Why does the left side of the heart carry deoxygenated blood?

lemmie just give you the whole story:

we breath in oxygen which goes to our lungs and diffuses into the blood in our pulmonary vein. this vein carries the oxygenated blood to the left atrium of our heart, then down to the left ventricle. This part of the heart is v. strong, as it then pumps the blood through the aorta to the muscles all over our body, which then take the oxygen to respire. From out body, the deoxygenated blood goes through the vena cava to the right atrium of our heart, then to the right ventricle that pumps that DEOXYGENATED blood up to the lungs to go all over again.

hope i helped :)