This is achieved by passage through the ductus venosus, which is estimated to shunt around 30% of umbilical blood directly to the inferior vena cava.Īt birth, these shunts need to close to allow the normal adult circulation to be established: The oxygenated blood entering the fetus also needs to bypass the primitive liver, this ensures that enough oxygen reaches the developing brain.
Any blood that does not pass through the foramen ovale enters the pulmonary trunk, which is linked to the distal arch of aorta by the ductus arteriosus. These two separate shunts allow the circulation to bypass the lungs. The lungs are bypassed by two separate shunts, firstly the foramen ovale between the two atria, which is responsible for bypassing the majority of the circulation. In the fetal circulation, vascular shunts are required to bypass the liver and non-functioning lungs. This allows blood to be shunted to the left side of the heart, bypassing the non-functional lungs. At all times, at least one hole is present in the septa to allow communication between the left and right atria. The timing of this process is carefully controlled. The presence of both the ostium secundum and foramen ovale allows a right to left shunt to be present in the developing heart.
Before the ostrium primum is closed a second hole, the ostium secundum, appears within the septum primum.įollowing this a second septum, the septum secundum, grows with a hole known as the foramen ovale present. The ostium primum is a hole present before the septum primum completes fusion with endocardial cushions. The septation of the primitive atrium involves the formation of two septa and three ‘holes’.įirstly, the septum primum forms and extends down towards the fused endocardial cushions to split the atrium into two. Septation of the heart into right and left channels occurs first, when endocardial cushions developing in the atrioventricular region expand to divide the heart. The long course of the left recurrent laryngeal nerve is clinically relevant, as it is susceptible to pathology in the chest (e.g. The distal part of the left 6 th aortic arch persists as the ductus arteriosum, and so the nerve remains in this position.
The most important of these is the recurrent laryngeal nerve (a branch from CN X) – which is associated with the 6th arch: Left arch forms the left pulmonary artery and the ductus arteriosus.Įach of the arches has a corresponding nerve during development. Right arch forms the right pulmonary artery Left arch forms part of the arch of the aortaĮither never forms, or forms incompletely and regresses Right arch forms the right subclavian artery The derivatives of the aortic arches in the adult are as follows: 1 st archĬontributes to the maxillary, hyoid and stapedial arteries.įorms the common carotid artery and part of the proximal internal carotid artery. The early arterial system begins as a bilaterally symmetrical system of arched vessels, which then undergo extensive remodeling to create the major arteries that exit the heart. This process produces a shape that is much closer to the fully developed heart. The bulbus cordis moves ventrally, caudally, and to the right (forward, down and right), and the caudal portion – the primitive ventricle – moves dorsally, cranially and to the left (backwards, up and left). The heart tube continues to elongate, and begins looping at around day 23 of development. Initially, the heart tube is suspended within the pericardial cavity by a membrane this subsequently degenerates to allow for further growth.įrom superior to inferior, the primitive heart tube is comprised of six regions: The development of the heart begins with the formation of the primitive heart tube following the folding of the embryo during the end of the third week.įirstly, lateral folding creates the heart tube by bringing together two precursor regions, then cephalocaudal folding positions the heart tube in the future thorax.
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