120 VERTEBRATE RESPIRATION 



pointed out in 1628 by William Harvey. The two circuits are not 

 separate, and blood enters both auricles from the inferior vena 

 cava because the foramen ovale remains open until just after 

 birth. The lungs only receive about 10% of the combined 

 ventricular output. Extra blood pumped out of the right 

 ventricle is shunted into the systemic circuit via the func- 

 tional ductus arteriosus. The foetal blood is oxygenated at 

 the placenta and passes through the liver in the umbilical vein 

 to become mixed with deoxygenated blood returning from the 

 rest of the tissues in the inferior vena cava. 60% of this blood 

 is deflected into the left auricle through the foramen ovale. 

 The remaining 40% enters the right auricle and becomes 

 mixed with deoxygenated blood returned in the superior 

 venae cavae and is pumped to the right ventricle. Blood from 

 the left side of the heart is forced out of the aorta and some 

 passes into the brachio-cephalic artery supplying the head and 

 anterior limbs but most flows into the descending aorta. 



In fig. 36 the percentage saturation of the blood with oxygen 

 is indicated for different parts of the circulation. These figures 

 were determined within a short period (5-15 seconds) of one 

 another and from them it is possible to deduce the proportions 

 in which any two blood streams become mixed with one 

 another. For example, the right auricle receives blood returning 

 in the inferior vena cava (67% saturation), and from the 

 superior vena cava (31 % saturation). After being pumped into 

 the right ventricle this blood passes into the pulmonary artery 

 where the blood is 52 % saturated. Hence it follows that the pro- 

 portion of inferior vena cava to superior vena cava blood in the 

 right auricle is approximately 2:1. Similarly, on the left side of 

 the heart blood from the inferior vena cava and pulmonary 

 vein becomes mixed in the proportion of about 4.5: 1. It can 

 also be calculated that 40% of the total ventricular output 

 passes along the ductus arteriosus from the pulmonary artery 

 to the main systemic aorta. In this way a very complete picture 

 of the circulation in the foetus has been developed as have the 

 changes following severance of the umbilical cord, some of 

 which are summarised below. 



