Three to five terminal bronchioles and the alveoli that they supply with air form a lobule. Many lobules unite to form the major subdivisions of the lung, called lobes. The left lung has two lobes, and the right lung has three. Each lobe has its own branches of bronchi and blood vessels, so a diseased lobe may be removed without sacrificing the usefulness of the other lobes.
Blood reaches the lung through two routes. Almost all of the blood comes through the pulmonary circulation. This blood has already circulated through the body tissues, where it has given up its oxygen and picked up carbon dioxide. A small amount of blood reaches the lungs through the bronchial circulation. This blood is rich in the oxygen and nutrients that the airway tissues--like all other body tissues--need.
Gas exchange in the lungs. In order to supply oxygen to the blood and remove carbon dioxide from it, the lungs must draw in fresh air and expel stale air. Fresh air is drawn in when the diaphragm and the muscles between the ribs contract. This action--called inspiration or inhalation--makes the chest volume larger and causes the lungs to expand. The expansion creates a slight vacuum in the lungs, and air from the atmosphere flows in. When the muscles relax, the lungs return to a smaller volume, and gas flows out into the atmosphere. This action is called expiration or exhalation.
Blood entering the lungs through the pulmonary circulation is dark-coloured, low in oxygen, and high in carbon dioxide. It is pumped by the right side of the heart into the pulmonary arteries, which lead to the lungs. The pulmonary arteries divide into smaller and smaller blood vessels, ending with pulmonary capillaries in the walls of the alveoli. The alveolar walls are so thin that oxygen and carbon dioxide move through them easily. Oxygen passes from the alveoli to the blood in the capillaries. At the same time, carbon dioxide leaves the blood and enters the alveoli. Once this exchange is completed, the blood becomes bright red and enters the pulmonary venous system. In this system, small vessels join to form larger vessels. The largest vessels, the pulmonary veins, carry blood to the left side of the heart. The oxygen-rich blood is then pumped by the left side of the heart back to the body tissues.
Other jobs of the lungs. Because the lungs must inhale the air from the environment, they are exposed to bacteria, viruses, dust, and pollutants. A sticky fluid called mucus lines the airways and traps most of these foreign substances. Tiny, hair like structures called cilia move together in a wavelike manner that pushes the mucus upward into the throat. There, the mucus and its trapped "invaders" are coughed up or harmlessly swallowed. Some substances do not get caught in the mucus and thus make their way into the alveoli. There, special cells called alveolar macrophages engulf the particles and carry them to the mucus or destroy them.
The lungs also help cleanse the blood of harmful substances. Blood flowing through the capillaries of the lung is filtered so that particles such as blood clots and fat globules are removed. Macrophages and other cells then break up and remove the trapped material.
Another function of the lungs can be compared to the operations of a chemical processing plant. Some cells make a fatty substance called surfactant, which coats the alveoli and allows them to expand easily. Other cells add, remove, or change materials in the blood that affect the function of the lungs or other organs.
Finally, the air exhaled by the lungs is used to make the vocal cords in the larynx vibrate. This action creates the sound necessary for speech.
Diseases of the lungs occur despite the defences provided by the mucus and macrophages. Sometimes, the number of harmful particles reaching the alveoli is so great that the macrophages cannot remove them all. In other cases, the particles can resist or destroy the macrophages.
Because lung diseases can result from many different causes, they are usually grouped by how they affect lung functions. Obstructive lung diseases, such as emphysema, asthma, and chronic bronchitis, cause the airways to become partly blocked or narrower, making it more difficult for air to move through them. Cigarette smoking and air pollution are major causes of these diseases. Restrictive lung diseases make it harder for the respiratory system to expand. They can cause a stiffening of the lung or chest wall or make the respiratory muscles unable to respond to nerve signals. Breathing in substances such as asbestos, silica, and coal dust can cause some types of restrictive disease. Pulmonary vascular diseases affect the circulation of blood in the lungs. For example, in pulmonary hypertension, the small blood vessels of the lung become narrower, making it difficult for the right side of the heart to pump blood.
Some diseases are difficult to categorize because they can harm the lungs in a number of ways. Some pollutants--particularly cigarette smoke--affect the cilia, causing the upward movement of mucus to slow or stop. Smoking cigarettes is also a major cause of lung cancer. Infectious lung diseases, such as tuberculosis and pneumonia, are caused by bacteria, viruses, or other organisms. These diseases are major killers in developing countries.
