1640 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



LEFT ATRIUM 



LEFT PULMONARY 

 FLOW 



PULMONARY 

 VASCULAR 

 RESISTANCE 



MINUTES FROM DELIVERY 90 



fig. ig. Changes in the circulation on ventilating the 

 fetal lung: a) artificial positive pressure ventilation of the lungs 

 caused a large fall of pulmonary vascular resistance, an increase 

 in pulmonary flow and a fall in pulmonary artery pressure. 

 b) Temporary occlusion of the ductus arteriosus caused a rise in 

 femoral pressure and a fall in pulmonary pressure and flow, 

 showing that blood had been flowing from the aorta into the 

 pulmonary trunk. The figures 35 and 79 indicate the carotid 

 arterial Os% saturation. (From G. S. Dawes. Changes in the 

 circulation at birth. Brit. Med. Bull. 17: 151, 1 96 1 . ) 



with the reduction in pulmonary arterial pressure 

 the flow is diminished and the wall constricts. Closure 

 of the ductus is not dependent upon its nervous con- 

 nections and will occur following inflation of the 

 lungs provided the oxygen tension is high and, like 

 the umbilical vessels, it will dilate when the blood 

 oxygen tension is low (27, 40); constriction can, 

 however, occur during asphyxia and this may be due 

 to the release of sympathetic amines. The responses 

 of the ductus arteriosus and the cord vessels are com- 

 mon to all unstriped muscle, and the exemption of 

 the neighboring aorta and pulmonary artery is due 

 to the preponderance of elastic fibers in the tunica 

 media of the latter vessels. 



In the fetal lamb in utero the right atrial pressure 

 is 1 to 1 1 2 cm H2O higher than the left atrial pressure 

 (fig. 18). This pressure difference occurs because 

 the pulmonary venous return to the left auricle is 

 small and only about one-ninth of that returning to 

 the right side of the heart; 75 per cent of the inferior 

 caval blood is directed bv the valve of the foramen 



ovale and redistributes the venous return to maintain 

 the left ventricular output and systemic and placental 

 blood flows. Following inflation of the lungs the de- 

 crease in pulmonary vascular resistance enables the 

 pulmonary blood flow to treble and as the interatrial 

 pressure difference is reversed the foramen oval closes 

 (77); Dawes and his colleagues consider that clamp- 

 ing the cord before the first breath, thus reducing 

 temporarily the inferior caval flow, might be sufficient 

 to lower the right atrial pressure and assist closure of 

 the foramen ovale. However, maintenance of its 

 closure will depend upon the increased pulmonary 

 venous return. In small animals the preponderance of 

 left-over-right atrial pressure is difficult to demon- 

 strate within the first 24 hours of birth, but develops 

 during the subsequent days and weeks; the main- 

 tenance of this pressure difference which is observed 

 throughout life is probably the combined influence of 

 filling and elasticity of the two ventricles. 



Pulmonary Vascular Resistance, Arterial 

 Pressure, and Blood Flow 



The way in which the first breath initiates the re- 

 duction in pulmonary vascular resistance is not yet 

 fully explained. Using a density flowmeter, Dawes 

 et al. measured the blood flow in the left pulmonary 

 artery of lambs delivered by Cesarean section; follow- 

 ing positive pressure ventilation with air, oxygen, or 

 nitrogen they observed a three- to fourfold increase in 

 pulmonary blood flow, a decrease in arterial pressure 

 and calculated a tenfold decrease in pulmonary vascu- 

 lar resistance (13, 78). Distention of the lungs with 

 warm saline was not found to increase the pulmonary 

 blood flow and no change in circulatory pattern 

 probably takes place during respiratory effects in 

 utero when amniotic fluid is known to enter the 

 lungs (65). Dawes' conclusion that the decrease 

 in pulmonary vascular resistance was primarily 

 due to the mechanical factors associated with ventila- 

 tion was questioned, recently, by Cook et al. (55) 

 following observations in newborn lamb preparations 

 in which the two lungs were ventilated separately 

 and the pulmonary vessels perfused at a constant 

 pressure; alveolar hypoxia and hypercapnia caused 

 vasoconstriction in the pulmonary circulation which 

 was more marked than that observed in the adult lung 

 (84) and ventilation with nitrogen alone gave variable 

 results — possibly on account of differences in local 

 ( !( )_. tension. Recent observations have shown that 

 both an increase in arterial and alveolar pO L > and a 

 reduction in pCOj contribute toward the increase in 



