The Lungs—A Marvel of Design
YOU can survive without food for weeks. You can live without water for days. But if you hold your breath, severe discomfort sets in after mere seconds. And just four minutes of oxygen deprivation can cause brain damage and death. Yes, oxygen is the prime need of the human body!
You probably do not have much control over the quality of the air you breathe. Nonetheless, you need air, and you need it now! How do you survive when the air is too cold or too hot or too dry or too dirty? How do you extract life-sustaining oxygen from such air, and how does the oxygen reach every part of your body? How do you rid your body of carbon dioxide, a gas? All of it happens thanks to your marvelously designed lungs.
The Lungs at a Glance
Your lungs are the two main organs of respiration. Ideally located inside your rib cage, they lie on either side of the heart. Your right lung has three sections, or lobes, and your left lung has two lobes. Each lobe is somewhat independent of the others. Because of this, surgeons are able to remove a diseased lobe without sacrificing the usefulness of the other lobes. At first glance the texture of the lung tissue may appear to resemble a sponge.
The lungs reach downward to the diaphragm, a powerful sheet of muscle that separates the chest cavity from the abdominal cavity. The diaphragm is the most important muscle of respiration, contributing to the constant inflation and deflation of the lungs. From the diaphragm, your lungs extend all the way up into the base of your neck. A thin membrane covers each lung. This membrane, or pleura, also lines the inside of the chest wall. The space between the two layers of the pleural membrane is filled with a lubricating fluid. This fluid enables the lungs and the rib cage to slide easily, without friction, during respiration.
Some 25 to 30 different types of cells in the lungs have now been identified by scientists. Various muscles and nerves, bones and cartilage, blood vessels, fluids, hormones, and chemicals all play key roles in the functioning of the lungs. Although certain aspects of the lungs are still not fully understood by scientists, let us acquaint ourselves with some of the many features that are understood.
A “Tree” of Airways
Your respiratory tract is essentially an interconnecting series of tubes and passageways. Before air reaches your lungs, it has quite a journey to make. First, the air flows from your nose or mouth into the pharynx, or throat. The pharynx is used both for the swallowing of food and for breathing. To prevent food and drink from entering your airways, a small movable lid known as the epiglottis blocks the entrance when you swallow.
The air then passes through the larynx, where your vocal cords are located. Next is the almost 4 1/2-inch-long [11.5 cm] trachea, or windpipe, reinforced by about 20 C-shaped bands of cartilage spaced throughout its length. The windpipe then branches into two one-inch-long [2.5 cm] tubes known as the main bronchi. One bronchus enters the left lung, the other enters the right lung. Inside the lungs these tubes further divide into more branches.
This branching occurs again and again inside the lungs until a structure resembling a tree is formed, with trunk, branches, and twigs. Of course, at each branching the airways become thinner and thinner. The air then enters the small branches, a network of miniature vessels called bronchioles, each with a diameter of about one twenty-fifth of an inch [1 mm]. The bronchioles lead to even smaller ducts, which send the air into some 300 million small air sacs known as alveoli. These air sacs are arranged in bunches and resemble hanging clusters of grapes or tiny balloons. It is here that the treelike system of airways ends and the air reaches its final destination.
The Final Threshold
When it reaches its final threshold, the air you breathe is contained within the extremely thin walls of the alveoli. They measure only 0.00002 inch [0.5 micron] across. The paper used in this magazine is about 150 times as thick as the walls of the alveoli!
Each one of these tiny alveoli is covered with a web of blood vessels known as pulmonary capillaries. These capillaries are so narrow that only one red blood cell can pass through at a time! And the walls are so thin that the carbon dioxide in the blood can seep through into the alveoli. The oxygen, in turn, passes in the opposite direction. It exits the alveoli to be absorbed by the red blood cells.
Each one of these red blood cells, or corpuscles, traveling in single file, remains in the pulmonary capillaries for about three fourths of a second. This is plenty of time for the carbon dioxide and the oxygen to exchange places. This movement of gases is by a process known as diffusion. The oxygenated blood then passes into larger veins in the lungs, eventually reaching the left side of the heart, from which the blood is pumped throughout the body as the fuel of life. All told, it takes about one minute for all the blood in your body to pass through this intricately designed system!
Now that the air has reached its final destination, how does it exit the lungs with its cargo of carbon dioxide? Is there a need for a second set of airways to be used in exhaling? A marvel in design, this “tree” of air ducts in your lungs is used both for incoming air and for outgoing. Interestingly, as you rid your lungs of carbon dioxide by exhaling, you can also make your vocal cords vibrate, thereby creating the sound necessary for speech.
As the air you breathe passes through your nose and mouth, it is actually being processed by a quality-control station. When the air is too cold, it is quickly heated to an adequate temperature. When the air is too hot, it is cooled down. What happens when the air is too dry? The walls of your nose, nasal sinuses, throat, and other passageways are lined with a fluid called mucus. When you inhale dry air, moisture in the mucus evaporates into the air. By the time the air reaches the farthest point in your lungs, it has a relative humidity of almost 100 percent. Interestingly, when you exhale, the air returns over half of its humidity to the mucus.
