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Finding its way into the smallest divisions of the lungs, called the alveoli, the air comes very near a large surface of blood. By this means the carbon dioxide diffuses out of the blood, and the free oxygen enters.
These columns are enclosed in spaces termed alveoli which are probably dilated lymph spaces, and which communicate freely with the lymph vessels. The cells composing the columns and filling the alveoli vary with the character of the epithelium in which the cancer originates.
In the lobule the air tube divides into a number of smaller tubes, each ending in a thin-walled sac, called an infundibulum. The interior of the infundibulum is separated into many small spaces, known as the alveoli, or air cells. The lungs are remarkable for their lightness and delicacy of structure.
If one of the smallest bronchial tubes be traced in its tree-like ramifications, it will be found to end in an irregular funnel-shaped passage wider than itself. Around this passage are grouped a number of honeycomb-like sacs, the air cells or alveoli of the lungs. These communicate freely with the passage, and through it with the bronchial branches, but have no other openings.
By this arrangement the air within the alveoli is brought very near a large surface of blood, and the exchange of gases between the air and the blood is made possible. It is at the alveoli that the oxygen passes from the air into the blood, and the carbon dioxide passes from the blood into the air. At no place in the lungs, however, do the air and the blood come in direct contact.
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