Fetal_and_Neonatoal_Circulation_Lecture_Notes (1).docx

Fetal and Neonatal Circulation Lecture Notes Session: 10/18/2023 Instructor: Spencer Mattingly Session Goals: Compare and contrast prenatal versus postnatal circulation, and how the major structures that facilitate it arose embryologically. Learning Objectives: 1) Describe the embryonic development...

Fetal and Neonatal Circulation Lecture Notes Session: 10/18/2023 Instructor: Spencer Mattingly Session Goals: Compare and contrast prenatal versus postnatal circulation, and how the major structures that facilitate it arose embryologically. Learning Objectives: 1) Describe the embryonic development of the arterial system as it relates to the aortic arch arteries. 2) Discuss congenital defects of the arterial system, including a patent ductus arteriosus, and coarctation of the aorta. 3) Describe the embryonic development of the venous system around the heart, including the fate of the major sinus venosus tributaries. 4) Discuss the congenital defects of the venous system, including an absent inferior vena cava, a lefts superior vena cava, and a double superior vena cava. 5) Compare and contrast prenatal versus postnatal circulation, including the fetal blood shunts and oxygenation levels. LO 1: Describe the embryonic development of the arterial system as it relates to the aortic arch arteries. Intraembryonic System arteries – supplying the body tissues of the embryo Aortic sac – expanded region of the embryonic aorta that receives blood from the truncus arteriosus and transmits it through the aortic arch arteries Aortic arch arteries – also called pharyngeal arch arteries. 6 paired vessels that give rise to major arterial structures around the heart, although some will regress during development. Develop in the neck around week 4. Dorsal aortae – two bilateral structures that receive blood from the aortic arch arteries, and will fuse caudally to formal a single dorsal aorta. Transmits arterial blood throughout the entire body of the embryo. Placental System arteries – sending blood to the placenta for oxygenation Umbilical artery – arising from the caudal aspect of the dorsal aorta and attached to the placenta Yolk Sac System arteries – sending blood to the yolk sac for nutrients Vitelline artery – arising from the dorsal aorta and sending blood to the yolk sac Development of the aortic apparatus Arch 1 – mostly regresses; contributes to the maxillary arteries in the head Arch 2 – mostly regresses; contributes to small arteries in the head (hyoid and stapedial Arch 3 – forms the common carotid arteries, while the dorsal aorta attached to it forms the internal carotid artery. The external carotid artery will form via angiogenesis from the common carotid (arch 3). Arch 4 – right arch will form the proximal part of the right subclavian artery; the left arch will form the arch of the aorta Arch 5 – completely regresses Arch 6 – sometimes called arch 5. Pulmonary arteries will form from the proximal part of this arch via angiogenesis. The distal part of the right arch will regress; the distal part of the left arch will remain as the ductus arteriosus. 7th intersegmental branch – the right side will be continuous with the fourth arch, forming part of the right subclavian artery. The left side will alone form the left subclavian artery, branching off the fourth arch (arch of the aorta). Aortic sac – the caudal aspect of the sac will be continuous with the fourth arch to form part of the arch of the aorta. The distal (cranial) aspect will expand out on the right side to form the brachiocephalic trunk, connecting arches 3 and 4 on the right side. Recurrent laryngeal nerves – branches of vagus nerve that innervate structures of the larynx. Initially they wrap around the 6th aortic arch arteries, but on the right side the distal part of this artery will regress, leaving the right recurrent laryngeal n. to wrap around the right 4th arch instead (the right subclavian artery). On the left side, it will remain trapped by the 6th arch (ductus arteriosus). LO 2: Discuss congenital defects of the arterial system, including a patent ductus arteriosus, and coarctation of the aorta. Patent ductus arteriosus – failure to close after birth. Blood will be shunted from high pressure to low (aorta to pulmonary trunk), which will cause a lower volume of blood in systemic circulation. This will lead to left ventricle hypertrophy. Coarctation of the aorta – narrowing