How the Body Works : Microanatomy of the Lungs

Presented here is the microanatomy of the lungs and its components. Alveolar sacs are the terminal portions of the breathing tube system. Composed of up to thirty tiny air pouches, or alveoli, they provide a lung a total surface area of about 850 square feet for the exchange of gases. Alveolar walls, composed of a single layer of cells, and the thin boundaries of capillaries---a barrier only one twenty-five thousandth of an inch thick---are all that separate blood and air. Exchange of carbon dioxide and oxygen across this barrier is completed in only a fraction of a second. Terminal bronchioles carry air to and from alveolar sacs. Cells lining bronchiole walls bear cilia---minute hairs whose beating action prevents large dust particles from entering the alveolar sacs. Macrophages on the inner alveolar surface ingest and destroy dust, soot and other foreign particles. These cells are the body's main line of defense against airborne bacteria drawn into the lungs. Connective tissue fibers form a frame around the alveoli and support their delicate walls. Lymph vessels carry excess tissue fluid away from alveoli. Pulmonary venules take oxygen-rich blood back to the heart. Pulmonary arterioles conduct carbon dioxide rich, oxygen depleted blood to the aveoli. Blood capillaries form a meshwork around the aveoli. As blood flows through the capillaries its loses carbon dioxide and takes up oxygen. and changes from dark to bright red in color.