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518 W. J. Ott<br />

Fig. 35.2. The oxygenated blood (2) is directed through<br />

the foramen ovale into the left atria, while the deoxygenated<br />

blood (1) passes into the right ventricle<br />

Fig. 35.4. a Gray-scale image of parasagittal scan of a 22-<br />

week fetus using color Doppler. The aorta and inferior vena<br />

cava (IVC) are shown. The arrow points to a segment of<br />

the ductus venosus as it enters the inferior vena cava just<br />

proximal to the right atria. b Doppler velocity flow in the<br />

ductus venosus of the fetus in a. Note the triphasic pattern,<br />

but that the flow is always forward throughout the<br />

cardiac cycle<br />

Fig. 35.3. Well-oxygenated blood (2) is then directed out<br />

the left ventricular outflow tract to the head and brain;<br />

while the deoxygenated blood (1) is directed via the ductus<br />

arteriosus down the aorta to the umbilical arteries for<br />

oxygenation in the placenta<br />

structure bounded on the right side by the Eustachian<br />

valve (or valve of the inferior vena cava) and<br />

on the left side by the foramen ovale flap. The<br />

atrial septum lies above the middle of the inferior<br />

vena cava in a crest-like structure known as the<br />

crista dividens (or septum secundum or limbus<br />

fossae ovalis). The inferior vena cava/foramen<br />

ovale complex can be described as a Y-shaped unit<br />

with a long branch to the left atrium and a short<br />

branch to the right atrium.<br />

2. This anatomical relationship results in two venous<br />

pathways for blood return to the fetal heart from<br />

the placental and lower body circulations. (a) A<br />

right inferior vena cava/right atrium pathway:<br />

blood flow from the right hepatic vein and right<br />

portion of the proximal inferior vena cava is<br />

directed along this pathway. (b) A left ductus<br />

venosus/foramen ovale pathway: blood flow from<br />

the umbilical sinus, ductus venosus, and left portion<br />

of the proximal inferior vena cava is directed<br />

along this pathway. The left and medial hepatic

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