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Pathology => Blood => Lungs
Lungs
INTRODUCTION
Lung, either of a pair of elastic, spongy organs used in breathing and respiration. Lungs are present in all mammals, birds, and reptiles. Most amphibians and a few species of fish also have lungs.
In humans the lungs occupy a large portion of the chest cavity from the collarbone down to the diaphragm, a dome-shaped sheet of muscle that walls off the chest cavity from the abdominal cavity. At birth the lungs are pink, but as a person ages, they become gray and mottled from tiny particles breathed in with the air. Generally, people who live in cities and industrial areas have darker lungs than those who live in the country.
STRUCTURE OF THE LUNGS
Air travels to the lungs through a series of air tubes and passages. It enters the body through the nostrils or the mouth, passing down the throat to the larynx, or voice box, and then to the trachea, or windpipe. In the chest cavity the trachea divides into two branches, called the right and left bronchi or bronchial tubes, that enter the lungs.
In the adult human, each lung is 25 to 30 cm (10 to 12 in) long and roughly conical. The left lung is divided into two sections, or lobes: the superior and the inferior. The right lung is somewhat larger than the left lung and is divided into three lobes: the superior, middle, and inferior. The two lungs are separated by a structure called the mediastinum, which contains the heart, trachea, esophagus, and blood vessels. Both right and left lungs are covered by an external membrane called the pleura. The outer layer of the pleura forms the lining of the chest cavity.
The branches of the bronchi eventually narrow down to tubes of less than 1.02 mm (less than 0.04 in) in diameter. These tubes, called bronchioles, divide into even narrower tubes, called alveolar ducts. Each alveolar duct ends in a grapelike cluster of thin-walled sacs, called alveoli (a single sac is called an alveolus). From 300 million to 400 million alveoli are contained in each lung. The air sacs of both lungs have a total surface area of about 93 sq m (about 1000 sq ft), nearly 50 times the total surface area of the skin.
In addition to the network of air tubes, the lungs also contain a vast network of blood vessels. Each alveolus is surrounded by many tiny capillaries, which receive blood from arteries and empty into veins. The arteries join to form the pulmonary arteries, and the veins join to form the pulmonary veins. These large blood vessels connect the lungs with the heart.
BREATHING
Although the words breathing and respiration are sometimes used interchangeably, they have distinct meanings. Breathing is the process of moving oxygen-rich air into and out of the lungs. Respiration refers to all of the processes involved in getting oxygen to tissues, including breathing, diffusion of oxygen from the lungs to the blood, transport by the blood, and diffusion from the blood to tissues. Respiration is essential for aerobic respiration, the process within cells in which nutrients and oxygen are used to build the energy molecule adenosine triphosphate (ATP). In aerobic respiration, body cells use oxygen to metabolize glucose, forming carbon dioxide as a waste product that is exhaled.
Because body cells are constantly using up oxygen and producing carbon dioxide, the lungs work continuously. An adult normally breathes from 14 to 20 times per minute, but vigorous exercise can raise the rate to 80 breaths per minute. A child's rate of breathing at rest is faster than an adult's at rest, and a newborn baby has a rate of about 40 breaths per minute. In general, smaller animals have faster breathing rates than larger animals. A rat, for example, breathes about 60 times per minute, while a horse breathes only about 12 times per minute.
The process of breathing is generally divided into two phases, inspiration and expiration. In inspiration, air is moved into the lungs. In expiration, air is forced out of the lungs. The lungs themselves have no muscle tissue. Their movements are controlled by the rib cage and the diaphragm. During inspiration the muscles around the rib cage contract, lifting the ribs upward and outward, and lowering the dome of the diaphragm until it forms a nearly flat sheet. As a result of these changes, the chest cavity expands. Because the lungs are attached to the chest cavity, they also expand. With the enlargement of the lungs, air pressure inside the lungs falls below the pressure of the air outside the body, creating a partial vacuum, and air from outside the body rushes into the lungs.
The amount of air normally taken into the lungs in a single breath during quiet breathing is called the tidal volume. In adults the tidal volume is equal to about 0.5 liters (about 1 pt). The lungs can hold about ten times this volume if they are filled to capacity. This maximum amount, called the vital capacity, is generally about 4.8 liters (about 1.3 gal) in an adult male, but varies from one individual to the next. Athletes, for example, can have a vital capacity of as much as 5.7 liters ( 1.5 gal). The vital capacity is reached only during strenuous exercise.
In expiration the muscles that lift the rib cage and lower the diaphragm relax. As a result, the rib cage and the diaphragm return to their original positions, and the lungs contract with them. With each contraction of the lungs the air inside them is forced out.
A person can alter the rate of breathing and can even stop breathing for a short time. But it is impossible to voluntarily stop breathing permanently because breathing, like the heartbeat, is an involuntary activity controlled by nerve centers in the brain stem, the lower part of the brain. These centers are connected with the muscles of the rib cage and diaphragm, and they increase or decrease the rate of breathing according to the needs of the body.
AEROBIC RESPIRATION
In the life-supporting process of aerobic respiration, oxygen from incoming air enters the blood; and carbon dioxide, a waste gas from the metabolism of food, is exhaled into the atmosphere. Air entering the lungs contains about 21 percent oxygen and 0.04 percent carbon dioxide. Air leaving the lungs contains about 14 percent oxygen and about 4.4 percent carbon dioxide. The composition of the air changes between inspiration and expiration, when the air is deep in the lung tissue.
The exchange of gases takes place when air reaches the alveoli. These small sacs are only one cell thick, and they are surrounded by blood capillaries that are also only one cell thick. Air diffuses through these cells into the capillary blood, which carries the oxygen-rich air to the heart to be distributed throughout the body. In the alveoli, at the same time, gaseous carbon dioxide diffuses from the blood into the lung and is expired.
DISEASES OF THE LUNG
The lungs are subject to a number of disorders, with symptoms that may include difficulty in breathing, chest pain, coughing, and wheezing. Lung cancer, most commonly caused by smoking tobacco, is the deadliest lung disease, and each year it kills more Americans than any other kind of cancer. Smoking tobacco may also lead to emphysema, which is caused by wholesale destruction of alveoli. Other lung disorders include pneumonia, or inflammation of the lungs, which is usually caused by viruses or bacteria; pneumothorax, or collapse of the lung; pulmonary edema, or fluid in the lung; and pleural effusion, or fluid on the pleura. Pulmonary embolism occurs when a blood clot or other foreign substance lodges in the lungs and blocks the major pulmonary arteries.
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