. The comparative physiology of respiratory mechanisms. Respiration. RESPIRATION IN AIR 69 C02 of the respiratory center which can easily be demon- strated by having a subject breathe, say, 3% C02 during rest and during work respectively (M. Nielsen, 1936). Experiments on a few subjects (Krogh and Lindhard, 1913) showed that in these the very beginning of muscular work brought about a sudden increase in ventilation, but this is not universal. During the first minute or two of constant work there is some fluctuation in ventilation, but a steady state is reached, usually after 3-5 minutes, in wh
. The comparative physiology of respiratory mechanisms. Respiration. RESPIRATION IN AIR 69 C02 of the respiratory center which can easily be demon- strated by having a subject breathe, say, 3% C02 during rest and during work respectively (M. Nielsen, 1936). Experiments on a few subjects (Krogh and Lindhard, 1913) showed that in these the very beginning of muscular work brought about a sudden increase in ventilation, but this is not universal. During the first minute or two of constant work there is some fluctuation in ventilation, but a steady state is reached, usually after 3-5 minutes, in which the ventilation remains remarkably constant corresponding to the rate of oxygen uptake. 2000 1500 500. 3 minutes Fig. 41. Curve showing oxygen absorption in human subject during initial stages and just after work. (Krogh and Lindhard.) During the initial stages of muscular work the uptake of 02 is always lower than in the steady state and it is a character- istic fact (Krogh and Lindhard, 1920) that the "oxygen-debt" (A. V. Hill, 1924) thus incurred cannot be paid off while the work goes on at the same or at an increasing rate, but partly when the work is reduced (E. Hansen, 1934) and completely when it stops (Fig. 41). This involves a gradual return of respiration and circulation to the resting values over periods from several minutes to about one hour, depending on the severity and duration of the work. With bursts of maximal work over 1/2 minute or less the maximum 02 uptake may occur after the work, and athletes can reach very high figures for the oxygen debt. Quite reliable figures of 9-11 liters O2 are on record (E. Hansen, 1934), corresponding to the resting. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Krogh, August, 1874-1949. Philadelphia, University of Pennsylvania press
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