Complex sentence: After the infant takes a few breaths of air, when born, the blood that fills the left atrium closes the hole up because the left arterial pressure is greater than that of the right arterial pressure.
Split into simpler sentences: After the infant takes a few breaths of air, when born, the blood that fills the left atrium closes the hole up. The reason for this is because the left arterial pressure is greater than that of the right arterial pressure.