• 21days – straight tube circulation begins
• 24 days – heart begins beating
• 4 weeks – outside of heart looks like real heart
• 5 weeks – atrial septum completed
• 7 weeks – ventricular septum and great artery septum complete
• 9 weeks – valves complete (complete heart)

Developmental Problems – anything abnormal occurring <21 days results in an abortion; >21 days results in congenital defect and abnormal blood flow

• paired heart tubes – connect to bilateral atrial channels
• Vitelline Veins – returns blood from the yolk sac; the left vitelline drains into the right vitelline vein and degenerates; the right vitelline forms the terminal portion of the inferior vena cava (IVC), the liver sinusoids and the portal system
• Vitelline Arteries – supply blood to the yolk sac and from the definitive celiac, superior mesenteric and inferior mesenteric arteries
• Umbilical Veins – drain blood from the chorion of the placenta; the entire right regresses; the left runs through the umbilical cord and connects with the ductus venosus to returns oxygenated blood from the placenta to the heart; forms ligament teres
• Umbilical Arteries – carries blood from the heart through the umbilical cord to the placenta
• Cardinal Veins – return venous blood from the embryo to the developing heart; the right anterior cardinal vein becomes the superior vena cava (SVC) and the right posterior cardinal vein becomes the azygous vein

• fate of cardiac cells is determined when the paired heart tubes are still in a horseshoe formation Þ very early
• as development proceeds the embryo folds longitudinally and transversely
• the heart tubes fuse first at the cranial end and end fusion at day 22
• the atrium will form the primitive RA and LA; the ventricle forms only the LV; the conoventricular area (AKA bulbus cordis) forms the RV and the infundibulum (outflow tract of RV to the pulmonic valves)
• 99% of the time it folds to the right, termed dextro (d) looping, so that the conoventricular is to the right and the ventricle is on the left
• the truncus arteriosus (cranially) which becomes the roots of the aorta and the pulmonary artery, and the sinus venosus (caudally), part of which is incorporated into the RV, develop about day 23
• Clinical Correlation: rarely, looping can occur to the left (levo looping) resulting in the RA emptying into the LV and the LA emptying into the RV; the pulmonary artery is connected to the LV and the aorta is connected to the RV Þ Ventricular Inversion
• each sinus horn receives blood from the vitelline, umbilical and common cardinal vein
• day 26-28 Þ the right sinus venosus is incorporated into the RA becoming the smooth portion of the chamber containing the sinus node and the presumed internodal
• the left sinus horn develops mainly into the coronary sinus
• day 28 Þ the outside of the heart looks similar to the fully formed heart

• common pulmonary vein forms from a single bud in the posterior medial portion of the primitive LA; it branches as it enlarges
• the lung buds develop from the intrapulmonary portion of the splanchnic vein plexus which connects with the common pulmonary vein; the intrapulmonary portion of the splanchnic vein plexus then loses its connection with the systemic veins
• the common pulmonary vein is then absorbed into the wall of the LA
• the final LA consists mainly of the original common pulmonary vein; the primitive LA contributes only to the auricle and a small portion of the mitral valve
• Clinical Correlation:
• (1) Cor triatriatum Þ a heart with three atrial chambers, the left atrium being subdivided by a transverse septum with a single small opening which separates the openings of the pulmonary veins from the mitral valve.
• (2) Systemic veins migrate with pulmonary veins and do not dissociate. Blood from the pulmonary vein enters the systemic circulation Þ Total Anomalous Pulmonary Venous Return
• (3) Abnormalities of drainage of the pulmonary veins (may drain to the IVC, SVC or RA).

• RA is fully formed and consists of mainly a trabeculated portions, smooth portion, crista terminalis and interatrial wall
• trabeculated portion Þ includes the auricle and derived from the primitive atrium
• smooth portion Þ called the sinus venarum and derived from the right horn of the sinus venosus
• crista terminalis Þ separates the smooth and trabeculated portions
• interatrial wall Þ forms from the septum primum and septum secundum and forms the foramen ovale

