Just as a fire needs oxygen to burn, the human body also needs a continuous

supply of this essential element for the process of combustion that goes on

constantly in every cell. We ordinarily do not think of a body metabolism as

combustion, yet that is what is: the controlled burning of carbohydrates, fats,

and proteins to provide energy. The job of the respiratory system is to furnish the

oxygen that combines with these fuels in each of the billions of cells, and to

carry away the waste product of carbon dioxide.

And so we breathe - fourteen times a minute, a pint of air per breath, more

than ten thousand quarts of air a day. Not all that comes in, of course, is

oxygen. Only about one - fifth of the air we breathe is this life - sustaining

element. But it makes up an important part of our bodies and our lives. At any

given moment, half the body\'s weight is oxygen. The overwhelming portion,

incidentally, is not in the form of a gas, the way we usually think of oxygen.

We loosely use the word "respiration" to describe the process of taking in

oxygen and letting out carbon dioxide. To be scrupulously accurate, respiration

refers to the ultimate exchange that takes place in the cells themselves, the

delivery of oxygen and the removal of carbon dioxide. This gas transfer, as it is

scientifically known, is at the heart of human life. If oxygen did not arrive, if the

carbon dioxide were not removed, our lives would be abruptly shortened. The

exchange must take place in every cell, including those distant from the

oxygen - rich atmosphere that surrounds us. Obtaining that oxygen and starting

it on its all - important trip through the body begins with those critical organs, the

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A Look At The Lungs

Although central to the vital and complex processes of all body cells, the

lungs are mechanically simple in form, function, and principle.

They are cone - shaped, pink in color, and weigh a little more than a pound

each. The normal lungs of a healthy male have a capacity of nearly ten quarts.

(Those of females are slightly smaller.) Lungs are hardy organs. If one is

diseasedand removed, the respiratory process continues adequately without it.

Of the two lungs, the right is larger as the heart takes more room on the left

side. Each lung is divided into lobes, which are fed by divisions of the bronchus,

leading from the trachea (windpipe). The right lung has three lobes, upper,

middle, and lower. The left has only two, upper and lower. The lobes are

separate from one another and are marked by grooves on the surface, known

as fissures. These give important information to doctors, as they can be seen on

a chest x - ray. By looking carefully at their position and observing, for instance,

whether they have moved up or down, they can tell whether you have suffered

collapse of part of your lung.

Locating The Lungs

The lungs lie within the flexible rib cage. They normally have only one fixed

attachment (at the larynx) and thus have considerable range of motion. The

bases of the lungs rest above the diaphragm, the principal muscle breathing.

Each lung is surrounded by a glistening membrane, the visceral pleura. The

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inner surface of the chest has a similar membrane, the parietal pleura.

Lubricate by fluids, these membranes glide smoothly against each other when

we breathe. Normally there is no space between them, but entry of bacteria,

penetrating wounds, or disease may separate the pleura.

Inside The Lung

The interior wall of the lung has a surface like a sponge. It is composed of

more than three hundred million tiny alveoli, each scarcely more than a

pinhead in diameter, and each open to the atmosphere at one end. The

combined surface area of these sacs is so great that if they were flattened out,

they would cover a tennis court. The alveolar walls are only a single cell thick,

immediately beneath them, also encased in a wall of single - cell thickness, is

the capillary bed of the lungs.

How The Lungs Work

If the lungs were removed from the chest, they would shrink like deflated

balloons. They are held open by surface tensions, which is created by fluid

produced by a thin lining around