The heart development and foetal circulation
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The primitive heart arises from what embryological tissue?
Describe the first stage of the primitive heart
The primitive hear arises in the mesoderm from the paired endocardial tubes, which are lined by endothelium. Two of these tubes fuse and become enclosed by thickened splanchinic mesoderm.
Describe the formation of the sigmoid heart
The primitive heart elongates, becomes anchored cranially and caudally and begins to form an s-shaped loop. The loop is the primitive ventricle.
Further elongation causes the heart to bend to the left/right and then dorsally/ventrally?
How do we change from a two chambered to four chambered heart?
The opening between the atrium and ventricles is divided into two openings by endocardial cushions. The inter ventricular septum grows from the floor of the ventricle upwards to divide the single ventricle into two. The interatrial septum comes down from the roof of the atrium to divide it into two atria.
What forms to separate the truncus arteriosus into the pulmonary trunk and aorta?
A spiral septum
What is the name of the hole in the atria that is present throughout foetal circulation?
What is the function of the foramen ovale?
This allows blood to be shunted from the right to left atria to bypass the pulmonary circulation, as the lungs are not functioning in a foetus
What vessel brings nourished, oxygenated blood to the foetus?
The umbilical vein
What foetal shunt is present in the liver?
The ductus venosus
What is the function of the ductus venosus?
It allows blood in the umbilical vein to bypass the liver and join the caudal vena cava
Describe blood flow through the foetal heart
Blood enters the right atrium from the caudal vena cava. It moves from the right to left atria, into the left ventricle and then out of the heart via the aorta.
What structure separates blood flow from the cranial and caudal vena cava into the right atrium?
An intervenous tubercle
What happens to blood that enters the right ventricle?
It exits the heart via the pulmonary trunk but is shunted into the aorta via the ductus arteriosus
Why does mixing of the oxygenated blood from the aorta and deoxygenated blood from the ductus arteriosus not result in problems with oxygen supply to different tissues?
The common carotids (branch of the aorta) go up to supply the brain with oxygen before mixing occurs with the blood from the ductus arteriosus, other tissues can cope with the lower oxygen tension of the mixed blood.
Describe the changes that occur in circulation when an animal is born
When an animal takes its first breath the lungs expand and blood now travels to the lungs to be oxygenated. This means the ductus arteriosus must close very quickly (within 2-3 days). It becomes the ligamentum arteriosum. At the same time the foramen oval has to close to stop shunting of blood between atria. The bigger volume of blood in the left ventricle (coming back from the lungs) causes the foramen oval flap to close and over time this flap seals against the wall of the atria. It becomes the fossa ovalis. The ductus venosus must also close.
What are the three main categories of congenital heart defects?
- Failure of closure of foetal structures (foramen ovale, ductus arteriosus, ductus vensosus)
- Septal defects
- Great vessel defects (dysplasia, stenosis, malposition/fusion of great vessels)
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