Session 3 - Development of the heart Flashcards
What is the first step in the septation of the heart?
- Divide the heart into left and right channels
- Endocardial cushions develop on the dorsal and ventral sides of the AV canal
- They proliferate and grow towards each other, fusing in the middle forming an island in the middle of the av canal
What is the end result of atrial septation?
-Development of two definitive atria from the primitive atrium
Describe atrial septation
1)Septum primum develops and grows down towards the endocardial island, as the septum grows an opening known as ostium primum develops
2)Before Ostium primum closes by fusion, a second opening develops by apoptosis known as ostium secundum
3)A second, cresent shaped septum then grows down towards the endocardial cushions, known as Septum secundum, and fuses with the island
4)The septum secundum contains an opening known as foreman ovalis
The two septa with their non-aligned holes is the septa of the atria
Describe ventricular septation
1) Large muscular septa grows upwards towards the endocardial island in the av canal from the inferior middle of the primitive ventricle
2) The muscular portion of the septa does not fuse with the island but growth ceases just below it, generating an opening known as the primary interventricular foreman
- The primary interventicular foreman is then closed by the development of the membranous portion of the septum, which grows down from the endocardial cushions and fuses with the muscular septum to complete ventricular septation
What is the name of the outflow septum?
-Conotruncal septum
Describe septation of the outflow tract
1) Endocardial cushion appears on both sides of the truncus arteriosus which are staggered
2) Growth of staggered cushions towards each other produces a spiral septum
3) Outflow tract is now two seperate tubes, one leading to RV (pulmonary trunk) and the other leading to LV (aorta)
How does the spiral septum of the outflow tract result in anatomical location of the PT and aorta?
-The Pulmonary trunk begins on the right, passes over the top of the aorta and finishes on the left
Describe the feotal circulation of deoxygenated blood
- Deoxyblood in SVC and IVC->RA->RV->PT
- Majority from PT->Ductus arteriosus-> Descending Aorta -> body
- Small amount from PT->PA->lungs->PV->LA->LV
Why does only a small amount of blood enter the lungs during foetal circulation?
-The lungs are collapsed which means there is a high resistance to flow so only a small amount enters
Describe foetal circulation of oxygenated blood
-Oxy blood from placenta -> Umbilical vein -> Ductus venosus->IVC -> RA-> streams across to foreman ovale-> LA->LV -> Aorta -> carotids supply brain, descending aorta supplies body
What must happen immediately after birth?
-Respiratory exchange must be established as the supply of oxygenated blood from the placenta has been interrupted
Describe the change in foetal circulation immediately after birth
- As respiration begins, the resistance to bloodflow through the lungs is markedly decreased, whilst the increased oxygen tension causes vasoconstriction of the smooth muscle of ductus arteriosus. This diverts blood from the RV to the pulmonary artery and thus lungs
- The oxygenated blood from the lungs supplies the body
- As the pressure in the LA rises above that of the right the Foreman ovale closes completing partioning of the heart
- The removal of the placenta causes the closure of ductus venosus and regression of the umbilical vein
How does change in LA pressure cause closure of the foreman ovale?
- In foetal life, the pressure in the RA is greater than the LA
- Bloodflow is therefore from right to left, forcing the septa of the heart apart, allowing blood to flow through the foreman ovale
- As LA pressure become greater than RA, septum secundum is forced against septum primum and the specific non-alignment of the holes means blood cannot flow through
- The septa become fused together over time
What becomes of the feotal shunts:
i) Foreman ovale
ii) Ductus arteriosus
iii) Ductus venosus
iv) Umbilical vein
i) Foreman ovalis
ii) Ligamentum arteriosum
iii) Ligamentum venosum
iv) Ligamentum teres hepatitis
What becomes of the following parts of the primitive heart tube:
i) Sinus venosus
ii) Primitive atria
iii) Pulmonary vein
iv) Bulbus cordis
v) Truncus arteriosus
i) Becomes absorbed into RA
ii) Become the auricles of the definitive atria
iii) Becomes absorbed into LA
iv) Becomes the outflow tract and RV
v) Forms the pulmonary trunk and aorta
