122 VERTEBRATE RESPIRATION 



ence in the relative volumes of blood returning to the two 

 auricles operates across the inter-auricular septum and causes 

 the foramen ovale to close. Its structure is such that it functions 

 as a valve allowing blood through to the left auricle but it closes 

 when the pressure on the left side is the greater. Differential 

 growth normally results in the complete healing up of this 

 aperture in the adult mammal. If it does not heal up the mammal 

 is said to have 'a hole in the heart' which leads to the condition 

 known as 'blue baby' which nowadays can successfully be cured 

 by cardiac surgery. A further significant effect of cutting the 

 umbilical vein and artery is to reverse the direction of blood 

 flow in the ductus arteriosus which can be heard as a 'murmur' 

 because of the turbulence set up in the flow through this duct 

 as it narrows. The reversed flow is the effect of a decrease in 

 resistance of the pulmonary circuit and an increase in resistance 

 of the systemic circuit due to the removal of the placental 

 circulation. Consequently, the pressure in the systemic circuit 

 exceeds that in the pulmonary and blood flows in the reverse 

 direction from systemic to pulmonary artery through the ductus 

 arteriosus. 



All the changes described above occur within minutes of the 

 delivery of a lamb and result in the establishment of the so- 

 called neonatal circulation (fig. 37). This is exactly the same as 

 the adult circulation with the exception of an open or patent 

 ductus arteriosus which shunts blood from the systemic to the 

 pulmonary circuit. It has been suggested that this circulation is 

 of great adaptive value in the early stages of mammalian exist- 

 ence because it counteracts the disadvantages of the inefficient 

 gaseous exchange occurring in the lungs at this time. Thus one 

 result of a fall in oxygen tension of the blood is by reflex action 

 to raise the blood pressure. The flow of blood from the systemic 

 to pulmonary circuit will therefore increase and the volume of 

 blood passing to the lungs will be augmented so that it exceeds 

 the volume circulating to the rest of the body. The importance 

 of the patent ductus arteriosus at this time is emphasised by the 

 marked lowering of the oxygen saturation of the blood if it is 

 experimentally occluded. Under these conditions the percentage 