This quality-control system also includes a sophisticated air filter. During the course of one day, about 2,500 gallons [9,500 L] of air pass through the lungs. This air is often loaded with infectious agents, toxic particles, fumes, or other impurities. However, your respiratory system is designed to remove most of these contaminants.
Initially, the hairs and mucous membranes in your nose do their part in catching larger particles of dirt. Then, you have millions of microscopic, hairlike projections that grow on the walls of your airways. They are called cilia. Like paddles, they wave back and forth at a rate of about 16 times a second, pushing dirty mucus away from the lungs. Your lungs also count on the services of special cells, called alveolar macrophages, designed to kill bacteria and trap dangerous particles.
Hence, the air you breathe is conditioned and filtered before it reaches the most delicate tissues of your lungs. Indeed, a marvel of design!
An Automatic System
Unlike food and water, oxygen can be drawn from the environment without any deliberate effort on your part. At a rate of some 14 breaths per minute, a healthy pair of lungs extracts oxygen from the air automatically. Even during sleep your lungs continue to work without your conscious supervision.
You also have the option of temporarily overriding this automatic system. Hence, you can deliberately control your breathing to some extent if you wish to do so. After all, would you want the mechanics of breathing to continue operating automatically while you swim under water? At a rate of 14 breaths per minute, would you have enough time to escape a smoke-filled room during a fire if you were unable to hold your breath? Of course, this automatic system cannot be bypassed for long periods of time. After some minutes at most, your lungs will inevitably return to their automatic mode.
But what activates the muscles to inflate or deflate your lungs during this automatic operation? The control center is located in the brain stem. Here special receptors monitor the level of carbon dioxide in the body. When there is an increase in carbon dioxide, messages are sent through a network of nerves, which in turn activate the appropriate muscles of respiration.
This gives remarkable flexibility to your respiratory system. The lungs can keep up with even abrupt changes in your activity. For example, during strenuous exercise, your body may use about 25 times as much oxygen and produce some 25 times as much carbon dioxide as it does when it is at rest. However, your lungs almost instantaneously adjust the frequency and the depth of your breathing in order to match your constantly changing oxygen requirements.
There are other sophisticated controls that enable the lungs to function properly. For example, some muscles used for respiration are also used in other functions, such as swallowing and speech. These functions are kept in balance so that they rarely interfere with your breathing. And all of this is done without any conscious effort on your part. Yes, automatically!
Of course, many things may go wrong with the lungs, especially when your resistance is low. To name just a few disorders, there are asthma, bronchitis, emphysema, lung cancer, pulmonary edema, pleurisy, pneumonia, tuberculosis, and a number of bacterial, viral, and fungal infections.
But these disorders are not the result of faulty or inadequate design of the lungs. Most lung diseases result from exposure to pollutants, dusts, and vapors dumped into the environment by man. Millions today suffer from lung cancer, bronchitis, and emphysema because of tobacco smoking and other self-inflicted abuses of the respiratory system.
Under normal circumstances, however, your lungs stand out as a marvel of design and as a living monument to the Grand Designer, Jehovah God! We are, indeed, as the psalmist put it, ‘in a fear-inspiring way, wonderfully made.’—Psalm 139:14.
[Box on page 22]
Why Does It Happen?
Sneeze: An involuntary and violent rush of air through the mouth and nose. Nerve endings in the nose cause you to sneeze in order to get rid of irritating particles in your nose. Cold air may also induce sneezing. A sneeze may produce an air speed of up to 103 miles per hour [166 km/hr] and expel up to 100,000 droplets of mucus and microorganisms. For this reason, unless you adequately cover your mouth and nose, your sneeze may be harmful to other people.
Cough: An abrupt expulsion of air, ridding the lungs of harmful substances when the lining of the respiratory tract becomes irritated. Coughing may also be a deliberate effort to clear the throat or the bronchi. Like sneezing, coughing can spread germs that cause disease.
Hiccup: A sudden, involuntary intake of air caused by a spasmodic contraction of the diaphragm. These abrupt contractions may be caused by irritation of the organs near the diaphragm. The spasm draws air into the lungs through the larynx. As the air is pulled into the larynx, it strikes the epiglottis, causing the vocal cords to vibrate. This produces the hic sound.
Snoring: A harsh sound made during sleep, usually caused by a person’s breathing through his mouth. Soft tissue in the roof of the mouth near the throat vibrates as air passes through. The lips, cheeks, and nostrils may also vibrate. If you sleep on your back, the mouth tends to hang open, and the tongue obstructs the air passage. Sleeping on your side may stop the snoring.
Yawn: A deep involuntary inhalation believed to be a response to a buildup of carbon dioxide in the lungs. Yawning has been referred to as a socially infectious habit because of the urge to yawn when you see or hear someone else yawning. Scientists cannot explain this phenomenon.
[Diagrams on page 23]
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Detail of a bronchiole