• Atrio-Ventricular Septum - the common atrioventricular canal separates the atria from the ventricles
• the dorsal endocardial cushion develops separates the common atrioventricular canal into left and right portions
• dorsal endocardial cushion develop superiorly and inferiorly and a ventral endocardial cushion will also develop
• the endocardial cushions will eventually make up the inferior portion of the atrial septum, the posterior superior portion of the ventricular septum and the medial portion of the tricuspid and mitral valves
• Atrial Septum
• septum primum grows down from the superior posterior wall to the endocardial cushions leaving an opening called the ostium (foramen) primum Þ allows from communication between the RA and the LA
• then the middle, superior portion of the septum primum degenerates forming the ostium (foramen) secundum; allows for communication between the RA and the LA; the ostium primum then closes by growth the dorsal endocardial cushion
• at the end of the 5^th week, the septum secundum grows down on the right side of the septum primum; it is not a complete septum and the ostium secundum is not completely covered
• the septum secundum halts before it reaches the floor of the atrium and leaves an opening called the foramen ovale; this occurs three weeks later; for blood to travel from the RA to the LA it must travel through two staggered openings Þ the foramen ovale at the floor of the atrium and the ostium secundum near the roof of the atrium
• Ventricular Septum
• more complex than the atrial septum and consists of the muscular and membranous (thinner) portions
• the muscular septum grows from the floor of the primitive left and right ventricles
• the membranous portion is formed from:
• muscular tissue
• inferior (most), dorsal and ventral endocardial cushions
• the conus which provides the largest amount of tissue
• Clinical Correlation: Ventral septal defect (VSD) accounts for 30% of all congenital heart disease; Þ hypoplasia of the conus Þ VSD; pulmonic valve and pulmonic arteries are small Þ aplasia of the conus Þ persistent truncus arteriosus Þ no separation of aorta and pulmonary trunk
• Truncal Septum – separates the aorta and the pulmonary artery so that by 7 weeks the LV opens into the aorta and the RV opens into the pulmonary artery
• occurs by truncal swelling forming bulbar ridges which form a spiral septum at 6 weeks
• Semilunar and Atrioventricular (AV) Valves- form separately
• the AV valves are completed by 7 weeks; the medial portion is formed by endocardial cushion tissue while the lateral portions are from lateral cushions which are not endocardial cushion in origin
• when the LV is on the left, the mitral valve is also on the left and the tricuspid valve is on the right
• when the LV is on the right (levo looping) the mitral valve is on the right
• the semilunar valve is completed by 9 weeks; the valve begins as swelling on either side of the proximal portion of the truncus and develops as the truncal septum develops
• the semilunar valves develop from the same origin of the tissue which separates the aorta and the pulmonary artery Þ they always go with the appropriate great artery

• start development about day 24 and are completed by day 38
• 1^st = part of maxillary artery
• 2^nd = stapedial artery and hypoid artery
• 3^rd = common carotid artery and proximal part of internal carotid artery
• 4^th = on left, aortic arch; on right, proximal part of the right subclavian artery
• 5^th = never fully develops and disappears
• 6^th = proximal part of the pulmonary arteries and on the left becomes the ductus arteriosus
• intersegmental arteries = subclavian arteries (origins of the subclavians are from the 4^th aortic arch)

• About half of the oxygenated umbilical vein blood passes into the hepatic sinusoids and portal vein and the other half passes through the ductus venosus to enter the hepatic portions of the IVC and thus the RA.
• Mixed IVC blood also contains less oxygenated blood from the lower part of the fetus so that the oxygen saturation of the IVC is lower than that of the umbilical vein.
• Most IVC blood passes through the foramen ovale to the LA where it is joined by a small amount of less oxygenated pulmonary venous blood. Thus relatively well oxygenated blood (80%) gets to the LV, ascending aorta and upper part of the body (including the brain).
• SVC blood is poorly oxygenated. It mixes with the coronary sinus blood to enter the right ventricle and pulmonary artery. Because the pulmonary arterioles are constricted owing to low pH, most of this blood goes through the ductus arteriosus to the descending aorta. Some of this blood provides oxygen for the lower part of the fetus, but most enters the umbilical artery because of the placenta’s remarkable low vascular resistance. Oxygenation then takes place in the placenta.
• The RV pumps 1.2-1.3/l over the LV in utero
• 7-8% of the total cardiac output goes to the lungs.
• 10-15% of the blood ejected by the RV goes to the lungs.
• pO2 of IVC blood entering the heart is 26-28; pO2 of SVC blood entering the heart is 12-14.
• Ascending aorta blood has a pO2 of 23-25; Descending aorta blood has a pO2 of 20-22.

• The placenta is removed from the circulation Þ immediate Ý systemic arterial resistance Þ Ý LA pressure
• With the baby’s first breath Þ immediate ß in pulmonary vascular resistance Þ Ý pulmonary blood flow Þ ß RA pressure
• Ý pO2 Þ constriction of the ductus arteriosus
• Ý LA and RA pressure Þ causes the septum primum to press against the septum secundum closing the foramen ovale
• The umbilical vein remains patent for many days, but eventually closes to become the ligamentum teres.
• The ductus venosus remains patent for many days, but eventually closes to become the ligamentum venosum.
• The intra-abdominal umbilical arteries persist as superior vesical arteries.
• The ductus arteriosus eventually closes to become the ligamentum arteriosum.
• The foramen ovale remains potentially open in most babies for many months, and it remains potentially open in 25% of normal adults.