At the end of each of the many tiny branches of our bronchial tree, we find openings to microscopic sacs. Each little sac is an alveolus, singular for alveoli. There may be several alveoli coming from one duct, forming a little clump. These groups of alveoli somewhat resemble a cluster of grapes that are all attached. It is in the alveoli that one of the most important transfers in our entire body takes place. It is here that the respiratory system comes into direct contact with the circulatory system, or blood vessels.