The respiratory system

The respiratory system includes the lungs and a series of airways that connect the lungs to the external environment.
The respiratory system can be functionally classified into two major subdivisions:
1- a conducting portion, consisting of airways that deliver air to the lungs
2- a respiratory portion, consisting of structures within the lungs in which oxygen in the inspired air is exchanged for carbon dioxide in the blood.
This portion of the respiratory system includes the nose, nasopharynx, larynx, trachea, bronchi,and bronchioles of decreasing diameters, including and ending at the terminal bronchioles.
These structures warm, moisten, and filter the air before it reaches the respiratory components,where gas exchange occurs.
A. Nasal cavity
• The nares are the nostrils; their outer portions are lined by thin skin, they open into the vestibule, the vestibule is the first portion of the nasal cavity, where the epithelial lining becomes nonkeratinized. Posteriorly, the lining changes to respiratory epithelium (pseudostratified ciliated columnar epithelium with goblet cells). The vestibule contains vibrissae (thick, short hairs), which filter large particles from the inspired air , it has a richly vascularized lamina propria (many venous plexuses) and contains seromucous glands.
• Olfactory epithelium: The olfactory epithelium is located in the roof of the nasal cavity, on either side of the nasal septum and on the superior nasal conchae. It is a tall, pseudostratified columnar epithelium consisting of olfactory cells, supporting (sustentacular) cells, and basal cells.It has a lamina propria that contains many veins and unmyelinated nerves and houses Bowman glands.
1. Olfactory cells are bipolar nerve cells characterized by a bulbous apical projection (olfactory vesicle) from which several modified cilia extend, olfactory cilia (olfactory hairs) are very long, nonmotile cilia that extend over the surface of the olfactory epithelium , act as receptors for odor.
2. Supporting (sustentacular) cells possess nuclei that are more apically located than those of the other two cell types, have many microvilli and a prominent terminal web of filaments
3. Basal cells rest on the basal lamina but do not extend to the surface, form an incomplete layer of cells are believed to be regenerative for all three cell types.
• Bowman glands produce a thin, watery secretion that is released onto the olfactory epithelial surface via narrow ducts. Odorous substances dissolved in this watery material are detected by the olfactory cilia. The secretion also flushes the epithelial surface, preparing the receptors to receive new odorous stimuli.
B. Nasopharynx
The nasopharynx, the posterior continuation of the nasal cavities, becomes continuous with the oropharynx at the level of the soft palate, it is lined by respiratory epithelium, whereas the oropharynx and laryngopharynx are lined by stratified squamous nonkeratinized epithelium. The lamina propria of the nasopharynx, located beneath the respiratory epithelium, contains mucous and serous glands as well as an abundance of lymphoid tissue, including the pharyngeal tonsil. When the pharyngeal tonsil is inflamed, it is called an adenoid.
C. Larynx
The larynx connects the pharynx with the trachea, the wall of the larynx is supported by hyaline cartilages (thyroid, cricoid, and lower part of arytenoids) and elastic cartilages (epiglottis, corniculate, and tips of arytenoids).The wall also possesses skeletal muscle, connective tissue, and glands.
• The vocal cords consist of skeletal muscle (the vocalis muscle), the vocal ligament (formed by a band of elastic fibers), and a covering of stratified squamous nonkeratinized epithelium.
? Contraction of the laryngeal muscles changes the size of the opening between the vocal cords, which affects the pitch of the sounds caused by air passing through the larynx.
? Inferior to the vocal cords, the lining epithelium changes to respiratory epithelium,which lines air passages down through the trachea and intrapulmonary bronchi.
• Vestibular folds (false vocal cords) lie superior to the vocal cords.
These folds of loose connective tissue contain glands, lymphoid aggregations, and fat cells.They are covered by stratified squamous nonkeratinized epithelium.