Final answer:
The assertion that all veins carry only oxygen deficient blood is false because pulmonary veins carry oxygen-rich blood from the lungs to the heart, and the umbilical vein carrying oxygenated blood is another exception in fetuses.
Step-by-step explanation:
The statement "Veins carry only oxygen deficient blood" is false. While most veins do carry deoxygenated blood, there are important exceptions in the circulatory system. Notably, the pulmonary veins are the vessels that return oxygen-rich blood from the lungs back to the heart.
During fetal development, there is also the unique case of the umbilical vein, which carries oxygenated blood from the placenta to the developing fetus. The circulatory system is designed to ensure that different areas of the body receive blood with the appropriate level of oxygenation. The superior vena cava and the inferior vena cava, for example, do carry deoxygenated blood back to the heart, but they are part of the systemic circulation, not the pulmonary circulation where the oxygen exchange occurs in the lungs.