The Path of Airflow in Humans Help (page 2)
Introduction to The Path of Airflow in Humans
Respiration literally means “the act of” (-tion) “breathing” (spir) “again” (re-). In actual usage, however, respiration is the process of gas exchange between two or more body compartments. Consider, for example, external respiration in mammals (Figure 18.1). External respiration is gas exchange (such as that of O 2 gas and CO 2 gas) that occurs between the blood in the pulmonary capillaries and the air within the alveoli (al- VEE -oh-lie). In mammals, each alveolus (al- VEE -oh-lus) is a “little cavity” (alveol) or microscopic air sac within the lung.
Internal respiration, in contrast, is gas exchange that occurs between the blood in the systemic capillaries and the fluid within the tissue cells. Internal respiration, then, is the way mammalian cells acquire oxygen and give off carbon dioxide.
During breathing, air is inhaled from the atmosphere, and it enters the millions of alveoli within the lungs. We know that breathing, itself, involves pulmonary functions: that is, those “pertaining to” (-ary) the “lungs” (pulmon). An appropriate term here is ventilation ( ven -tih- LAY -shun), the “process of” (-tion) “fanning or blowing” (ventil). A ventilation system in a large building, for example, blows stale air out, and sucks fresh air in. A similar situation exists for the lungs. Therefore, pulmonary ventilation is the sucking of air into the lungs, followed by the blowing of air out of the lungs.
Inspiration Versus Expiration
The technical terms concerning pulmonary ventilation are inspiration ( IN spir- ay -shun) on the one hand, expiration ( EKS -pir- ay -shun) on the other hand. Inspiration is the process of sucking air into the lungs, while expiration is the process of blowing air out of the lungs.
Summarizing these relationships, we can say that:
PULMONARY VENTILATION = INSPIRATION + EXPIRATION
RESPIRATION = The Process of Gas Exchange between Various Body Compartments
Pulmonary ventilation is the way that the body takes in fresh air (by inspiration) and gets rid of stale air (by expiration). And respiration (external and internal) is the means by which O 2 and CO 2 are exchanged between various compartments while the air is within the body.
These very basic inter-relationships are diagrammed for humans and other mammals within Figure 18.1.
“When humans take in air by inspiration, what is the sequence of body structures through which the air passes?” the curious-minded reader might well ask. The answer is depicted within Figure 18.3.
Upper and Lower Respiratory Pathway
The Upper Respiratory Pathway
Lying above both lungs is the upper respiratory pathway. This pathway begins with two cavities – the nasal ( NAY -sal) cavity and the oral ( OR -al) cavity.
The nasal cavity lies within the “nose” (nas), while the oral cavity is situated behind the “mouth” (or). These are the first two body structures that inhaled air usually enters. From both of these cavities, the air goes back into the pharynx (throat).
Situated at the lower end of the pharynx, one sees the larynx ( LAIR -inks). The larynx or “voice box” is a box-shaped collection of cartilage plates held together by dense fibrous connective tissue. Like the bow of a ship, the laryngeal (lah- RIN -jee-al) prominence is a projection of cartilage sticking out from the front of the voice box. Stretched across the interior of the larynx are the two vocal ( VOH -kal) cords. The vocal cords are two straps of highly elastic connective tissue, which vibrate with the passage of air through the larynx. These vibrations create the “voice” (voc) sounds.
The glottis ( GLAHT -is) is the name of the tapered, “tongue” (glott)- shaped opening between the two vocal cords. Closely related to the glottis is the epiglottis ( EH -pih- glaht -is). The epiglottis is a highly flexible flap of cartilage literally located “upon” (epi-) the glottis. The epiglottis thus serves as a flexible lid over the top of the larynx or voice box. When a person swallows, the food or liquid normally pushes the epiglottis shut. This usually prevents food or liquid from entering the larynx and the rest of the respiratory pathway.
Below the larynx is the trachea ( TRAY -kee-ah) or main “windpipe.” The trachea is stiff and noncollapsible, due to the presence of horseshoes of cartilage within its walls.
The Lower Respiratory Pathway
As the trachea branches, the lower respiratory pathway is created. The right and left primary bronchi ( BRAHN -kigh), the first branches, then enter the two lungs. The primary bronchi just keep branching. Eventually, a set of bronchioles ( BRAHN -kee- ohls ) or “little bronchi” emerges. Much like an inverted (upside down) tree, the respiratory tree thus consists of a succession of ever-smaller and more numerous branches – the larynx, trachea, bronchi, and bronchioles.
At the furthest tips of the bronchioles, hang clusters of thousands of alveoli. (Picture the many olives suspended from the branches of an inverted olive tree.) Each alveolus is essentially a collapsible, extremely thin-walled air sac. It is across the walls of the alveoli, and their close neighbors, the pulmonary capillaries, that external respiration occurs. By this means, remember, oxygen contained in the inhaled air finally enters the bloodstream.
[ Study suggestion: Look very carefully at Figure 18.3. Which structure mentioned in this section do you think represents the so-called “Adam’s apple” in males? Why?]