Development of Blood Vessels Blood vessel formation (angiogenesis) starts at the beginning of the third week. Blood vessels first start to develop in the extraembryonic mesoderm of the yolk sac, connecting stalk, and chorion. Blood vessels begin to develop in the embryo about two days later.

Production of Blood Production of blood (hemopoiesis or hematopoiesis) begins first in the yolk sac wall about the third week of development. Erythrocytes produced in the yolk sac have nuclei. Blood formation does not begin inside the embryo until about the fifth week. Erythrocytes produced in the embryo do not have nuclei (eunucleated). Hematopoiesis inside in the embryo occurs first in the liver, then later in the spleen, thymus, and bone marrow.

Vitelline circulation refers to the system of blood flowing from the embryo to the yolk sac and back again. The yolk-sac is situated on the ventral aspect of the embryo; it is lined by endoderm, outside of which is a layer of mesoderm. It is filled with fluid, the vitelline fluid, which possibly may be utilized for the nourishment of the embryo during the earlier stages of its existence.

Blood is conveyed to the wall of the sac by the primitive aortæ, and after circulating through a wide-meshed capillary plexus, is returned by the vitelline veins to the tubular heart of the embryo. This constitutes the vitelline circulation, and by means of it nutritive material is absorbed from the yolk-sac and conveyed to the embryo.

Cardinal veins Cardinal veins Scheme of arrangement of parietal veins. During development of the veins, the first indication of a parietal system consists in the appearance of two short transverse veins, the ducts of Cuvier, which open, one on either side, into the sinus venosus. Each of these ducts receives an ascending and descending vein. The ascending veins return the blood from the parietes(hollow organs) of the trunk and from the Wolffian bodies, and are called cardinal veins.

The fetal circulation In the fetus the four-chambered heart circulates blood through the fetal body, through the placenta and through the vessels supplying the gut (previously the vitelline vessels) The blood arriving at the right side of the heart through the superior and inferior venae cavae has come from the fetus (body and head), from the digestive system and liver, and from the umbilical vein. The blood in the umbilical vein is highly oxygenated.

The blood mixes in the right atrium and is directed towards the interatrial septum where it passes through the foramen ovale into the left atrium. From the left atrium it passes into the left ventricle and so to the systemic circulation through the aorta.

Any blood which passes into the right ventricle leaves that chamber though the pulmonary trunk to pass to the aorta through the ductus arteriosus, but also to a small extent to the lungs.

The aorta thus distributes blood to the head, the body including the gut, and to the placenta through the paired umbilical arteries. At the placenta the blood is oxygenated and collected in the single umbilical vein. The umbilical vein travels to the liver where a connection, the ductus venosus, shunts most of the blood into the inferior vena cava.

This circulation ensures that the head and upper limbs receive oxygenated blood, that the body receives fairly well oxygenated blood with some addition of venous blood from the head and lungs.

Since at this stage the lungs are not inflated, the resistance in the pulmonary circulation is high and the blood is diverted back into the systemic circulation through the ductus arteriosus. The ductus venosus diverts blood from the umbilical vein bypassing the liver, directly into the IVC.

Changes at birth At birth the lungs will inflate and the placenta will become disconnected from the source of oxygenation. With the first breath the resistance in the pulmonary circulation falls as the lungs inflate. Blood in the right ventricle is able to pass through the pulmonary circulation where oxygenation occurs. Oxygenated blood from the lungs returns to the left atrium.

Cutting and tying the umbilical cord closes both the umbilical arteries and the umbilical vein. Other significant changes in the circulation occur over the next hours and days. The ductus arteriosus starts to constrict and is usually permanently closed by three months after birth. As pressure in the left atrium rises, the septum primum is forced against the septum secundum, closing the foramen ovale.

Eventually the two septa fuse to permanently separate the two atria. Absence of flow in the umbilical vein causes it to become ligamentous - the ligamentum teres, although a narrow channel remains. The ductus venosus becomes the ligamentum venosum and the blood from the portal vein must pass through the liver before reaching the IVC.