The answer is that <span>all
veins carry blood back to the heart, except for the pulmonary vein. the
vena cava empty into the right side of the heart, so they technically
bring blood to the
</span>main atrium.
I believe that the answer is: Most organisms are made up of one or more cells Cell Theory was first developed in 1839 by Schleiden and Schwann, they stated that all organisms are composed of one or more cells.
