458 PRACTICAL PHYSIOLOGY 



Lactic acid is, therefore, present in relatively greater amount in the 

 blood than in the urine under ordinary conditions, being to a consider- 

 able extent kept back by the kidneys. 



When, however, lactic acid in the blood rises above a limiting value, 

 which in man is less than 70 mg. per 100 c.c , it is actively excreted by 

 the kidneys, so that the urine may often contain 500 mg. per 100 c.c. 

 even when fairly dilute. This result is in man easily brought about by 

 violent exercise such as running. 



EXPERIMENT I. Collect the urine of a man at rest, or engaged in 

 quiet work, during half an hour, and determine the lactic acid in the 

 urine by the distillation method. Then let him run for at least two 

 minutes at a pace sufficient to produce marked dyspnoea. Collect and 

 measure the urine after half an hour. Determine the lactic acid in 

 20 c.c. of this urine. A qualitative determination is sufficient, as the 

 reaction obtained with the urine passed at rest is very small. The 

 urine passed at rest will not give the thiophene test. That passed after 

 the exercise will give the test after the following preliminary treatment. 

 The urine is rendered alkaline with sodium carbonate, evaporated and 

 extracted with alcohol. The alcohol is evaporated and the residue dis- 

 solved in a little water, strongly acidified with phosphoric acid and 

 extracted with many times its volume of washed ether in a separating 

 funnel. The ether is separated and shaken up with dilute sodium car- 

 bonate solution, again separated and used for extracting the acid liquid 

 a second time. This process is repeated two or three times. The 

 alkaline liquid so obtained is boiled with animal charcoal till colourless, 

 filtered, and evaporated to dryness. The residue is then dissolved in 5 

 to 10 c.c. pure sulphuric acid and the thiophene test applied. In spite 

 of boiling with charcoal a reddish brown colour is usually obtained on 

 adding the sulphuric acid, which masks the reaction to a certain 

 extent. 



Violent exercise also increases the acidity of the urine (see p. 431) and 

 the proportion of ammonia nitrogen to the total nitrogen. The 

 amount of lactic acid in the blood is increased, and the alkalinity of 

 the blood reduced at the end of a short period of violent exercise, but 

 both have returned practically to normal at the end of half an hour. 



When more moderate exercise is taken, such as walking, there is no 

 increase in the lactic acid of the urine, even if the exercise be continued 

 for many hours, so that the production of lactic acid in exercise clearly 

 depends on the intensity of the muscular activity. There is no 

 evidence that the blood is less well supplied with oxygen than usual 

 during a short period of violent exercise, so that the muscles are 

 presumably not short of oxygen. The production of an increased 



