text: why are capillaries from the circulatory system small and thin ? it is based on the circulatory system in your body. capillaries are extremely small vessels located within the tissues of the body that transport blood from the arteries to the veins. capillary walls are thin and are composed of endothelium (a single layer of overlapping flat cells). oxygen, carbon dioxide, nutrients and wastes are exchanged through the thin walls of the capillaries. capillaries are so small that red blood cells can only travel through them in single file.