of the aorta in the area around the ligamentum arteriosum. Can be pre- or post-ductal depending on whether it is proximal or distal to the opening of the ductus arteriosus. Pre-ductal – narrowing occurs before (proximal) to the ductus arteriosus. Prenatally, circulation develops as normal, since systemic blood is able to bypass the narrowing via the ductus. Upon closure of the ductus after birth, however, the baby will become symptomatic (systemic blood is not able to bypass the narrowing). Post-ductal – narrowing occurs after (distal) to the ductus arteriosus. Prenatally, collateral circulation must be established for systemic blood to bypass the narrowing. This is achieved via the internal thoracic and intercostal arteries. Blood will flow from the: ascending aorta > subclavian arteries > internal thoracic arteries > anterior intercostal arteries > posterior intercostal arteries (reversed flow) > thoracic aorta (distal to narrowing). This will cause enlarged and tortuous internal thoracic and intercostal arteries because of the higher volume of blood. Usually does not become symptomatic until adolescence. LO 3: Describe the embryonic development of the venous system around the heart, including the fate of the major sinus venosus tributaries. Placental System veins – draining oxygen rich blood from the placenta Umbilical vein – right umbilical veins regresses, left vein drains into IVC Ductus venosus – fetal blood shunt that allows the umbilical vein to partially bypass the liver Yolk Sac System veins – draining nutrient rich blood from the yolk sac Vitelline veins – form the hepatic sinusoids (network of veins draining the developing liver). Also form the hepatic portion of the IVC. Intraembryonic System veins – draining the body tissues of the embryo Common cardinal veins – junction of the anterior and posterior cardinal veins that drain into the sinus venosus. The right common cardinal vein becomes the superior vena cava; the left side regresses Anterior cardinal vein – draining tissues of the cephalic region Form the internal jugular and brachiocephalic veins Posterior cardinal vein – draining tissues of the caudal region. Has 3 main subcomponents: Supracardinal veins – draining the thoracic wall; forms the azygos system Subcardinal veins – draining the kidneys and abdominal wall; right side forms the renal veins and renal portion of the IVC; left side regresses Sacrocardinal veins – draining the lower limbs; right side forms the sacrocardinal portion of the IVC and proximal part of the left common iliac vein; left side regresses LO 4: Discuss the congenital defects of the venous system, including an absent inferior vena cava, a lefts superior vena cava, and a double superior vena cava. Double inferior vena cava – left sacrocardinal veins persist; forms a left-sided IVC that drains into the left renal vein Absent inferior vena cava – subcardinal veins anastomose with supracardinal veins rather than the IVC. This means the renal veins and everything inferior to it will drain into the azygos system, rather than a hepatic IVC. Left superior vena cava – left common cardinal vein persists; right side regresses. Forms a left-sided SVC that drains into the (enlarged) coronary sinus Double superior vena cava – both common cardinal veins persist; the right SVC develops as normal, the left side forms a left SVC that drains into the coronary sinus LO 5: Compare and contrast prenatal versus postnatal circulation, including the fetal blood shunts and oxygenation levels. Fetal circulation: “Oxygen rich circulation” - placenta > umbilical vein > ductus venosus > IVC > right atrium > foramen ovale > left atrium > left ventricle > aorta “Oxygen poor circulation” - head/brain (and upper limb) > superior vena cava > right atrium > right ventricle > pulmonary trunk > ductus arteriosus > aorta There is some mixing between these two paths, since oxygen poor blood from the lower limbs is also draining into the IVC along with the umbilical vein. Plus, a small amount of blood will mix in the right atrium due to the paths crossing. Neonatal circulation: Body tissues > SVC and IVC > right atrium > right ventricle > pulmonary arteries > lungs > pulmonary veins > left atrium > left ventricle > aorta Fetal circulation remnants: Umbilical artery = medial umbilical ligaments Umbilical vein = ligamentum teres hepatis Ductus venosus = ligamentum venosum Foramen ovale = fossa ovalis Ductus arteriosus = ligamentum arteriosum