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Atria is a large part of the heart so too big for cells; arteries & veins lead away or to the heart.
Capillaries are the smallest vascular structures that allow materials to be exchanged easily between cells.

Answer:

Capillaries

Explanation:

Capillaries are the smallest blood vessels in the human body. They connect arterioles (smallest arteries) to the venioles (smallest veins). They are made of inner endothelial layer and outer epithelial layer, thus they are only two layers thick. They play important role in circulation. The capillaries transport gasses, fluids, and nutrients in the body tissues and also facilitates exchange of gases (oxygen and carbon dioxide), diffusion of fluids and nutrients into the body tissues and transport of waste materials.    

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