Cardiovascular System

Birth initiates major changes in the cardiovascular system when the circulation of fetal blood through the placenta ceases and the infant's lungs ex­pand and begin to function. The fetal shunts that allowed the blood to bypass the lungs (foramen ovale, ductus arte-riosus) and the liver (ductus venous) close and cease to func­tion. At birth, the size of the heart is large in relation to the chest cavity. The size and weight of the heart double the first year. Initially, the right ventricle is more muscular than the left ventricle, but this reverses in infancy. The heart rate gradually slows, and systolic blood pressure rises.