1112 PHYSIOLOGY 



But in ordinary quiet respiration many parts of the lungs remain practically motion- 

 less, so that in some of the alveoli there is little or no renewal of the air. When 

 alveoli are cut out from the respiratory movements, there appears to be a corresponding 

 diminution in the blood flow through the capillaries surrounding them. A certain amount 

 of blood will therefore ' escape oxidation in traversing the lungs, and this will be 

 mingled with the large mass of blood which has undergone complete oxidation in passing 

 round the ventilated alveoli. Hence in the arterial blood of an animal at rest, haemo- 

 globin is only about 90 per cent, saturated, corresponding to a tension of about 70 

 mm. Hg., which may be looked upon as the normal tension of oxygen in arterial blood. 

 During muscular exercise more and more of the alveoli are involved in the increased 

 respiratory movements, so that the percentage saturation of the haemoglobin and the 

 tension of oxygen in the blood may be actually increased during the hyperpncea which 

 accompanies this condition. 



The blood, thus laden with oxygen, travels to the left side of the heart, 

 and from there is sent through the arteries to all parts of the body. It 

 must be remembered that neither in the lungs nor in the tissues does the 

 haemoglobin come in actual contact with the source of the oxygen, nor 

 with the cells which it is to supply. In both cases the interchange is effected 

 through the intermediation of the plasma and, in the tissues, of the lymph 

 as well. Since the tissue elements are constantly using up oxygen, they 

 absorb any oxygen that is present in the surrounding lymph. There is in 

 consequence a descending scale of oxygen tensions from red blood corpuscle 

 through plasma, vessel wall, lymph, and tissue element. The cell draws 

 from the lymph, ajid the lymph from the plasma, so that the oxygen tension 

 in the plasma sinks. This has the same effect as if we put the red corpuscles 

 in a mercurial pump and lowered the pressure of gas. The immediate result 

 is an evolution of oxygen, which is taken up by the plasma, to be in turn 

 passed on to the lymph and the tissue cell. 



The passage of oxygen out of the capillaries into the tissue cells must 

 thus be proportional to the difference of tension between the oxygen in the 

 capillaries and that in the cells. If Pb is the oxygen pressure in the capillary 

 blood and Pt that in the tissues, the rate of flow of oxygen from capillaries 

 to tissues must be proportional to Pb Pt. If the blood is to lose oxygen 

 during the whole of this passage through the tissues, we may take for Pb 

 the tension of the oxygen in the venous blood as it leaves the tissues. This 

 is generally about 30 mm. Hg. Pt, the tension of oxygen in the tissues, 

 may be determined in various ways. Verzar's method is based on the 

 following argument. If the oxygen pressure in the tissues is nothing, Pb Pt 

 will equal 30 = 30. In this case any diminution in the tension of the 

 oxygen in the blood will diminish the steepness of fall of pressure between 

 capillaries and tissues, and there must be a corresponding diminution in the 

 consumption of oxygen by the tissues, as determined by a comparison of 

 the oxygen contents in the blood flowing to and away from the tissue 

 respectively. If however Pt is positive, e. g. 20 mm. Hg., Pb Pt equals 

 30 20 = 10. Here a drop in the oxygen tension of the blood will give 

 rise to a corresponding drop in the tension within the tissues : for instance, 

 if the tension in the blood is diminished to 20 mm. Hg., that in the tissues 

 will drop to 10 mm. Hg., and tho difference on the two sides of the capillary 



