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RIGHT ATRIUM

The cranial portion of the right venous valve (valve of sinus venosus) along with the septum spurium will form the crista terminalis in the wall of the adult right atrium. The crista terminalis extends ventrally along the right border between the orifices of the superior and inferior venae cavae and marks the boundary between the primitive atrium and sinus venosus. The primitive atrial portion of the right atrial wall is located ventral to the crista terminalis and contains musculi pectinati. The sinus venosus portion is located dorsally and has a smooth wall.

The caudal portion of the right venous valve fuses with the septum between the orifices of the inferior vena cava and coronary sinus.

The intermediate portion of the right venous valve becomes the valve of the inferior vena cava and valve of the coronary sinus. Highly oxygenated blood returning from the placenta is shunted by way of the valve of the inferior vena cava toward the foramen secundum to the left atrium.

LEFT ATRIUM

The common pulmonary vein is absorbed into the dorsal wall of the left atrium giving separate orifices for the right and left pulmonary veins. Subsequently the right and left pulmonary veins are also absorbed into the dorsal wall of the left atrium as far as their upper and lower tributaries. Their incorporation into the left atrial wall produces a total of four pulmonary vein orifices.

The primitive atrium with the musculi pectinati in its wall is greatly reduced on the left side becoming mainly the auricle of the left atrium. The absorbed pulmonary veins form most of the left atrial wall that is smooth and located dorsally.

INTERATRIAL SEPTUM

The septum secundum thickens. Its caudal, free edge becomes crescent shaped and is directed toward the orifice of the inferior vena cava, which also helps shunt the highly oxygenated blood from the placenta to the left atrium.

The lower free edge of the septum secundum eventually overlaps the foramen secundum converting it into an oblique opening bound by the septum secundum and septum primum. The interatrial opening becomes the foramen ovale that remains patent until birth. After the foramen is closed by the thin septum primum, the depression on the right side of the interatrial septum is called the fossa ovalis. The arching free edge of the septum secundum is the annulus ovalis.

ARTERIES

The majority of the definitive, named arteries are present by the end of the eighth week. Those in the extremities lag in attaining the adult pattern. The right and left coronary arteries are prominent, first branches of the aorta (ascending part). They arise immediately above the aortic semilunar valve. Each artery courses primarily in its respective part of the atrioventricular (A-V) sulcus sending major branches into the interventricular sulcus.

VEINS

VEINS IN THE LIVER

After the ductus venosus closes at birth, its afferent branches in the liver become branches of the portal vein thereby causing blood from the gut to flow to the hepatic sinusoids. The efferent veins that drain the sinusoids become tributaries of the hepatic veins that drain into the inferior vena cava.

DURAL SINUSES

As the otic capsule expands, the sinuses dorsal to the capsule enlarge and connect the tributaries of the middle and posterior cerebral plexuses through the primitive transverse and sigmoid sinuses. The anterior cerebral plexus drains predominantly into the middle cerebral plexus. That portion of the primary head vein medial to the trigeminal ganglion becomes the definitive cavernous sinus, which establishes new connections with the primitive sigmoid sinus known as the superior and inferior petrosal sinuses. The midline portions of the cerebral plexuses enlarge to produce new sinuses (superior sagittal, inferior sagittal and straight) that connect with the primitive transverse sinus. The primitive sinuses move with the expanding cerebral hemispheres attaining the definitive position together.

PULMONARY VEINS

The right and left pulmonary veins disappear as they become incorporated into the left atrial wall. Their upper and lower tributaries become the upper and lower right and upper and lower left pulmonary veins.

LYMPHATICS

The mesenteric lymph sac in the upper lumbar segments will dilate to form the definitive cisterna chyli. The left jugular lymph sac and cisterna chyli grow toward one another to form the major lymph channel of the body, the thoracic duct.

CIRCULATORY CHANGES AT BIRTH

Two important events occur at birth that profoundly influence the newborn’s circulatory system: a) the flow in the umbilical vessels is interrupted when the placenta breaks away from the uterine wall and b) the newborn begins breathing thereby opening the capillary lung field, which greatly increases the pulmonary circulation. The physiological changes are rapid, occurring within a few days. The anatomical changes usually take place over several months.

When the blood flow through the lungs increases, the pulmonary venous return to the heart increases thereby raising the pressure in the left atrium. This presses the thin septum primum against the thick septum secundum and functionally closes the foramen ovale. Anatomical closure produces the fossa ovalis. Simultaneously the muscular wall of the ductus arteriosus contracts increasing the flow to the lungs through the pulmonary arteries. Within three months its lumen is obliterated thereby producing the ligamentum arteriosum.

The muscular wall of the umbilical arteries contracts when blood flow through them ceases. Anatomical closure converts their distal segment on the deep side of the ventral abdominal wall into the medial umbilical ligaments. Their proximal segment gives rise to the superior vesical arteries to the bladder. The umbilical vein and ductus venosus close a short time after the umbilical arteries. Anatomical closure changes the umbilical vein into the ligamentum teres that courses from the deep side of the umbilicus, through the edge of the falciform ligament, to the portal vein where the latter enters the liver (hilum). The ductus venosus becomes the ligamentum venosum between the portal vein and the inferior vena cava.

Source: Atlas of Human Embryos.