptake) and heart rate is essentially rectilinear.
Stroke volume increases from a resting value of
104 ml to near maximum values even during
low-intensity aerobic exercise. The increase in
cardiac minute volume as higher levels of
oxygen uptake are attained is accounted for by
the increase in heart rate.
Meanwhile, the arteriovenous oxygen differ-
ence continues to increase due solely to the
lowered concentration of oxygen in systemic
mixed venous blood leaving the active tissues.
The arterial concentration remains constant at a
value of approximately 20 ml · l–1blood, indicat-
ing that pulmonary capillary blood becomesbasic exercise physiology 11
Table 1.3Representative data for steady-state oxygen uptake and circulatory variables at rest and during various
intensities of constant-intensity exercise (for an 80-kg athlete). The maximum aerobic power for the athlete is
4.5 l · min-^1 and maximum fHis 195.
V.
o 2 Q.
fH a-vo 2 diff.
(l · min-^1 ) (l · min-^1 ) (beats · min-^1 ) SV (ml) (ml O 2 ·l-^1 )Rest 0.25 6.4 60 104 40
- 1.00 12.3 100 123 81
- 2.00 14.8 120 123 136
- 3.00 17.2 140 123 174
Intense aerobic exercise 3.50 19.7 160 123 178
Intense aerobic exercise with 4.00 22.1 180 123 180
anaerobic contribution
- 3.00 17.2 140 123 174