A large number of changes take place during the perinatal period (Table 3.15). As already discussed, there is loss of the low resistance placental circuit, a decrease in pulmonary vascular resistance and greatly increased pulmonary blood flow, transformation of the heart from two pumps working in parallel, to one involving a series arrangement, closure of the umbilical vessels, ductus venosus, ductus arteriosus, and foramen ovale, and an increase in arterial PO2 and hemoglobin saturation.
In addition, there is a gradual decrease in the thickness of the right ventricle wall relative to the left ventricle wall. This is because of falling afterload (lowered pulmonary resistance) on the former and rising afterload (increasing resistance & blood pressure) on the latter (Figure 3.10). The decline in relative right ventricle mass is directly related to the fall in pulmonary artery pressure.
There is a decrease in the ratio of heart weight to body weight as growth occurs, reflecting the relatively smaller cardiac workload in the maturing animal. Heart rate also usually declines, finally reaching the adult rate. There is a continued changeover from the fetal type (Hbf) to the adult (HbA) type of hemoglobin.
There is a gradual decrease in cardiac output and O2 consumption on a per g body weight basis. Finally, the considerable resistance or tolerance of asphyxia (anoxia) exhibited by the fetus is lost soon after birth.
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