570 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



oxygen at the capillary wall in quantities which are 

 not reflected by the uptake at the mouth. 



It is a matter of intuitive judgment which can be 

 proven mathematically (40, 131) that the more 

 nearly constant the general physiological condition 

 of the subject during the time of sampling, the more 

 adequate is the calculation of blood flow. The 

 constancy should apply to alveolar and blood gas 

 tensions, oxygen uptake, and respiratory quotient, 

 and is likely to be violated when the subject exercises 

 or changes the inspired gas mixture (anoxia) (33). 

 In order, therefore, to insure that the oxygen uptake 

 at the mouth is as nearly as possible like that which 

 occurs at the tissue cells, it is necessary to hold the 

 subject in a steady state for 10 to 15 min before the 

 samples are taken as well as during this time. 



The necessity for the steady state is emphasized 

 when the results of calculating the blood flow with 

 CO2 as a reference substance are compared with 

 those using Oo. The CO2 results are very much more 

 variable because slight changes in ventilation will 

 work an easier transfer of CO2 from lung blood to 

 lung air than is the case with oxygen. There would 

 thus be changes in the CO2 content of pulmonary 

 air that would not show at once in the expired air 

 or in the A-V difference. Moreover, important 

 quantities of CO2 are combined with the tissue fluids 

 and with sequestered plasma in the alveolar capillaries 

 which are also released or stored with changed 

 ventilation (12, 61). 



The existence of a steady state does not rule out the 

 general idea that oxidations may occur in the lungs 

 or in the blood passing through the lungs. This idea 

 was disposed of in 191 3-1 5 when Henriques showed 

 that the injection method (see below) measured a 

 large enough blood flow to account in terms of the 

 A-V difference for the gas exchange (75, 76). The 

 question arose again in 1920 when it was suspected 

 that lactic acid disappeared oxidatively on passage of 

 blood through the lungs. The question received con- 

 tradictory answers until the possibility of pulmonary 

 oxidations was again laid to rest by Mitchell & 

 Cournand (100). 



There are cyclic changes in the circulation rate 

 which depend upon respiration and the cardiac cycle 

 (in the presence of shunts) as well as upon spontaneous 

 vasomotor activity which are refractory to maneuvers 

 aimed to bring about a steady state (131, 135). If the 

 samples are taken at a constant rate, there will be no 

 means of weighting the samples taken when the flow- 

 is fast as compared with those taken when the flow is 

 slow. The A-V difference and oxygen uptake should 



be instantaneously integrated, but such a calculation 

 can only be set down symbolically on paper. Practical 

 tests (123) and a priori considerations (58) indicate 

 that this concept is not likely to introduce a serious 

 error. 



In certain pulmonary diseases, the natural bron- 

 chial blood flow of 10 to 12 ml per min be- 

 comes greatly augmented. These conditions include 

 congenital pulmonary stenosis, bronchiectasis, and 

 situations in which a large pulmonary artery is oc- 

 cluded. Under such conditions numerous small 

 branches of the bronchial artery enter the alveolar 

 circulation and drain from the lung by the pulmonary 

 vein. The amount of such bronchial flow may reach 

 20 per cent of the total flow in bronchiectasis and is 

 said to be able to sustain life in cases of pulmonary 

 atresia with intracardiac shunting. (Compare Chapter 

 14). In the absence of intracardiac shunting such 

 blood flow serves no useful purpose and cannot be 

 measured by the Fick procedure. It can be measured, 

 however, as the difference between dye injection 

 calculations made from curves taken from the pulmo- 

 nary artery (excluding the bronchial circulation) and 

 the brachial artery (including it). The dye is injected 

 into the right atrium (40). 



The most important diagnostic use of the Fick pro- 

 cedure is in the evaluation of intracardiac shunts in 

 congenital heart disease. These uses will be taken up 

 in Chapter 14. 



RESPIR.-KTORV METHODS 



In the nineteenth century and the first quarter of 

 the twentieth century it was, as mentioned above, not 

 considered safe to enter the human heart to obtain 

 mi.xed venous blood samples. At this time also, the 

 human artery was rarely punctured. Credit is due to 

 Loewi & von Schrotter (90) for thinking in 1903 of 

 using the lungs as an aerotonometer to measure the 

 tension of the mixed v-enous blood and hence of its gas 

 content. Not only were these authors the first to use 

 the lungs in this manner, but also they were the only 

 ones to apply the principle quite impeccably. Using a 

 balloon cannula, they blocked off" a part of one lung 

 and allowed time for complete equilibrium of the air 

 in that part of the lung with the mixed venous blood 

 perfusing the alveoli. The C.O2 or oxygen of this air 

 was analyzed and its tension measured. This was ap- 

 plied to the relevant dissociation curve and the gas 

 content of the mixed venous blood determined. A 

 similar procedure using alveolar air was used to 



