(This page is pertinent to both lecture and lab.)
Our experiment in lab followed both blood lactate levels and the volume of gas exhaled (Ve) as our subjects expended progressively more energy on a bicycle ergometer. But before looking at the results, let's review the sensors that influence ventilation.
Question: What is the name of the most important sensor controlling ventilation, where is it found, and what is the factor in the blood that it is monitoring?
Question: The second sensor is the peripheral chemoreceptor. Where is it found? Also, what are two factors in the blood that it monitors?
Now let's look at the correlation between lactate in the blood and ventilation. The results from one of our subjects (Tuesday 12:30) are shown below.
First, observe that there is lactate in the blood normally, even when a person is resting. The lactate in the blood is not a problem if the acid-base situation is adjusted by the kidneys. Indeed, lactate is an excellent fuel for exercising muscles. As glucose is degraded to lactic acid, only a small amount of the energy is converted to ATP. Most ATP forms in the mitochondria.
Also, observe the ventilation in the first two minutes, in which it is a little above what one might. This is not surprising. It represents the anticipation that almost always occurs near the onset of exercise. Feedforward effects on the respiratory and cardiovascular system are usually very noticeable. In fact, the effect is unusually small in this subject. The anticipation ensures that blood gases and the arterial pressure do not change with the onset of exercise in a healthy person.
Next, ventilation increases linearily, but rather slowly, as the level of work increases. At sea level there is abundant oxygen in the air, and not much effort is required to keep the breathing at a level which will supply this oxygen.
But then, above about 11 minutes, the breathing begins to rapidly increase as the work levels increase. Observe that the lactate level also increases at about this time.
Finally, observe the last lactate measurement, which was taken about two minutes after the subject had to stop. At the very peak of exercise, lactate was being formed rapidly in the muscle fibers. Then in the ensuing minutes this was dumped into the blood, raising the lactate levels to their highest level a couple of minutes after the subject had to stop.
Now let's consider the effect of the observed changes in ventilation on the blood gases. The figure to the right shows the expected changes in PaO2 and PaCO2 as the level of exercise increases.
Observe that the PaO2 actually increases somewhat past the lactate threshold (vertical dotted line), while the PaCO2 decreases. This, of course, is not what one expects intuitively. At the extremes of exercise, it feels as if one is struggling to get enough oxygen. In reality, the hemoglobin in blood leaving the lungs is essentially saturated with oxygen at all levels of exercise.
These changes in the blood gases are the result of the great increase in ventilation that results from the addition of lactic acid to the blood.