1. APPEARANCE
The CVS is the First major system to function in the embryo.
The primordium of the heart Begins at (18) days.
It appears as: Aggregation of Splanchnic Mesenchymal cells in the Cardiogenic Area (ventral to the pericardium).

2. ANGIOBLASTIC CORDS
The cells arrange themselves to form: Two Longitudinal Cellular Cardiac Primordia.
This is under the influence of the embryonic endoderm.

3. ENDOCARDIAL HEART TUBES
The cords canalized and become thin walled.
As a result of lateral folding:
The tubes approach each other and begin to fuse.

4. SINGLE HEART TUBE Fusion of the tubes begin:
At the Cranial end of the developing heart.
It extends Caudally.
A Single Endocardial Tube is formed.

5. HEART BEATS The First Beats are detected at: 22- 23 Days. Blood Flow:
Starts during the 4th week.
This can be detected by Doppler ultrasonography.

6. PERICARDIUM It is part of the Intraembryonic Celomic Cavity.
It lies over the Horse Shoe –Shaped Cardiogenic area.

7. POSITION The heart and pericardial cavity come to lie:
Ventral to the foregut.
Caudal to the oropharyngeal membrane.
This is a result of head folding.

8. MESOCARDIUM
The cardiac tube is attached to the dorsal side of the pericardial cavity by
Dorsal Mesocardium.
Ventral Mesocardium. is Never formed.

9. PERICARDIAL SINUSES Transverse Sinus:
It is between the right and left sides of the pericardial cavity.
It is formed by:
Degeneration of the Central part of the Dorsal Mesocardium.

10. STRUCTURE OF THE WALL Endocardium: It is the Endothelial tube.
Primordial Myocardium:
It is the Splanchnic Mesoderm surrounding the pericardium.
Cardiac Jelly:
A gelatinous layer which separates the two.
Epicardium :
Derived from cells in the external surface of the sinus venosus.

11. CARDIAC LOOP It elongates and forms Constrictions and Dilatations :
Truncus arteriosus.
Bulbus cordis.
Ventricle.
Atrium.
Sinus venosus.

12. ROTATION OF CARDIAC LOOP
The Bulbus Cordis and Ventricle grow faster than the others.
The cardiac loop bends upon itself.
The Cranial part bends:
Ventrally, Caudally and to the Right.
The Atrial portion shifts :
Dorso Cranially and to the Left.

13. BULBOVENTRICULAR LOOP
It is U-shaped.
It has :
Cranial Arterial End:
Truncus Arteriosus.
Aortic sac.
Aortic Arches.
It is fixed through the pharyngeal arches.
Caudal Venous End
It is fixed to the Septum Transversum.

14. SINUS VENOSUS A separate chamber. It opens into:
The center of the dorsal wall of the primitive atrium.
It has:
Right and Left Horns of the same size.

15. TRIBUTARIES OF HORNS Each Horn receives (3) Veins:
1. Common Cardinal (from the embryo).
2. Umbilical (from the placenta).
3. Vitelline (from the yolk sac).

16. CHANGES OF S.VENOSUS The Sinoatrial Orifice moves to the Right
Because of:
Obliteration of the following veins:
Right Umbilical.
Left Vitelline.
Left Common Cardinal.

17. FATE OF S. VENOSUS The left horn becomes : Coronary Sinus.
The right horn: Incorporated into the Right Atrium.

18. RIGHT ATRIUM It has: Smooth part: Incorporated from Sinus Venarum
Rough trabeculated part:
Derived from the primordial atrium.
The two parts are demarcated by
The Crista and Sulcus Terminalis (internally and externally).

19. SINOATRIAL ORIFICE
Has right and left venous valvular folds (venous valves).
Right Valve:
Cranial part : Crista Terminalis
Caudal part : valves of Coronary Sinus and IVC.
Left Valve : Incorporated into the Interatrial Septum.

20. LEFT ATRIUM The Rough Trabeculated Left Auricle:
From the primordial atrium.
The smooth part: From incorporation of the primordial Pulmonary Vein.

21. PULMONARY VEINS The (4) pulmonary veins are formed from: Gradual
incorporation of the pulmonary vein and its main branches into the wall of the expanded atrium.