The path (nasal cavity, pharynx, larynx, trachea, and bronchi) until the alveoli is called airway.
As the diaphragm contracts, the volume of the thoracic cavity increases (Fig. 3-24) and air is drawn into the lungs. The intercostal muscles are auxiliary (Fig. 3-23).
< Nose, pharynx >
The olfactory receptors are located in the upper region of the nasal cavity and is connected with the I (olfactory nerve) (Fig. 13-67).
The first airway, nasal cavity, is warm and humid, owing to the large surface area of the nasal septum and the nasal conchae.
The paranasal sinuses are air-filled spaces that are continuous with the nasal cavity. The paranasal sinuses are equivalent to the medullary cavity (Fig. 1-2).
Of the paranasal sinuses, maxillay sinus is the largest. Due to the position of its opening, gravity cannot drain the maxillary sinus contents when the head is erect.
The pharynx (nasopharynx, oropharynx, laryngopharynx) leads the inspired air to the larynx (Fig. 4-15).
The pharynx belongs to both digestive system (Fig. 4-21) and respiratory system.
The nasopharynx is connected to the tympanic cavity by the auditory tube (Fig. 4-19). Opening of the auditory tube is required to equalize the pressure between the tympanic cavity and surrounding atmosphere (Fig. 4-19).
< Larynx >
The hyoid bone is a horseshoe-shaped bone situated between the mandible and thyroid cartilage of larynx (Fig. 5-10). The hyoid bone is the insertion of the suprahyoid muscles (Fig. 3-19) and infrahyoid muscles (Fig. 3-21).
Fig. 5-10. Laryngeal cartilages.
The larynx is comprised of the epiglottic, thyroid, arytenoid, and cricoid cartilages. Only the cricoid cartilage is circular, and primarily contributes to maintaining the larynx airway. The horseshoe-shaped hyoid bone also helps keep the airway.
Fig. 5-11. Bilateral vocal cords.
The vocal cord has the vocal ligament between the thyroid and arytenoid cartilages. Since the arytenoid cartilages are bilateral (Fig. 5-10), the vocal ligaments and vocal cords are bilateral also.
At rest, the vocal cords are located apart from each other. This is why we do not make any sound when breathing in ordinary time. However, when the vocal cords stick together by the intrinsic muscles of larynx, the exhaled air vibrates the vocal cords for phonation.
If one pushes down one’s own thyroid cartilage while making voice, the tone will get lower.
The thyroid cartilage in male is more protruded than that in female. Consequently, a man’s vocal cords are long and thus produce low pitch sound like long strings of a contrabass. On the other hand, a woman’s vocal cords are like short strings of a violin.
< Lung >
Fig. 5-15. Pleura covering lung.
A squashed balloon surrounding the lung is the pleura, and a space in the balloon is the pleural cavity. The pleural cavity contains a few serous fluid (Fig. 8-35).
Due to the left-shifted heart (Fig. 10-8), the right lung is larger than the left one.
Fig. 5-17. Lobes of lung.
The large right lung has two (horizontal, oblique) fissures, whereas the small left lung has one (oblique) fissure. So, the right and left lungs are divided into three (superior, middle, inferior) lobes and two (superior, inferior) lobes each by the fissures.
The visceral pleura (Fig. 5-15) is protruded into the fissure of the lung. Within the fissure, the pleural cavity also contains serous fluid which lubricates neighboring lobes during the inhalation and exhalation (Fig. 5-2).
Each lobe is constituted by segments. In total, the right lung has 10 segments, whereas the left lung has 9 segments.
One bronchus, one pulmonary artery, and two pulmonary veins are connected to each lung. One pulmonary artery and two pulmonary veins are directly from the heart (Fig. 10-9) (Fig. 10-19).
The larynx is connected to the trachea (Fig. 4-15), which bifurcates into two main bronchi.
Fig. 5-22. Trachea, bronchi.
One can feel not only the laryngeal cartilages (Fig. 5-10), but also the tracheal cartilages at the front of one’s neck. Likewise, bronchi have cartilages which ensure structural integrity and maintain airway. In bronchioles, these cartilages decrease in size, and then finally disappear.
The main bronchus divides into lobar bronchi, and further into segmental bronchi. The segmental bronchi consecutively divide into bronchioles. The bronchioles go on to divide until they become alveoli (Fig. 5-25).
Suppose that the lung were the United States. Then, the lobes would be the states, and the segments would be the cities (Fig. 5-19). In succession, the main bronchus, lobar bronchus, and segmental bronchus would be the president, governor, and mayor.
Like the main bronchus, a pulmonary artery (Fig. 10-9) divides to become lobar arteries and segmental arteries.
Accompanying the segmental bronchus, a segmental artery occupies the center of a segment. However, its continuing vein is located between the segments, rather than at the center of a segment. Therefore, it is called the intersegmental vein (Fig. 5-22). This architecture is made for efficient use of space of the lung.
The pseudostratified ciliated columnar epithelium in the bronchi (Fig. 16-18) secretes mucus where dusts and other small particles are attached. The mucus is moved toward the mouth by the cilia and coughed-up.
Between the alveolus and the adjacent capillary (Fig. 10-1), oxygen and carbon dioxide are passively exchanged by diffusion, due to the difference between their partial pressures. For the fluent exchange, the alveolus and capillary are lined by the simple squamous epithelium (Fig. 16-17).
Cross-sectional area of the entire alveoli is larger than that of the trachea like the capillary (Fig. 10-77). Therefore, airflow in the alveoli is slow, which enables gas to be exchanged unhurriedly.