Cardiac Embryology & Fetal Circulation PDF

CARDIAC EMBRYOLOGY & FETAL CIRCULATION  BASIC CARDIAC EMBRYOLOGY  The heart is the first organ to function within an embryo.  It forms from a pair of two heart tubes The cardiac tube itself consists of three layers: epicardium, myocardium and endocardium. The outermost layer and bou...

CARDIAC EMBRYOLOGY & FETAL CIRCULATION  BASIC CARDIAC EMBRYOLOGY  The heart is the first organ to function within an embryo.  It forms from a pair of two heart tubes The cardiac tube itself consists of three layers: epicardium, myocardium and endocardium. The outermost layer and boundary of the pericardial cavity is the epicardium. The myocardial mantle follows as the next inner layer. Together they form the myoepicardium. The considerable distance from the myocardial mantle to the endocardial tube is filled with cardiac jelly and the cardiac lumen is coated with endocardial cells.  WEEK 3  Heart tubes are forced to the midline into a single endocardial heart tube composed of the following parts:  aortic sac  truncus arteriosus  bulbus cordis  interventricular sulcus  primitive ventricle  atrioventricular sulcus  primitive atrium  sinus venosus WEEK 3 WEEK 3 1) sinus venosus  SVC, IVC, pulmonary veins, CS, posterior portion of RA & LA 2) primitive atrium  RA & LA 3) atrioventricular sulcus  divides primitive Cranial atrium end & primitive ventricle 4) primitive ventricle  apical portion of LV & inlet portion of RV 5) interventricular sulcus  divides primitive ventricle & bulbus cordis 6) bulbus cordis  apical portion of RV, RVOT & LVOT, AO, main PA 7) AO sac will rise the aortic arch Caudal  SINUS VENOSUS The sinus venosus is responsible for the inflow of blood to the primitive heart. It empties into the primitive atrium and receives venous blood from the right and left sinus horns. Vitelline veins - return poorly oxygenated blood from the yolk sac (The yolk sac lies outside the embryo connected by a yolk stalk to the midgut of the fetus) Umbilical veins - carry oxygenated blood from the placenta Common cardinal veins - return poorly oxygenated blood from the body of the embryo  SINUS VENOSUS The sinus venosus & primitive atrium SINUS VENOSUS communicate via sinoatrial orifice The sinus venosus has a right sinus horn and a left sinus horn The Left horn forms the coronary sinus The right horn becomes absorbed into the right WEEK 4 CARDIAC LOOPING Once the heart tubes are completely fused the heart begins to beat on day 22 (Terry Reynolds Bk says day 23) The heart grows rapidly, forcing the tube fold into an s-shaped loop It folds to the right dextro looping(d-looping) The bulboventricular loop is formed  CARDIAC LOOPING Around day 23 of development. The bulbus cordis moves 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 Aortic arch & arteries  THE AORTIC ARCH & ARTERIES  In the fetus, simultaneous with cardiac looping is a process which results in the formation of six arches which make up the aortic arch system. These arches develop from the aortic sac portion of the primitive heart tube and come and go with the ever changing structures of the fetal heart. These six arches are functional although only arches number three, four, and six persist in the adult heart and become the following:  Number three becomes the carotid arteries  Number four forms the aortic arch  Number six forms the pulmonary arteries and ductus arteriosus WEEK 4 The septum primum grows from the wall of primitive atrium & descends towards the endocardial cushions The ostium primum (the opening) remains  Growth of the endocardial cushions within the atrioventricular canal separates the canal from the left & right sided orifices The MV & TV begin to form The ventricular septum appears as a small ridge on WEEK 4 (continued)) There is only one great vessel leaving the heart. This is the truncus arteriosus The truncus Pulmonar arteriosus is y trunk situated on the roof of the primitive atrium The aorta and main pulmonary artery will evolve WEEK 5 The septum primum grows downward between the LA & RA The septum primum fuses with the endocardial tissues The septum secundum grows parallel with the septum primum WEEK 5 WEEK 6 Septum secundum ceases to grow and forms the foramen ovale. Superior and inferior endocardial cushions fuse Atrioventricular valve cusps form allowing the right atrium to communicate with the right ventricle and the left atrium to communicate with the left ventricle. WEEK 7 Remnant of septum primum acts as flap at Foramen Ovale (FO) & controls blood flow between atria At birth, LAP increases forcing septum primum against the septum secundum closing FO The coronary sinus is formed CORONARY SINUS A venous channel about 2cm in length that receives blood from the coronary veins and empties into the right atrium. WEE K7 The final step of ventricular septation takes place when secondary interventricular foramen is completely obliterated by membranous septum leaving 2 distinct ventricles WEEK 7 Within the truncus arteriosus, bulbar ridges grow toward each other & fuse in the middle The truncus spirals downward towards the ventricles The spiral effect is directly involved in the rotation & division of truncus arteriosus, forming the AO & main PA At same time, AOV & PV cusps are developing FETAL SHUNTS  The fetal circulatory system uses two right to left shunts, (the Foramen Ovale and the Ductus Arteriosus)which are small passages that direct blood that needs to be oxygenated.  