D. veins carry blood to the heart back from the rest of the body
The circulatory system is a closed system composed of blood, blood vessels, and the heart, whose function is to carry oxygen, nutrients, and other molecules (e.g. hormones) to the cells, and remove waste products (e.g., carbon dioxide). There are three types of blood vessels that have specific roles in the circulation system: arteries, veins, and capillaries.
Veins are blood vessels that transport blood low in oxygen (O2) from the body back to the heart for its reoxygenation. Arteries carry O2 blood away from the heart. Both veins and arteries represent two closed systems of interconnected tubes which begin and end at the heart.
Finally, capillaries are blood vessels that connect arteries with veins, which are involved in the exchange of nutrients and gases (O2 and CO2) from the blood into the body tissues. The arteries deliver the O2-rich blood to the capillaries, and subsequently, capillaries deliver waste-rich blood (i.e., byproducts of cellular reactions) to the veins for transport back to the lungs and heart.
In resume, veins are blood vessels involved in the transport of deoxygenated blood towards the heart.
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