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HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



arterial trunks. The circulatory pattern established at 

 this time persists throughout the remainder of fetal 

 development. This circulatory system must function 

 through fetal life while receiving oxygen from the 

 mother, and must be capable of rapid accommodation 

 to independent existence immediately after delivery 

 of the fetus. The lungs are collapsed and have no 

 respiratory function, and the resistance to the flow 

 of blood through the vessels of atelectatic lung tissue is 

 high. Since before birth the vascular resistance in the 

 pulmonary vasculature is greater than that in the 

 systemic circulation, most of the blood flow is diverted 

 around the lungs. The foramen ovale and ductus 

 arteriosus act as bypasses allowing the blood from the 

 systemic veins to enter the systemic circulation without 

 passing through the lungs. 



If there were no interatrial communication, left 

 ventricular output would be equal only to the amount 

 of blood flowing through the lungs. It might be 

 expected that under these conditions the left ventricle 

 might not develop normally because of the small 

 amount of blood it would pump. Patten (185) 

 described the heart from an infant in whom the 

 foramen ovale had sealed prematurely; the left 

 ventricular cavity was very small and the wall was 

 poorly developed. 



Lind & Wegelius (163) have described by means of 

 angiocardiography the blood flow through the 

 circulatory system of human fetuses. Blood returning 

 from the placenta flows through the umbilical vein 

 and enters the ductus, venous, and vascular networks 

 of the liver. On entering the vena cava this blood 

 merges with the systemic venous blood. A larger 

 proportion of blood from the inferior vena cava is 

 directed across the atrial chamber, through the 

 foramen ovale, and enters the left atrium. Most of the 

 blood from the superior vena cava, however, passes 

 into the right ventricle from the right atrium. Using 

 radioi.sotope techniques Everett & Johnson (97) 

 reported that only about one-fourth of the blood from 

 each of the vena caval streams becomes mixed. 



Since there is little pulmonary venous return, most 

 of the oxygenated blood from the placenta flows 

 directly into the left side of the heart and thence into 

 the ascending aorta. Thus the heart and brain receive 

 blood with maximal oxygen content. Blood flow 

 through the pulmonary circulation probably increases 

 as the lungs develop, but never approaches the flow 

 through the systemic circuit, and because the pul- 

 monary vascular resistance is greater than that in the 

 systemic arteries (owing mainly to the collapsed 

 lungs) a large proportion of the relatively desaturated 



blood entering the right ventricle and pulmonary 

 artery is shunted through the patent ductus into the 

 descending aorta. 



The volume of blood flow through the umbilical 

 vessels has been mea.sured directly by several methods, 

 and is approximately 130 ml per kg per min in a 

 mature fetal lamb (18). This constitutes more than 

 half the combined output of both ventricles. Pul- 

 monary blood flow is comparativelv low and amounts 

 to only 30 ml per kg per min at the most. 



The mean arterial pressure in a mature fetal lamb 

 is about 65 mm of mercury. Pulmonary arterial 

 pressure is just above that in the descending aorta. 

 The pressure in the umbilical vein is about 15 mm 

 of mercury and that in the great veins 2 to 5 mm of 

 mercury. There is, therefore, a pressure head of 50 

 mm of mercury to drive blood through the placenta 

 from the umbilical arteries to the umbilical vein, 

 and a pressure head of 10 mm of mercury or more to 

 drive blood through the liver from the umbilical vein 

 to the inferior vena cava. 



Poslnatal Circulation 



The changes that occur in the circulation at birth 

 result from four events: /) removal of the placenta, 

 3) expansion of the lungs, j) rise in partial pressure 

 of arterial oxygen, and ./) deli\ery of the fetus from a 

 warm intrauterine environment into the cold external 

 world. Probably the most important of these events 

 is the "first breath," with the resultant inflation of 

 the lungs and increased blood flow to them. On the 

 basis of studies by Ardran et al. (11) in the lamb it 

 would seem reasonable to assume that in the human 

 infant with the first breath there is a prompt drop in 

 pulmonary resistance with a probable increase in 

 systemic resistance, particularly if the umbilical cord 

 is clamped simultaneously. With this increased 

 pulmonary flow there is an increase in left atrial 

 pressure, and following clamping of the umbilical 

 cord a decrease in inferior vena caval pressure. This 

 causes the flap on the foramen ovale to be closed and 

 prevents the venoarterial shunt through it. However, 

 crying during the first several days of life may increase 

 the pres.sure in the right atrium enough over that in 

 the left atrium to reinstitute the venoarterial shunt 

 and to produce transitory systemic arterial desatura- 

 tion (192). The studies of Crehan and associates (75) 

 showed that arterial oxygen saturation of infants 

 increased rapidly, so that at 1 7 min after the first 

 breath the values were within the range obtained for 

 normal adults. Anatomic obliteration of the potential 



