1. The airway (the channel that transmits gas) of the lungs is like an inverted tree. The main trunk is the trachea, which sends out many branches, the thinnest of which is called the fine bronchus. The fine bronchioles are divided into clusters of air sacs called alveoli. Each of these small air sacs (alveoli) is surrounded by tiny blood vessels (capillaries). 2. The normal lung is an elastic, spongy organ made up of many of these air sacs. The tissue that connects and supports these small air sacs is called the interstitium. Imagine a barrel filled with balloons, the balloons are the alveoli or air sacs, the place where the balloons touch each other represents the “interstitium”. Oxygen (O2) from the outside air is conducted through the airways to the alveoli by breathing, and then enters the bloodstream through the “gap” between the alveoli and the capillaries (the so-called interstitium), while waste products such as carbon dioxide (CO2) leave the bloodstream in the opposite direction through this gap and are excreted out of the body.