Circulatory system

FETAL CIRCULATION

Special Structures in Fetal Circulation
  1. Placenta: where gas exchange takes place during fetal life.
  2. Umbilical arteries: Carry de-oxygenated blood from the fetus to placenta. 2 in number.
  3. Umbilical vein: Brings oxygenated blood coming from the placenta to the fetus. 1 in number.
  4. Foramen Ovale: connects the left and right atrium. It pushes/ shunts blood from the right atrium to the left atrium.
  5. Ductus Venosus: Carry oxygenated blood from umbilical vein to inferior vena cava, by passing fetal liver.
  6. Ductus Arteriosus: carry oxygenated blood from pulmonary artery to aorta, bypassing the fetal lungs.
Flow Chart of Fetal Circulation

Detail:
The umbilical vein carrying the oxygentaed blood (80% saturated) from the placenta, enters the fetus at the umbilical and runs along the free margins of the falciform ligament of the liver. In the liver, it gives off branches to the left lobe of the liver and receives the deoxygentaed blood from the portal vein. The greater portion of the oxygenated blood mixed with some portal venous blood, short circuits the liver through the ductus venosus to enter the inferior venacava and thence to the right atrium of the heart. The O2 content of this mixed blood is then reduced. Although both the ductus venosus and hepatic portal, fetal trunk blood enters the right atrium through the inferior venacava, there little mixing occours. the terminal part of the inferior venacava receives blood from the right hepatic vein.
In the right atrium, most of the well oxygenated (75%) ductus venosus blood is preferentially directed into the foramen ovale by the valve of inferior venacava and crista dividens and passes into the left atrium. Here it is mixed with small amount of venous blood returning from the lungs through the pulmonary veins. this left atrial blood is passed on through the mitral opening into left ventricle.
Remaining lesser amount of blood (25%) after reaching the right atrium via superior and inferior venacava (carrying venous blood from the cephalic and caudal parts of the fetus respectively) passes through tricuspid opening into the right ventricle.
During ventricular systole, the left ventricular blood is pumped into the ascending arch of aorta and distributed by their branches to the heart, neck, brain and arms. The right ventricular blood with low oxygen content is discharged into the pulmonary trunk. Since the resistance in pulmonary arteries during fetal life is very high, the main portion of the blood passes directly through the ductus arterious, into the descending aorta by passing the lungs where it mixes with the blood from the proximal aorta. 70% of the cardiac output (60% from right and 10% from left ventricle) is acrried by the ductus arteriosus to the descending aorta. About 40% of the combined output goes to the placenta through umbilical arteries. The deoxygenated blood leaves the body the way of two umbilical artries to reach the placenta where it is oxygenated and gets ready for recirculation.
The mean cardiac output is comparatively high in fetus and is estimated to be 350ml per kg minute.

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