The terminal bronchi and alveoli are located at the very end of the conducting zone and the beginning of the respiratory zone in the respiratory system. The bronchi (or bronchus) are the air passages into the lungs that begin at the end of the trachea. There are two bronchi, one for each lung. The bronchus divide into smaller branches known as segmental bronchi, which divide again into bronchioles, and then again into terminal bronchioles.
Each terminal bronchiole separates to create respiratory bronchioles that have alveoli, which are small, balloon-like sacs at the end of the small air passages in the lungs (the bronchiole). Oxygen is inhaled and absorbed into the bloodstream through the thin walls of each alveolus, by way of the pulmonary veins. Carbon dioxide from the pulmonary artery is exhaled as a waste product of the lungs.
The greater the surface area the lungs have for gas exchange, the greater is their efficiency to absorb oxygen. The 700 million (or more) alveoli found in both lungs, if flattened out, would cover an area of some 50-100 square yards. This is approximately the size of a tennis court, and is all neatly folded and bundled into the chest cavity. Each alveolus has a wall that is only one cell thick. A capillary wall has about the same thickness. The distance between air and blood is about 1/1000th of a millimeter. The oxygen is transported by the red blood cells, which squeeze single file through the pulmonary capillaries. Red cells are packed with hemoglobin, or red pigment, that attracts the oxygen. Carbon dioxide is diffused in the same way back through the capillaries and alveolar walls to be exhaled.
The enormous surface area of the alveoli and the short diffusion distance between alveolar air and capillary blood quickly allows the blood to achieve equilibrium with gases of the alveolar air. This function is further increased by the fact that each alveolus is surrounded by a capillary network so extensive that it forms an almost continuous sheet of blood around each alveolus.