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When does the circulatory system begin to form?

Begins to develop and function by 4th week.


What is the difference between the adult and foetal cardiovascular system?

  1. The placenta is a low resistance, high flow pathway and site of gasm nutrient and waste exchange. 
  2. Lungs not breathing air, fluid filled, high resistance. 
  3. Two right ot left shunts of blood (foramen ovale and ductus arteriosus). 
  4. RV and LV act in parallel - both pump into the systemic circulation (RV via ductus arteriosus, a right to left shunt).


Umbilic cord


  • Umbilical arteries: which carry embryonic/foetal blood to the placenta. From internal iliac aa. 
  • Umbilical veins: returning embryonic/foetal bloof to the embryo/foetus. To liver and ductus venosus. 

Foetal and maternal blood fo not mix, but exchange gases, nutrients and wastes. 



Foramen ovale

In interatrial septum. 

Oxygenated blood from IVC directed by valve of IVC from RA through foramen ovale to LA. 


Ductus arteriosus

Blood from pulmonary trunk to aorta, just past L subclavian a. 


From which layer of the embryo does the CVS develop?

Almost entirely from mesoderm. 


Overview of timeline of heart development

Late 3rd - early 4th week: cardiogenic field in mesoderm forms paired endocardial tubes, which fuse in midline as the embryo folds to form heart tube, which begins to beat. 

4th week: heart tube folds into a shape that resembles adult heart (ventricle anterior, atrium posterior). 

5th - 8th weeks: septa, valves develop, separating heart into 4 chambers. 

9th week: mature foetal form achieved. 


Segments of the heart tube (4th week)

  • (veins)
  • Sinus venosus (caudal end of heart)
  • Primitive atrium
  • Primitive ventricle
  • Bulbus cordis (cranial end of heart)
    • Caudal part of bulbus cordis
    • Conus cordis
    • Truncus arteriosus
  • (arteries)


Adult derivative of sinus venosus

Part of right atrium, coronary sinus


Origin of left atrium

Most from pulmonary veins


Embryological derivative of primative atrium

Right and left auricles. 


Embryological derivature of primative ventricle

Most of left ventricle, part of right ventricle. 


Embryological derivature of bulbus cordis

Caudal part - most of right ventricle.

Conus cordis - outflow tracts of ventricles. 

Truncus arteriosus - pulmonary trunk, ascending aorta. 


Formation of cardiac loop. 

4th week.

  • Bulbus cordis (cranial end) moves ventrally, caudally and to the right. 
  • Atrium (caudal end) moves dorsally, cranially and to the left. 
  • Atrial and venous ends approximate. 




Heart tube rotates in opposite direction, resulting in mirrow image of normal (heart moves on right side). 

Isolated dextrocardia often associated with transposition of the great vessels. 


Situs inversus

All thoracic and abdominal organs mirrow image of normal - usually normal functioning. 


Division of the heart tube into chambers

Single heart tube becomes chambered heart in the 5th - 8th weeks

  • Atrioventricular valves separates atria from ventricles
  • Interatrial septum - separates RA adn LA
  • Interventircular septum - separates RV and LV


Interatrial septum formation 

Septum primum

  • Foramen (osteium) primum
  • Foreamen (ostium secundrym)

Septum secundum forms definitive interatrial septum. 


Foramen ovale

Opening in septum secundum between RA dn LA; septum primum serves as a one-way valve. Blood from IVC directed by valve of IVC in RA through foramen ovale to LA. 


Atrial septal defects

Septum primum or secundum defects can lead to patent foramen ovale (stays open) after birth. This results in shunt of blood from higher to lower pressure side after birth (LA to RA). 


Atrioventricular canal and outflowo tract realignment

  1. Primitive atrium opens into primitive ventricle (future LV) via atrioventricular (AV) canal. 
  2. Caudal bulbus cordus (future RV) to conus cordis (outflow tract) to truncus arteriosus (future aorta and pulmonary artery)
  3. AV canal moves to the right and outflow tract to left - so centred on LV/RV


Interventricular septum formation

The interventricular septum divides primitive ventricle and caudal bulbus cordis into LV and RV. 

  • Muscular interventricular septum: grows cranially from near the apex - largest part of septum. 
  • Endocardial cushions help form membranous interventricular septum (near AV valves). 


Ventricular septal defects

Most common 


Development of AV valves

Endocardial cushions (EcC): swellings of subendocardial mesenchyme atrioventricular (AV) canal region. 

4: right, left, superior/anterior, inferior/posterior

Divide AV canal into right and left AV canals

From the AV valves and the membranous IV septum.


Persistent common AV canal

Separate right and left atrioventricular valves do not form, due to endocardial cushion problems. 

Atrial and ventricular septal defects occur with this abnormality.


Division of conus arteriosus and truncus arteriosus

Conotruncal cushions (also called bulbar ridges) and endocardial cushions contribute. 

Form conotruncal septum which separates:

  • Conus cordis into RV and LV outflow tracts
  • Truncus arteriosus into pulmonary trunk and ascending aorta


Aortic and pulmonary valve development

Formed by conotruncal cushions. 


Endocardial and conotruncal cushion problems

Incorrect development of endocardial and conotruncal cushions can lead to a variety of problems such as:

  • Transposition of the great vessels
  • Persistent truncus arteriosus with VSD
  • Tetralogy of the flalot
  • Stenotic valves


Transposition of the great arteries (TGA)

  • Left ventricle to pulmonary trunk
  • Right ventricle to aorta 


Persistent truncus arteriosus with VSD

Common pulmonary trunk/ascending aorta

Mixing of pulmonary and systemic circulations