Respiratory zone

Branching from the terminal bronchioles are the respiratory bronchioles. This is the first generation of airways to have alveoli in their walls. Finally, there are the alveolar ducts which are completely lined with alveolar sacs. This region, from the respiratory bronchioles through the alveoli, is referred to as the respiratory zone, which comprises most of the lungs and has a volume of about 3000 ml at the end of a normal expiration. 

Alveolar ventilation. Alveolar ventilation is less than the total ventilation because the last portion of each tidal volume remains in the conducting airways therefore, that air does not participate in gas exchange. As mentioned at the beginning of the chapter, the volume of the conducting airways is referred to as anatomical dead space. The calculation of alveolar ventilation includes the tidal volume adjusted for anatomical dead space and includes only air that actually reaches the respiratory zone ... 

An alveolar region or respiratory zone, which includes the alveolar ducts and the alveoli where the gas exchange takes place. 

The transition zone consists of the respiratory bronchioles (generations 17 to 19), which contain alveoli. At the terminal end, the respiratory zone is composed of parenchyma that contains the alveolar ducts and about 300 million alveoli (alveolar sacs) to provide the gas-exchange surface. Since the surface area expands to such a large extent within the very last generations of bifurcations, the inhalation airflow rapidly slows down to zero velocity so that the movement of gas molecules and the exchange occurs entirely by diffusion (Stocks and Hisloop 2002). 

Pulmonary deposition of an aerosol preparation is determined primarily by its size. Aerosols with a mass median aerodynamic diameter of 1-5 xm produce the best therapeutic results and are the target particle size for inhalation therapy. These small particles penetrate deep within the respiratory tract to ensure drug deposition in peripheral airways. The cross-sectional area (cm ) of the lung increases dramatically at the level of the respiratory zone therefore, the velocity of gas flow during inspiration rapidly decreases at this level. Moderate-sized particles (5-10 (xm) frequently settle out by sedimentation in larger more central airways because the velocity of gas falls rapidly in the region of the terminal bronchioles. 

Blood vessels supplying the conducting or central airways (i.e., the bronchial circulation) are part of the systemic circulation. By contrast, the blood supply to airways of the respiratory zone involve the pulmonary circulation. The separation of these vascular networks can be almost complete. For example,... 

The different zones of the airways, conducting and respiratory zones, possess different physiological functions and are distinguished by their roles in the exchange of gases. 

Conducting airways do not contribute to the gas exchange and can be considered to be merely a conduit between the external environment and the respiratory zone (vide infra). The volume of air accommodated by the conducting airways represents the anatomic dead space and is air not directly available for gas exchange. Aside from serving as a conduit to the respiratory zone, the conducting airways perform two other functions gas buffering and humidification. 

In the respiratory zone, the epithehum becomes thinner and changes Irom simple cuboidal epithehum to simple squamous [1]. The alveolar epithehum is made up of two types of cells type I and type II cells. Type I cells maintain the alveolar side of the blood—air barrier and cover about 95 % of the alveolar surface. Type II ceUs cover the rest of the alveolar surface and produce surfactants (ie, compounds that reduce surface tension) and act as stem cells for type I ceUs. 

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