Fetal Circulation: (cont.)
Forty-four units pass into the right atrium proper, while 26 units go directly through the foramen ovale to the left atrium. A small flow from the coronary sinus (3 units) and a larger flow from the superior vena cava (20 units) also enter the right atrium, making a total of 67 units. This blood then enters the right ventricle and is pumped out through the pulmonary artery (Figure 2.13).
Because pulmonary vascular resistance is high, only 7 units flow to the lungs, the remaining 60 units being shunted to the descending aorta through the ductus arteriosus. This blood of somewhat lower oxygen content perfuses the lower body for the most part.
The blood which passed through the foramen ovale into the left atrium is joined by 7 units of blood returning from the lungs. This lowers the P02 and oxygen content to some extent. This blood (33 units) is then pumped out of the left ventricle and into the aorta. It enters the subclavian, carotid, and coronary arteries and mainly perfuses the head, neck, upper body, and myocardium. About one-third finds its way to the descending aorta.
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