Lungs and Respiration

Function. During a normal day, we breathe nearly 25,000 times. The more than 10,000 liters of air we inhale is mostly oxygen and nitrogen. In addition, there are small amounts of other gases, floating bacteria and viruses. It also contains the products of tobacco smoke, automobile exhaust, and other pollutants from the atmosphere in varying amounts. The lungs have both "respiratory" and "nonrespiratory" functions. The respiratory function of the lungs is "gas exchange." This is the term for the transfer of oxygen from the air into the blood and the removal of carbon dioxide from the blood. The nonrespiratory functions of the lungs are mechanical, biochemical, and physiological.

The lungs provide the first line of defense against airborne irritants and bacterial, viral, and other infectious agents. They also remove volatile substances and particles of matter generated within the body. The lungs control the flow of water, ions, and large proteins across its various cellular structures. Together with the liver, they remove various products of the body's metabolic reactions. The lungs also manufacture a variety of essential hormones and other chemicals that have precise biological roles.

Structure. The lungs are shaped like cones and textured like a fine-grained sponge that can be inflated with air. They sit within the thoracic cage where they stretch from the trachea (windpipe) to below the heart. About 10 percent of the lung is solid tissue, the remainder is filled with air and blood. This unique structure of the lung is delicate enough for gas exchange and yet strong enough to ma in-tam its shape and enable it to perform the many functions vital for keeping us healthy. Two "plumbing" systems, the airways for ventilation (exchange of air between the lungs and the atmosphere) and the circulatory system for perfusion (blood flow), are coordinated by special muscles and nerves.

This arrangement enables the lung to perform its primary function of rapidly exchanging oxygen from inhaled air with the carbon dioxide from the blood. Air enters the body through the nose or the mouth, and travels down the throat and trachea into the chest through a pair of air tubes called bronchi (plural for bronchus). The bronchi divide and subdivide into successive generations of narrower and shorter branching tubes of unequal length and diameter. The final destination for inhaled air is the network of about 3 million air sacs, called alveoli, located at the ends of the lungs' air passages.

Between the trachea and alveoli, the lungs look like an inverted tree. The first (main) branching of the trachea leads to the left and right lungs. The two lungs fill most of the chest cavity. Between the lungs are located the heart, the major blood vessels, the trachea, the esophagus (tube leading from the throat to the stomach), and lymph nodes. The thorax (chest wall) surrounds and supports the lungs. Movement of the air into the lungs is controlled by the respiratory muscles of the thorax. These muscles, collectively called the ventilatory apparatus, include the diaphragm (the muscle that separates the chest and abdominal contents) and the muscles that move the ribs. When the respiratory muscles contract, the chest enlarges like a bellows sucking in air (inhalation). As air fills the lungs, they expand automatically. The lungs return to their original (resting) size when we exhale.

The performance of the ventilatory apparatus is coordinated by specific nerve sites, called respiratory centers, located in the brain and the neck. The respiratory centers respond to changes in oxygen, carbon dioxide, and acid levels in the blood. Normal concentrations of these chemicals in arterial blood are maintained by changing the breathing rate. The right lung is slightly larger than the left lung and is divided into three sections or lobes; the left lung has only two lobes. Each lobe is subdivided into two to five bronchopulmonary segments. The segments are further subdivided into lobules served by smaller branches of the bronchi.

The outside of the lung and the inside of the chest cavity are lined by a single continuous membrane called the pleura. The portion of the pleura surrounding the lungs is called the visceral pleura, while the portion lining the chest cavity is called the parietal pleura. The potential space between the lungs and the inside of the chest cavity is called the pleural space or pleural cavity. The pleural space is moistened with a fluid that lubricates the pleurae as they slide back and forth on each other during ventilation.

Normally the pleural space contains only a small amount of fluid and is free of any gas, blood, or other matter. Blood vessels, bronchi, and nerves come together at the entrance of the lung called the hilum. Bronchopulmonary lymph nodes, important for the drainage of the lungs, are located here. The extensive nervous system of the lungs extends from the hilum to almost all of the lungs' structural units.

The Conducting Airways

The first 16 subdivisions of the bronchi ending in terminal bronchioles are called the conducting airways. Terminal bronchioles are the smallest airways without alveoli. They further divide into respiratory bronchioles, ending in alveolar ducts. Respiratory bronchioles have occasional alveoli budding from their walls, while alveolar ducts are completely lined with alveoli. The last seven branchings of the bronchioles where gas exchange occurs are called the respiratory zone. The terminal respiratory unit of the lung from the respiratory bronchiole to the alveolus is called the acinus.

exchange between inhaled air and blood takes place in the alveoli. Blood is brought to the alveoli through a fine network of pulmonary capillaries where it is spread in a thin film. The barrier separating the air and blood is extremely thin, 50 times thinner than a sheet of tissue paper. A large surface area (80 square meters, as large as a tennis court) is available for gas exchange. In the resting state, it takes just about a minute for the total blood volume of the body (about 5 liters) to pass through the lungs. It takes a red cell a fraction of a second to pass through the capillary network. Gas exchange occurs almost instantaneously during this short period.

Infection results when there is alteration in normal host defense mechanisms or diminution in the general immune status of an individual, or when an immunocompetent individual is exposed to a virulent organism which overwhelms the host defenses.

"The Lungs in Health, and Disease" National Institute of Health, National Heart, Lung, and Blood Institute.

Lung Disease

• Every year, close to 361,000 Americans die of lung disease. Lung disease is America's number three killer, responsible for one in seven deaths.
• Lung disease is not only a killer, most lung disease is chronic. More than 25 million Americans are now living with chronic lung disease.
• 84.8 million Americans -- children and adults -- have some form of chronic respirators disease. 25.6 million of these suffer from such lung diseases as emphysema, asthma and chronic bronchitis. Center for Diseases Control and Prevention.