Lecture Fourteen- Wednesday, February 3 , 2010 rd
VIII VENTILATION DURING EXERCISE
A. Tidal Volume and Breathing Frequency during Exercise
Comparisons of the components determining pulmonary minute ventilation (VE)
during progressive exercise up to maximum:
When exercise starts, both tidal volume (VT) and breathing frequency (f) increase
proportionally (Figure 46 a & b). However, VT plateaus; therefore, high ventilatory
rates during hard exercise are due to incremental increases in f. Because of the
increased breathing frequency, inspiratory and expiratory times decrease during
progressive exercise but expiratory times fall relatively more than inspiratory time
(Figure 46 c & d). Consequently, peak expiratory flow rate increases more than peak
inspiratory flow rate.
see pictures in slides
B. Minute ventilation and metabolic rate during exercise
In both untrained and trained subjects, minute ventilation (VE)(dot) increases
linearly with metabolic rate (VO2)(dot) up to about 50% to 65% of VO2(dot) max
(Figure 47). Thereafter, VE(dot) increases at a rate disproportionately greater than
the change in VO2(dot). Note that an effect of endurance training is to delay the
ventilatory inflection point (Tvent).
see picture in slides
Is Ventilation a limiting factor in aerobic performance at sea level? (VE/Q)
-Resting values of VE can increase 35 folds during exercise (from 5L/min to 190
L/min, in a fit individual).
-Resting values of cardiac output (CO) can increase 5-6 folds during exercise (from
5L/min to 25-30 L/min, in a fit individual).
-The VE/Q 1 at rest. Because VE can increase more than Q during exercise, there is
an increase in VE/Q. The increase in this ratio is one reason why ventilation is not
believed to limit aerobic performance.
-In a less fit individual, the absolute values of VE and Q will be less but the ratio will
increase to a similar extent.
Is Ventilation a limiting factor in aerobic performance at sea level?
(alveolar surface area)
-The alveolar surface area is 50m (1/2 of a single tennis court).
-The average blood volume is 5L.
-4% of this 5L is in the pulmonary system at any one time during maximal exercise.
-Therefore, there is a large capacity for gas exchange.
C. Control of Ventilation during Exercise
The Central Chemoreceptors During Exercise:
During exercise, there is an alkalotic (pH) response in the medullary ECF. This
decreases the ventilatory response. Therefore, the role of the central
chemoreceptors is important at rest but not so much during exercise.
Peripheral chemoreceptors are mainly sensitive to changes i