Page 87 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Physical Work The smaller the muscle mass involved in the work,
the higher the increase in blood pressure. Hence, the
There are three types of muscle work: blood pressure increase in arm activity (cutting
! Positive dynamic work, which requires to hedges) is higher than that in leg activity (cycling). In
muscles involved to alternately contract and patients with coronary artery disease or cere-
brovascular sclerosis, arm activity is therefore more
relax (e.g., going uphill). dangerous than leg activity due to the risk of myo-
! Negative dynamic work, which requires the cardial infarction or brain hemorrhage.
muscles involved to alternately extend while
braking (braking work) and contract without a Muscular blood flow. At the maximum work
Nerve and Muscle, Physical Work Many activities involve a combination of two put. Hence, no more than 10 kg of muscle
level, the blood flow in 1 kg of active muscle
load (e.g., going downhill).
rises to as much as 2.5 L/min (! p. 213 A),
! Static postural work, which requires con-
equivalent to 10% of the maximum cardiac out-
tinuous contraction (e.g., standing upright).
(" 1/3 the total muscle mass) can be fully ac-
or three types of muscle work. Outwardly
tive at any one time. Vasodilatation, which is
directed mechanical work is produced in dy-
required for the higher blood flow, is mainly
namic muscle activity, but not in purely pos-
achieved through local chemical influences
tural work. In the latter case, force!distance =
(! p. 212). In purely postural work, the in-
0. However, chemical energy is still consumed
crease in blood flow is prevented in part by the
and completely transformed into a form of
squeezes its own vessels. The muscle then
times the duration of postural work).
fatigues faster than in rhythmic dynamic work.
In strenuous exercise, the muscles require up
During physical exercise (! C1), the ventila-
2 heat called maintenance heat (= muscle force fact that the continuously contracted muscle
to 500 times more O 2 than when at rest. At the
.
same time, the muscle must rid itself of meta- tion (V E) increases from a resting value of ca.
bolic products such as H , CO 2, and lactate 7.5 L/min to a maximum of 90 to 120 L/min
+
– 1
(! p. 72). Muscle work therefore requires (! C3). Both the respiratory rate (40–60 min
drastic cardiovascular and respiratory max; ! C2) and the tidal volume (ca. 2 L max.)
changes. contribute to this increase. Because of the high
.
In untrained subjects (UT), the cardiac out- V E and increased CO, oxygen consumption
.
put (CO; ! p. 186) rises from 5–6 L/min at rest (V O 2 ) can increase from ca. 0.3 L/min at rest to a
.
to a maximum of 15–20 L/min during exercise maximum (V o 2 max) of ca. 3 L/min in UT (! C4
(! p. 77 C). Work-related activation of the and p. 76). Around 25 L of air has to be venti-
.
.
sympathetic nervous system increases the lated to take up 1 L of O 2 at rest, corresponding
heart rate up to ca. 2.5 fold and the stroke to a respiratory equivalent (V E/V O 2 ) of 25.
.
.
volume up to ca. 1.2 fold (UT). In light to mod- During physical exercise, V E/V O 2 rises beyond
erate exercise, the heart rate soon levels out at a the endurance limit to a value of 40–50.
.
new constant level, and no fatigue occurs. Very Increased O 2 extraction in the tissues also
strenuous exercise, on the other hand, must contributes to the large increase in V O 2 during
soon be interrupted because the heart cannot exercise. The decreasing pH and increasing
achieve the required long-term performance temperature shift the O 2 binding curve
(! B). The increased CO provides more blood towards the right (! p. 129 B). O 2 extraction is
for the muscles (! A) and the skin (heat loss; calculated as the arteriovenous difference in
! p. 222.). The blood flow in the kidney and in- O 2 concentration (avDo 2 in L/L blood) times the
testine, on the other hand, is reduced by the blood flow in L/min. The maximum O 2 con-
.
sympathetic tone below the resting value sumption (V O 2 max) is therefore defined as:
.
(! A). The systolic blood pressure (! p. 206) V O 2 max = HRmax · SVmax · avD O 2 max
rises while the diastolic pressure remains con-
stant, yielding only a moderate increase in the where HR is the heart rate and SV is the stroke
.
mean pressure. volume. V O 2 max per body weight is an ideal
measure of physical exercise capacity (! p. 76).
74
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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