Answer: from the pulmonary veins into the left atrium.
Explanation:
After oxygenation of blood in the lungs, the right and left pulmonary veins carry oxygenated blood and empties it into the left atrium. The left atrium then empties the blood into the left ventricle through the left atrioventricular valve.
With contraction, the ventricle pumps the blood to the systemic circulation through the aorta. The blood flow into the aorta is guarded by the aortic valve.