The purpose of these shunts is to bypass certain body parts in particular, the lungs and liver that are not fully developed while the fetus is still in the womb.  The other shunt present in fetal circulation is the  THE FORAMEN OVALE In the fetal heart, the foramen ovale is a passageway for blood to enter the left atrium from the right atrium. Ductus Arteriosus  Most of this blood is bypassed or shunted away from the lungs through the ductus arteriosus to the aorta  Most of the circulation to the lower body is supplied by blood passing through the ductus arteriosus Fetal shunt Adult remnant Foramen ovale Fossa ovalis Ligamentum Ductus arteriosus arteriosum Ligamentum Ductus venosus venosum Ligamentum teres Umbilical vein (hepatis) FETAL CIRCULATION 1.Oxygenated 2.The ductus venosus this shunt blood & that passes highly nutrients from oxygenated IVC the mothers placenta flows through the umbilical vein 3.blood from the liver to the inferior vena cava 5.This mixes with deoxygenated 4. The enriched blood returning to 6.The IVC blood travels brings through the the heart deoxygenate Eustachian from the d blood from Valve (EV) upper body the lower into the RA through the body 7.Some 8. The of the majority blood in of blood (FO) the RA flows into flows the RA into the through RV the FO and into the LA, then enters the LV 9.From the LV, 10.Blood blood in the travels aorta into the travels Aorta to the body where the oxygen is  11.The blood that is pumped into the RV is then travels into the pulmonary artery Because the lungs are fluid filled instead of air filled, the vessels in the lungs are narrow creating higher resistance of blood flow into Only 8% of blood flow travels out of the PA to the lungs, providing nutrients for the developing lung tissues  12.Due to the high resistance in the pulmonary circulation and the low resistance in the systemic circulation (DA) 14. Then out 13.Blood the aorta to pumped the body into the PA from the RV will flow into the DA 15) After the oxygenated blood nourishes the fetus’s organs and tissues, the deoxygenated blood then enters the umbilical arteries and flows into the placenta to be enriched with oxygen and nutrients DUCTUS ARTERIOSUS (DA) ENRICHED BLOOD 1)placenta enriched blood  umbilical vein  portal system 2)small amount of enriched blood  portal system & mixes with liver’s depleted blood 3)majority of enriched blood  ductus venosus  IVC (depleted blood in IVC mixes with enriched blood from ductus venosus) 4)IVC enriched blood  EV  RA 5)small amount of enriched blood in RA mixes with depleted blood from SVC  RV & PA 6)majority of blood in RA  FO  LA (enriched blood in LA mixes with small amount of blood returning from lungs) 38 7)enriched blood enters LV & AO DEPLETED BLOOD 8)depleted blood  SVC  RA 9)in RA, depleted blood from SVC mixes with small amount of enriched blood from IVC  RV & PA 10)small amount of blood in PA  lungs & returns to LA via PV4 11)majority of blood in PA  DA  AO; depleted blood from DA mixes with enriched blood from proximal AO  abdominal AO, common iliac arteries, internal iliac arteries 12)internal iliac arteries  2 umbilical arteries  placenta 39 AT BIRTH  At birth, the umbilical cord is clamped and the baby no longer receives oxygen and nutrients from the mother.  When the placenta is removed from the babies systemic circulation the systemic vascular resistance begins to rise. With each breath the alveoli in the lungs begins to expand and the surrounding vessels dilate in the response to the presence of oxygen.  Pulmonary pressures will decrease  AFTER BIRTH  Increased pulmonary blood flow increases blood returning to LA the LAP increases and forces FO to close (fossa ovalis).  DA closes almost immediately after birth due to muscular contraction (ligamentum arteriosum).  Umbilical arteries close shortly after birth due to smooth muscle contraction produced by increased O2 levels; AFTER BIRTH  Once the umbilical arteries close, the umbilical vein & ductus venosus follow  The umbilical vein becomes the ligamentum teres  The ductus venosus becomes the ligamentum venosum  AT BIRTH The closure of the ductus arteriosus and foramen ovale completes the transition of fetal circulation to newborn circulation. FAILURE OF SHUNTS CLOSING AFTER BIRTH  If these shunts fail to close after birth and remain open, they are said to be patent (open) PATENT FORAMEN OVALE (PFO) PATENT DUCTUS ARTERIOSUS (PDA) PATENT DUCTUS VENOSUS