Question

need to know both a and b

A. discuss the embryonic development of the heart (be sure to mention the roles of mesoderm, cardiogenic area, cardiogenic cords, endocardial tubes, primitive heart tube, and the 5 regions of the primitive heart tube.) (truncus ateriosus, bulbus cordis, primitive ventricle, primitive atrium.)

B. compare fetal circulation with adult circulation, be sure to list the various shunts thst much close immediately after birth.

Answer 1

The heart is mesodermal in origin. It develops from primitive heart tube, which forms from mesenchyme in the cardiogenic area of the embryo. This tube forms the endocardium of the heart. The splanchnic mesoderm surrounding the primitive heart tube forms myocardium and epicardium.

The mesenchymal cells in the cardiogenic area located ventral ( anterior) to the developing pericardial cavity fuse to
form two angioblastic cords called cardiogenic cords.

These cords get canalized to form two endothelial heart tubes.

These tubes fuse with each other in a craniocaudal ( from above to down)
direction to form a single primitive heart tube.

However, the caudal ends of two heart tubes fail to fuse with each other. As a result, the caudal end of the heart tube remains bifurcated.

The heart tube forms five dilatations. From cranial-
to-caudal end, these are as follows:
1. Truncus arteriosus
2. Bulbus cordis
3. Primitive ventricle
4. Primitive atrium
5. Sinus venosus

Fate of the embryonic dilatations of the
primitive heart tube

Embryonic dilatation   Adult derivatives
1. Truncus arteriosus---->Ascending aorta
                                            Pulmonary trunk.

2. Bulbus cordis ----->Smooth upper part        of the right ventricle (conus arteriosus)
Smooth upper part of the left ventricle
(aortic vestibule).

3. Primitive ventricle ---->Trabeculated part of the right ventricle
Trabeculated part of the left ventricle.

4. Primitive atrium ---->Trabeculated part of the right atrium
Trabeculated part of the left atrium.

5. Sinus venosus ---->Smooth part of the right atrium (sinus
venarum)
Coronary sinus
Oblique vein of the left atrium.

The fetal circulation is different from that of adult circulation.
There are three basic reasons for this difference:

1. Blood in the fetus is oxygenated by placenta and not by lungs.

2. During fetal life, the lungs are collapsed, hence the resistance to blood flow through the lung is much higher. As a
result, only minimal amount of blood passes through the lungs to supply oxygen and nutrients to the lungs.

3. Portal circulation is of little significance.

Fetal circulation
*Before birth, the oxygenated blood (80% saturated with O2) from the placenta returns to the fetus by the left umbilical vein.In the liver, the left umbilical vein joins the left branch of portal vein. Here, most of blood from left umbilical vein bypasses the sinusoids of the liver by passing through ductus venosus—a channel that connects portal vein to the inferior vena cava.

After a short course, the inferior vena cava ---->the right atrium. In the inferior vena cava, richly oxygenated blood mixes with the deoxygenated blood returning from the lower limbs. Thus, most of blood from the inferior vena cava passes through foramen ovale into the left atrium.Small amount of blood remains in right atrium and it mixes with the deoxygenated blood from the superior vena cava and passes to the right ventricle.

Blood from the right ventricle passes through pulmonary trunk, and right and left pulmonary arteries.Due to high resistance in pulmonary tissue during fetal life only a small fraction of blood enters the pulmonary circulation, and most of it passes into the aorta through the ductus arteriosus (DA)—a channel that connects the left pulmonary artery to arch of aorta.

The left atrium receives mainly rich oxygenated blood from the right atrium through the foramen ovale. Only a small amount of deoxygenated blood enters the left atrium from the lungs through the pulmonary veins. Blood from the left atrium passes to the left ventricle. Left venticle to Aorts Thereafter the arch of aorta receives poorly oxygenated blood from the pulmonary trunk through the ductus arteriosus. . The lower part of the body is thus supplied with relatively less oxygenated blood as compared with the upper part of the body.

Fetal structure       
1.Umbilical vein forms Ligamentum teres

2. Ductus venosus forms Ligamentum venosum

3. Foramen ovale forms Fossa ovalis

4. Ductus arteriosus forms Ligamentum arteriosum

5. Right and left umbilical
arteries forms
•Superior vesicle arteries
• Medial umbilical ligament