Supplement to '' Nature,'' July 14, 1923 



lungs by the movements of respiration, thence diffuses 

 through the lungs into the blood, which is pumped 

 round the body to the active limbs and muscles. The 

 amount of oxygen, however, which can be carried by 

 the blood is comparatively small, namely, only about 

 Jth of its total volume. 



The efficiency of the mechanism by which the oxygen 

 is carried round in the circulating blood depends very 

 largely on the efficiency and capacity of the heart. 

 For prolonged vigorous exercise a powerful and efficient 

 heart is essential. If, however, the lungs be too small 

 the oxygen pressure in them will fall too rapidly when 

 a given amount of oxygen is carried away by the blood, 

 and the smaller the lungs the shorter will be the time 

 (for a given blood-flow) during which each drop of blood 

 lingers in them in contact with the air. The smaller 

 the lungs, therefore, the less opportunity will the blood 

 have of collecting its required oxygen : the smaller the 

 lungs and the less efficient their ventilation, the lower 

 will be the pressure of oxygen in the arterial blood. 



Now the heart is an extremely vigorous and hard- 

 working organ, and it has the first call upon the oxygen 

 which is carried by the blood. The coronary artery 

 takes blood directly from the aorta, and carries it 

 round the heart muscle itself. If the lungs be small, or 

 their ventilation inadequate, or their walls too im- 

 permeable, the pressure of oxygen in the arterial blood 

 will begin to fall ; consequently the heart itself will 

 get a lower pressure of oxygen — it will slow .up or give 

 a less effective beat, the blood-flow will be slowed, and 

 the oxygen pressure in the blood will rise again to 

 another higher value. Thus a balance will be reached 

 in which each unit in the double mechanism is working 

 at its limiting capacity, and one will find in athletes, 

 who are capable of long-continued effort, that there is 

 a combination of (a) a vigorous and efficient heart, 

 and {b) capacious lungs capable of rapid and extensive 

 ventilation. 



A vigorous output of blood by the heart requires a 

 vigorous return of blood to the heart. On the venous 

 side of the small capillaries which feed the muscles 

 with oxygen, there is little pressure left to drive the 

 blood along to the heart. In the veins, therefore, the 

 flow of blood is largely determined by the activity 

 and movements of the body. The veins are provided 

 with valves, and the alternating movements of the 

 limbs and muscles help to pump the blood along the 

 veins. If the body be rigid the arteries and capillaries 

 are constrained and the blood-flow is hindered, while the 

 veins get none of the rhythmic changes of pressure 

 which tend to pump the blood along them, and so they 

 fail to supply the heart with blood. Such exercise as 

 holding oneself up with arms bent, in a gymnasium, on 

 a pair of rings, is not in itself violent, and would not. 



if it could be continued, require an amount of oxygen 

 comparable with running, even at so slow a pace as 

 eight miles an hour. In such exercise, however, an 

 extremely violent contraction in the very muscle thai 

 requires the energy almost entirely prevents the supply 

 of blood to it, no oxygen is received, lactic acid rapidly 

 accumulates, and exhaustion sets in. 



Similarly, in such types as rowing, in which part ' 

 the body is in a state of stress during a large part of ll, 

 time and the rhythmic movements are relatively slow, 

 the supply of oxygen is more difficult. Consequent 1- 

 rowing appears to strain the heart more often tha 

 other kinds of athletic effort. For an easy and vigoroi: 

 circulation no exercise seems to compare with running 

 on the flat ; here the movements are very rapid and 

 the muscles are rigid during only a fraction of each 

 cycle ; consequently the blood can run through very 

 easily, and it gets helped along in the veins by the 

 jolts and jerks and shakes which the body receives, 

 and by the rapid rhythmic pressures which are applied 

 to the veins by the movements of the limbs. Thus 

 from the point of view of taking as much exercise in 

 a given time, with as little strain on the heart as 

 ■ possible, runmng is probably superior in type to any 

 kind of exercise. 



The function of the heart in exercise is so important 

 that a vivid appreciation is desirable of the extra- 

 ordinary tasks it sometimes undertakes. A subject of 

 II J stone weight succeeded in taking in about 4*2 

 litres of oxygen in a minute, while running round a 

 track at about 9 miles per hour carrying a bag and 

 breathing through valves and mouth-piece. Now the 

 amount of oxygen which the blood can take in and give 

 out, as it circulates once through the body, is certainly 

 not more than about ith of its own volume. Hence at 

 least 7 times 4*2 litres of blood per minute, i.e. about 

 30 litres, were circulating round his body during this 

 experiment. The largest water-tap in an ordinary 

 house has an output which is poor when compared 

 with that of a human heart. It is little wonder that 

 the heart goes wrong sometimes : the wonder is that this 

 happens relatively so seldom. 



An Example from Athletics. — The way in which the 

 capacity of the body for exercise depends upon the 

 supply of oxygen, actual or potential, can be illustrated 

 by an example from athletics. A certain subject is 

 capable of taking in about 4*2 litres of oxygen per 

 minute ; let us assume that his maximum oxygen 

 credit is 13*2 litres, as found by Lupton in another 

 subject. Suppose that at the end of a race his oxygen 

 supply, actual or potential, is completely exhausted. 

 Then clearly if he runs for a minute he has (4'2 -1- 13'2) = 

 17*4 litres to spend altogether : if he runs for two 

 minutes (2 x 4*2 -1- i3'2) = 2i*6 litres altogether, or io"8 



