The Respiratory System and the Lungs - The trachea and the lungs

The right lung (your right) is somewhat bigger than the left. The lungs hang in the chest attached to the windpipe or trachea .

The Trachea

The trachea itself branches off at the back of the throat, on pharynx , where the epiglottis prevents food from entering the trachea and channels swallowed food along its proper route, the esophagus. The top part of the trachea forms the voice box or larynx , made up of vocal cords—actually two flaps of cartilage, muscle, and membranous tissue that protrude into the windpipe—whose vibrations in response to air exhaled from the lungs give us our voice.

Below the larynx, the trachea descends five or six inches to a spot just about directly behind your breastbone, where the first of many thousands of branchings into bronchi, bronchioles , and alveoli occurs. C-shaped rings of cartilage give the trachea both support and flexibility. Running your finger down the front of your neck, you can feel the bumps made by the cartilage rings.

Above the base of the trachea in midchest the lungs arch on either side like giant butterfly wings, then fall to fill out the bottom of each side of the thoracic cavity.

The Pleural Membranes

Both lungs are encased in moist, clinging, tissue-thin membrane called the pleura , which also lines the inside of the thoracic cavity where it comes into contact with the pleural coating of the lungs. The slippery pleural membranes hold tightly to each other, because there is an air lock or vacuum between them, but at the same time are free to slide over each other. The principle is the same as that illustrated by moistening the surfaces of two pieces of plate glass and placing the moistened surfaces together: the two pieces of glass will slide over each other but will resist being pried apart, because a partial vacuum exists between them.

The Pleural Cavity

The vacuum space between the pleura of the lung and the pleura of the thoracic cavity—although it is normally not a space at all—is called the pleural cavity . Each lung has its own pleural membrane: that of one lung does not interconnect with the other, so that one pleura may be injured without affecting the other.

It is extremely fortunate for us that the pleural linings both stick fast and can slide along each other's surfaces. Although the lungs are virtually without muscle, they are extremely elastic and in their natural condition are stretched fairly taut, held to the sides of the thoracic cavity by the suction of the pleura.

Collapsed Lung

Should this suction be broken and the pleural linings pull apart, the lung would shrink up like a deflated balloon. Such a condition, caused by the rush of outside air into the pleural cavity, is known medically as pneumothorax and causes a lung collapse. Violent injuries such as gun and stab wounds, various lung diseases, and obstructions of the breathing tubes can cause a lung or portion of a lung to collapse.

In the surgical procedure called artificial pneumothorax , a physician deliberately injects air between the pleural linings to collapse a portion of a lung. This is done to rest a lung in severe diseases such as tuberculosis, or to control heavy bleeding within the thoracic cavity.

Pleurisy

The intense chest pains called pleurisy are caused by inflammation of the pleura. The pleural linings lose their slipperiness and the increased friction stimulates pain receptors in the pleural lining of the chest. There are, however, no pain receptors in the lungs’ pleural linings nor in the lungs themselves: this is why pain is not an early warning signal of lung cancer.

The Bronchi

Just behind the breastbone and just in front of the heart, the trachea divides into the right bronchus and the left bronchus, leading respectively to the right and left lungs. These are the primary two bronchi or bronchial tubes . Each is the main trunk of a bronchial tree that serves its respective lung. Soon after leaving the trachea, each bronchus branches repeatedly into smaller tubes called bronchioles , which in turn branch into alveolar ducts, which terminate finally with the hundreds of millions of microscopic air sacs called alveoli, discussed at the beginning of this section. The alveoli are the site of the all-important exchange of carbon dioxide and oxygen.

Lobes and Segments

The larger right lung has three distinctive sections, or lobes —upper, middle, and lower. The left lung has only an upper and lower lobe. The lobes themselves are divided into smaller segments. Medically, these lobes and segments are important because they are somewhat independent of each other and can be damaged or removed surgically, as in operations for lung cancer, usually without damaging the function of adjacent, healthy segments or lobes.

The fact that a lung segment, lobe, or even an entire lung can be removed implies that we have plenty of reserve lung tissue, and this is indeed the case. When we are at rest, we use only about one-tenth of our total lung capacity. The total surface area exposed within our lungs to outside air is, amazingly, 600 square feet. This compares to a mere 20 square feet of skin surface. To appreciate the incredibly intricate, lacelike finery of the lungs’ structure, we need only know that those 600 square feet of surface area are contained within two organs that together weigh only two-and-a-half pounds.