Anatomy and Physiology: Pharynx
At the back of the nose, we start our journey down the pharynx. The first stop is in the nasopharynx, followed by the oropharynx, which is right behind the mouth, the hypopharynx, and then the laryngopharynx, which is the last stop. Just as some infections are only a finger's length away (a cautionary note to nosepickers of all ages), some infections not only make it past the nose, but they make it all the way down into the lungs.
The Big Picture
Remember, thinking about the body one system at a time is always a mistake. Even in the most traditional textbooks you will find diagrams of the respiratory system with a heart in view. That, quite simply, illustrates the fact that the lungs are exchanging gases with the cardiovascular system. But such details as the epiglottis at the top of the larynx illustrate this sys-tem's connection with the digestive system, and the tonsils within the pharynx illustrate the connection with the lymphatic system. If you learn to look for it, you will see that every body system is connected with every other one!
As an extra measure of protection, the pharynx is lined with several tonsils—the adenoids in the nasopharynx, the palatine tonsils at the back of the mouth, and the lingual tonsils at the back of the tongue—that also help in the fight against infection. This is crucial, because bacterial infection inside the brain can be fatal. The brain is limited in terms of fighting off infection, and so the tonsils, which are a part of the lymphatic system (refer again to Figure 13.2), fight the infection before it can ever get to the brain!
One interesting thing about the tonsils is their shape. Rather than having a distinct, smooth outer edge, the tonsils have a series of open pits called crypts. These crypts are designed for catching bacteria and viruses as they pass through the pharynx. It's as if the tonsils are saying, “Go ahead … make my day!”
A child's body is exposed to a remarkable range of pathogens. This exposure helps the body's immune system develop, especially given the memory capabilities of the body's B cells (so named because they are made in the bone marrow). Undue concern with infection, think of the ever-growing passion for antibacterial soaps, has led to parents overprotecting their children. Without proper exposure to foreign antigens, the body's immune system fails to develop properly, leading to an increase in the incidence of asthma and fatal food allergies; these problems are rare among people raised near large mammals, with the ever-present manure. The irony of this is that the best way to develop protection from illnesses is to roll around in cow dung as a child!
The other cool structure in the throat is the epiglottis (see Figure 13.4), because eating is a potentially life-threatening situation! This is because the trachea needs to be held open to allow an almost constant airway (except when swallowing). This is done by the cartilaginous rings around the trachea, which you can feel if you gently rub your fingers down the center of your throat. If food travels down through the glottis (larynx) to the trachea, the airway can get blocked and the person can choke to death. Only by coughing, or, in extreme cases the Heimlich maneuver, can you remove food that has gone down, well, the wrong tube. The open tube of the trachea makes the existence of the epiglottis crucial.
Flex Your Muscles
Memorizing the definition of the word epiglottis is nowhere near as effective as analyzing the word: epi- means above, and the glottis is the larynx (voice box), which is at the top of the trachea. As such, knowing the location of the flap-like epiglottis, as implied by the name, reveals its function: to block the entrance to the glottis when swallowing, and thus forcing the food down the esophagus.
You might remember a cool little bone called the hyoid. The hyoid bone is found at the very top of the larynx; its function is to hold the opening to the airway open. In front of the larynx, below the hyoid bone, is a thick area of cartilage called the thyroid cartilage. As you can imagine, right in front of that you can find the thyroid gland.
Sound is based on a few mechanically simple properties. In a nutshell, sound is based on the vibration of air molecules. That vibration is caused when air is forced past the vocal folds, or vocal cords, which are flaps inside the larynx. The movement of air causes the vocal folds to vibrate. The tighter the folds, the higher frequency the vibrations and thus the higher pitch to the sound; by relaxing the vocal folds, the frequency of the vibrations and the pitch of the sound will be lower.
Think again about the thyroid cartilage (or Adam's apple). Sexual dimorphism in humans is the idea that males and females are of a different size and body shape. One area of that size difference is the size of the thyroid cartilage. Because the thyroid cartilage is right in front of the larynx, it makes sense that the larynx would be a different size as well. Just as the cello produces a deeper sound then the smaller violin, a larger larynx gives men a deeper voice.
The larynx alone, however, is not responsible for the difference in sound between one person's voice and another. Have you ever played acoustic or electric guitar? Is there a difference in the volume and quality of the sound of an electric guitar when it's plugged in and when it isn't? Why is an acoustic guitar able to produce a louder sound without electricity? Remember, the body of an electric guitar is solid, whereas that of an acoustic guitar is hollow and has a circular opening. The hollow portion of the acoustic guitar acts as a resonating chamber for the sound; the nature of that chamber alters not only the volume but also the quality of the sound.
Think back again to the skeleton. Remember learning about the sinuses? Those hollow chambers in the frontal, ethmoid, sphenoid, and maxillary bones act as resonating chambers for the voice. Since the shape of the sinuses is partly genetic, it makes sense that you sound a little like your parents. Since those sinuses are also lined with mucous membranes, it also makes sense that when you have a cold or bad allergy the extra mucus in the sinuses will change the quality of your voice, just as the champagne dulls the clink of the glasses during a toast.
Excerpted from The Complete Idiot's Guide to Anatomy and Physiology © 2004 by Michael J. Vieira Lazaroff. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.