William Harvey's
On the Motion of the Heart and Blood in Animals
Harvey shed great light on the passage of blood from the arteries to
the veins when he discovered two very important differences in the vascular
anatomy of the fetus: that there are blood pathways from the vena cava
to the pulmonary vein and from the pulmonary artery to the aorta that
are not present after birth. In the fetus, there is a small orifice
called the foramen ovale in the septum that allows some blood to pass
directly from the right atrium to the left atrium when the atria contract.
When the right ventricle contracts, it sends blood out through the pulmonary
artery, where, in adults, it would flow to the lungs. In the fetus,
however, there is a shunt called the ductus arteriosus which allows
the blood ejected from the right ventricle to bypass the lungs and enter
directly into the aorta. These two shunts prevent most of the blood
from flowing to the lungs, which are not operational in the womb for
obvious reasons.
Upon birth, the lungs begin to function, which triggers the
closing of the two shunts, and the circulatory system begins to function
in the manner observed in postnatal and adult subjects. What this means
is that because the blood no longer flows directly from the venous to
arterial system, the newly functional lungs, no longer shunted out of
the system, must be the point of connection between the venous and arterial
systems. Unsure of the purpose of the blood flowing through the lungs,
Harvey speculates that it is to cool the blood, but he says that this
is a different subject.