Arterial system:
Each of the pharyngeal arches has its own artery and cranial nerve. These arteries are called aortic arches that originate from the root of the aorta (the aortic sac), and end into the dorsal aorta. There are 6 pairs of aortic arches, in human embryo, the 5th arch often underdeveloped or absent.The first aortic arch: largely disappears, remnant of it forms the maxillary artery.
The second aortic arch: also disappears, remnant of it forms the hyoidal and stapedial arteries.The third aortic arch: forms the common carotid artery and the beginning of the internal carotid artery. Continuation of the internal carotid is derived from the dorsal aorta distal to the third arch.
The forth aortic arch: forms the arch of the aorta on the left side between the Lt Common carotid artery and the Lt 7th intersegmental artery (which is the primordium of the Lt Subclavian artery). On the Rt side, the 4th aortic arch forms the beginning of the Rt subclavian artery, the continuation of it is derived from the Rt dorsal aorta connecting the 4th, 5th, and 6th aortic arches and also by the 7th intersegmental artery.
The sixth aortic arch: (the pulmonary arch) it forms the pulmonary arteries. The connection of the RT sixth arch and the Rt dorsal aorta will disappears, while the connection of the 6th arch with the dorsal aorta forms the ductus arteriosus on the left side.
During the development of the aortic arches, there will be disappearance of the following:
1.the bilateral dorsal aortae between the 3rd and the 4th arches (called the carotid arch).2.the Rt dorsal aorta between the origin of the Rt 7th intersegmental artery and the junction with the left dorsal aorta.
Also during this developemtal period, the heart descends from the neck to the thoracic cavity, as a result of that, the following two changes occur:
1.the Lt subclavian artery (originating from the 7th intersegmental artery) become nearer to the left common carotid artery.
2.the recurrent laryngeal nerve (which is the nerve of the 6th pharyngeal arch) hooks around the distal part of the left 6th arch (forming the ductus arteriosus) to reach the larynx. On the right side, the nerve hook ascends superiorly because of the disappearance of the distal part of the 6th aortic arch, thus hooking around the Rt subclavian artery.
Venous System.
Three systems can be recognized:(a )the vitelline system, which develops into the portal system;
( b ) the cardinal system, which forms the caval system ; (c )the umbilical system, which disappears after birth.
The complicated caval system is characterized by many abnormalities, such as double inferior and superior vena cava and left superior vena cava.
Changes at Birth.
During prenatal life the placental circulation provides the fetus with its oxygen, but after birth the lungs take on gas exchange. In the circulatory system the following changes take place at birth and in the 1st postnatal months:
The ductus arteriosus closed; becomes ligamentum arteriosum
The oval foramen closed; becomes fossa ovalis
The ductus venosus closed; becomes ligamentum venosum
The umbilical vein obliterated and becomes the ligamentum teres hepatis
The umbilical arteries form the medial umbilical ligaments.