134 



VERTEBRATE RESPIRATION 



SO that steady running is not necessarily the best over these 

 distances. Most of the better times over 440 yards have been 

 done by running the first part much faster than the second. Pay- 

 ment of the 'oxygen debt' may not be wholly concerned with 

 oxidising lactic acid, for this decreases slowly relative to the fall 

 in the extra O2 consumption. The extra O2 may be used to re- 

 synthesise energy-rich phosphate bonds from sources other than 

 the oxidation of lactic acid. 



100 200 AOO 800 1500 3,000 5,000 10,000 2Q000 



Abscissae; distance (m ) (logarithmic scale) 

 Ordinates: average speed (m./sec) 



Fig. 39. 



Decline in speed with increase in distance run, plotted on the basis 

 of world records in 1955 (after Bannister, 1960). 



As we have indicated, long-distance running reaches a 'steady- 

 state' with respect to aerobic respiration because the anaerobic 

 processes contribute less and less to the total energy required as 

 the distance increases. Anaerobic mechanisms probably con- 

 tribute nearly a half of the total energy during a mile race, but 

 less than a quarter for 3 miles. During such races the muscles 

 may increase their O2 consumption 50 times and this entails 

 important modifications of the respiratory and circulatory 

 systems. These result in increases of O2 intake at the lungs, 

 cardiac output (from 4-5 to 30-40 Htres/min), and muscle blood 

 flow. 



