Through the veins, the deoxygenated blood returns to the heart’s right atrium after it is being used up by tissues. When that used up blood leaves the tissue of the body, the veins where it passes through enlarges until it will reach the heart.
Veins that could be seen in the human body tissues are microscopic, but the largest veins are large enough to be visible to the naked eye - it could be seen entering the right atrium.
The venous system somewhat functions reversely from the arterial system. The arteries branch a set of smaller arteries, and the veins which constitute these arteries receive smaller tributaries to the larger veins. Veins coming from the entire body will conjoin into one or two large veins before emptying the heart back. The entrance of the blood to the right atrium is made by the superior and inferior vena cavas. In the human body, it is the veins that are more numerous. These are situated close to the skin, and are called superficial veins. These veins are vital for getting blood and for injection.
Deep veins are running near the arteries, and these are responsible for supplying blood to the body when the veins empty. The veins are named according to their location and the organs in which they supply blood to.