1672 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



BLOOD FLOW X HbOg CAPACITY 

 DIFFUSING CAPACITY 



fig. 5. Nomogram illustrating the respiratory and circulatory interplay involved in delivering 

 an adequate supply of oxygen to the arterial blood. The dashed-line rectangle represents the situa- 

 tion at rest; the solid-line rectangle represents the situation during exercise. At rest, this subject has 

 an oxygen requirement (VuJ of 250 nil/min. Starting in the left upper quadrant, and moving from 

 quadrant to quadrant in a counterclockwise direction, Vo, is shown to be met by a cardiac output of 

 5 liters/min, an oxygen capacity of 20 ml/ 100 ml, a Hb flow/capillary area of 5 and a diffusing 

 capacity of 20 ml/min/mm Hg. Also pictured are the corresponding arteriovenous differences in 

 oxygen content and saturation of blood (A-V AO?), as well as the mean alveolar-capillary diffusing 

 gradient for oxygen (mean A alveolar-capillary Po 2 ). During exercise, as the oxygen requirement 

 increases (Vo« = 1250 ml/min), these variables undergo appropriate change. A similar nomogram 

 could be constructed for oxygen uptake in the tissues. [Based on Barcroft (9) and Lilienthal el al. 

 (266).] 



■- NORMAL 

 — EXERCISE 



pressure are qualitatively similar: the systolic pres- 

 sures in the pulmonary and systemic circulations are 

 identical; the pulmonary arterial diastolic pressure is 

 lower than the systemic arterial diastolic pressure, due 

 to the presence of a spiral valve between the systemic 

 artery and the ventricle (352). In the turtle, the pul- 

 monary arterial pressure is of the order of 35/12 

 (352). When systemic vascular resistance increases, 

 blood is diverted through the ventricular septal 

 defect to the pulmonary circulation (443). 



The ventricular septum is complete and the two 

 circulations are entirely separate in the homeothermic 

 mammals and birds. Among the mammals and birds, 

 pulmonary arterial pressures have been measured in 

 a wide variety of species including man, dog, cat, 

 guinea pig (114), chicken (352), and calf (199). In 

 the chicken and calf, the pulmonary arterial pressure 

 is generally of the same order of magnitude as in the 

 clog, cat, and man, i.e., of the order of 20 to 30 mm 

 Hg systolic and 10 to 12 mm Hg diastolic; in the 



