GENERAL FEATURES OF THE RESPIRATORY SYSTEM

A. Components and Basic Functions of the Respiratory System: The respiratory system includes the lungs, airways tie, pharnyx, larynx, trachea, bronchi) and associated structures. Specialized for gaseous exchange between blood and air, including the uptake of oxygen and release of carbon dioxide, it is functionally divisible into 2 major parts: the conducting and respiratory portions.

1. Conducting portion. The walls of this system of tubes are specialized to carry air to and from the site of gas exchange without collapsing under the pressures created by the ventilating mechanism. This portion also conditions the air, warming, moistening, and cleaning it to enhance gas exchange. It includes the nasal cavity, nasopharynx, larynx, trachea, bronchi, bronchioles, and terminal bronchioles.


2. Respiratory portion. This portion is distinguished by alveoli, small, saccular structures whose thin walls enable the gas exchange between air and blood. Alveoli occur in clusters at the end of the bronchial tree. These clusters extend (like rooms from a hallway) from the walls of respiratory bronchioles, alveolar ducts, and atria and alveolar sacs.


B. Wall Structure: Like the digestive tract, the tubelike respiratory tract has layered walls whose lining epithelium derives from endoderm. The wall layers include an epithelium, a lamina propria that contains mucous glands as well as cartilage that prevent the tract from collapsing under pressure, smooth muscle that regulates the luminal diameter, and an adventitia that contains collagen and elastic fibers. Respiratory epithelium

a. General features. The epithelium lining most of the tract is ciliated pseudostratified columnar with goblet cells; it is generally referred to as respiratory epithelium. As the respiratory tract undergoes branching and its luminal diameter decreases, the epithelium gradually drops in height and loses first goblet cells and then cilia as it approaches the alveoli.

b. Epithelial cell types :

(1) Ciliated columnar cells predominate in the tract. Each has about 300 motile cilia on its apical surface; there are associated basal bodies in the apical cytoplasm.
(2) Mucous goblet cells are the second most numerous type. They secrete the mucus that covers the epithelium and traps and removes bacteria and other particles from inspired air. Cilia projecting from columnar cells sweep the contaminated mucus toward the mouth for disposal. (3) Brush cells. Also columnar, these cells lack cilia; they often have abundant apical microvilli. Two types are present: One resembles an immature cell and apparently serves to replace dead ciliated or goblet cells; the other has nerve endings on its basal surface and appears to be a sensory receptor.
(4) Basal cells. These small round cells lie on the basal lamina but do not reach the lumen. They appear to be stem cells that can replace the other cell types.

c. Metaplasia refers to the change in tissue organization or type undergone by epithelia in response to changes in the physical or chemical environment. For example, a smoker's respiratory epithelium typically develops more goblet cells in response to high pollutant levels and fewer ciliated cells in response to carbon monoxide. These changes, which are reversible, frequently cause congestion of the smaller airways.