Major Lower Respiratory Tract Organs:
The pharynx carries the air into the larynx. The entire function of the larynx is to produce sounds. The larynx is commonly known as the voice box. The larynx contains vocal cords within it, which are laryngeal folds that project into the space within the larynx (you'll learn more about these in lab). You see, as air travels out under pressure through the larynx, the vocal cords in the larynx vibrate. This vibration causes the air running through it to vibrate, which can also be thought of as forcing the air to move in waves. When you learned about sound and hearing, you learned that sound is simply air travelling in waves. Therefore, the general idea of the larynx is to force air into waves to produce sounds. If the vocal cords vibrate quickly, the sound waves will be fast, and a high-pitched sound will be made. If the vocal cords vibrate slowly, the sound waves will be slow, and a low-pitched sound will be made. Finally, if the air is not forced through the larynx under pressure, the vocal cords will not vibrate at all, and no sound will be made (normal exhaling).
The larynx is highly structured. You see, in order to have vocal cords vibrate at different frequencies (speeds), we have to be able to adjust the tension on the vocal cords. We do that by having muscle attached to the cords, regulating their tension. When we want to make a high-pitched sound, we increase the tension on the vocal cords to force them to vibrate faster. Mainly because of this need for regulation of the vocal cords with muscle, we need a strong, supportive structure for the larynx. This structure is provided by a number of pieces of cartilage, as seen in this picture below.
The thyroid, cricoid, corniculate, and arytenoid cartilage are all supportive cartilaginous structures for the larynx. The entire larynx is also supported by attachment to the hyoid bone.
One other item in this picture is the "epiglottic cartilage." This piece of cartilage does not support the larynx. Instead, the epiglottis functions to block off the air passageways of the lower respiratory tract when swallowing. You see, since both food and air are in the pharynx, and the pharynx leads to the larynx, there has to be a way to prevent the food in the pharynx from entering the larynx, yet still a way to allow air to enter the larynx. The epiglottis is a movable structure, performing this function. We will focus more on the epiglottis in lab.
You have already learned about the trachea in lab, a long time ago, during A&P1. You learned that the trachea is an air passageway in the throat, maintained open by rings of hyaline cartilage that run 3/4 of the way around the trachea. The cartilaginous rings are on the anterior side of the trachea, preventing closure of the trachea from normal outside pressures (strangling is a much higher pressure and closes off the trachea anyway).
One last note about mucous membranes. Almost the entire lower respiratory tract is also covered by a mucous membrane. The cilia on these membranes now pushes the mucus rostrally, toward the pharynx. In this manner, any unwanted materials that make it all the way into deeper respiratory tract areas will still get trapped in the mucus, and the filthy mucus will still get sent to the digestive system.
The trachea is the pipe needed to send the air from the nose down toward the lungs. However, the lungs are extremely large, and there are two of them; there is only one, central trachea. So, the trachea must branch into numerous, smaller tubes in order to fully supply the lungs with air.
This branching of the air tubes is accomplished by the "bronchial tree."
The first branching from the trachea is into two tubes, one to each lung, called the primary bronchi (bronchus is the singular). Each primary bronchus branches into smaller and smaller bronchi until the tubes are tiny (called bronchioles) and run into the dead ends of the lung tissue, called alveoli. These are discussed below, in the section on lungs.
As the air passageway gets smaller, from the primary bronchi to the bronchioles, the cartilage surrounding it decreases. In the smallest bronchioles, the cartilage is entirely gone, and only elastic connective tissue and smooth muscle remain to support them.
All the rest of the terminology on the bronchial tree will be covered in lab.
If you look at a histological section through the lungs, you will see that they are full of holes. The bulk of lung tissue is open space! Kind of the way a sponge looks, but even more porous.
Basically, lung tissue is the dead ends of the bronchial tree (the alveoli). There are also lots of blood vessels running through the lungs, since the pulmonary circuit runs through picking up oxygen. You will even see bronchioles, since the bronchioles run right into and through the lungs.
Macroscopically, as if you were holding a lung in your hand, you would see that the lung is made up of a few lobes. You will see the lobes of the lungs in lab. Microscopically, as if you were looking through a microscope, you are looking at alveoli. So, let's go on and figure out alveoli.
In this figure from your book (Figure 19.14, page 749), on the left hand side of the figure, you can see the tiny final branches of the bronchioles. At their very ends, they branch extensively into tiny little dead-end-circles. I think it looks kind of like broccoli flowers. If you consider it to be like broccoli, keep in mind that to be really analogous, the broccoli would have to be hollow all the way through.
The dead-end-circles are the alveoli (singular: alveolus). The top-most group of alveoli in the left-side of the figure is shown covered in blood capillaries. Directly under it, the group of alveoli there have only the arterioles and venules depicted on them. The grouping below that is shown cut in half. Here you can see that the terminal bronchiole is called an alveolar duct at the point where it enters the region of alveoli. At the right hand side of the figure, in the box, a single alveolus is drawn, covered in its pulmonary capillaries.
If you consider that thousands of alveoli receive air from a single bronchiole, as can be seen somewhat from the picture above, you should be able to picture how the millions of alveoli (and the pulmonary capillaries that run by them) can make up the large tissue of the lungs. So, even though the alveoli are teensy, millions of them all together can make a large, sponge-like tissue. Can you picture this?
The alveoli are composed of simple squamous epithelium. They do not have a mucous membrane covering them on the inside, because any unwanted materials should have been trapped long before reaching the alveoli. You will see later in this unit that the simple squamous epithelium of the alveoli allow for gas exchange with the blood.
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