(9) RESPIRATORY SYSTEM

• Three parts of the Respiratory System:

  1. Ventilating mechanism
  2. Conducting portion
  3. Respiratory portion
• Ventilating mechanism - This mechanism that creates pressure differences that move air into (inspiration) and out of (expiration) the lungs, includes the diaphragm, rib cage, intercostal muscles, abdominal muscles, and elastic connective tissue in the lungs
• Conducting portion - This portion includes the nasal cavity, nasopharynx, larynx, trachea, bronchi, bronchioles and the terminal bronchioles.
• Inspiration (inhalation) - is active, involving muscle contraction. To inhale, the intercostal muscles lift the ribs while the diaphragm and abdominal muscles lower the floor of the thoracic cavity.
• Expiration (exhalation) - is more passive: Relaxing the muscles allows the elastic fibers to retract, contracting the lungs and forcing air out.
• mucociliary blanket - which covers the mucosal surface of the conducting airways, serves to trap inhaled particles and convey them and cellular debris out of the system.
• Respiratory portion - is the site of actual exchange of gases and is the distal continuation of the conducting portion.
• In those regions of the lungs where the gaseous exchanges take place, the epithelium is simple squamos and the goblet cells are absent.
• bronchioles - represent the transitional structures between the conducting and respiratory portions of the respiratory system
• respiratory bronchioles contain thin-walled outpocketings, the alveoli.
• 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.
• Mucous goblet cells - are the second most numerous types. They secrete the mucus that covers the epithelium and traps and removes bacteria and other particles from the inspired air.
• Brush cells - are also columnar, these cells lack cilia; they often have abundant apical microvilli.
• Two types of brush cells 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.
• Basal cells - are small round cells that lie on the basal lamina but do not reach the lumen. They appear to be stem cells that can replace the other cell types.
• Small granule cells - resemble basal cells, but they contain many small cytoplasmic granules and exhibit DNES (diffuse neuroendocrine system) activity.
• Metaplasia - refers to tissue organization or type undergone by epithelia in response to changes in physical and chemical environment
• Lamina Propria - consists of connective tissue and contains mucous glands in the upper tract (from the nasal cavity to the bronchi).
• Smooth muscle - begins in the trachea, where it joins the open ends of the C-shaped tracheal cartilage. In the bronchi, many layers of smooth muscle cells encircle the walls in a spiral pattern. From this point, the thickness of the muscle layer gradually decreases until it disappears at the level of the alveolar ducts.
• Nasal cavity -This cavity is divided by the nasal septum into two bilaterally symmetric cavities that open to the exterior through the nares (nostrils).
• Each nasal cavity consists of two chambers - a vestibule and a nasal fossa that differ in position, size and wall structure.
• Three regions of the nasal cavity

  1. Vestibular region
  2. Respiratory region
  3. Olfactory region
• Vestibular region - represents the point of reflection of the skin as the mucous membranes of the nasal cavity. It is lined by non-keratinized stratified squamos epithelium. Pigment cells may be present. The muscularis mucosa is absent. The lamina propria - tunica submucosa is coarse areolar connective tissue that blends with the underlying fascia of muscle or the fibrous layer of the associated investments of bone or cartilage.
• Respiratory region - comprises the bulk of the nasal cavity, lined by ciliated pseudostratified columnar epithelium with goblet cells. Muscularis mucosa is absent. The lamina propria-tunica submucosa is areolar connective tissue.
• Olfactory region - is a specialized area for olfaction that is located on the ethmoturbinates, dorsal turbinate and nasal septum. It may be yellow, brown, gray or black in color due to the accumulation of pigment. The lining epithelium is ciliated pseudostratified columnar. It is very thick and may have as many as 15 layers of nuclei.
• The cell types in the olfactory region:

  • Supportive (sustentacular) cells
  • Basal cells
  • Neurosensory olfactory cells
• Supportive (sustentacular) cells - are tall with broad apices and narrow bases. The nucleus is oval, vesicular and located towards the apex of the cell.
• Basal cells - are cuboidal in shape.
• Neurosensory olfactory cells - are modified neurons, the basal process of which continues to the brain as axons of the first cranial nerve. The nucleus is round, vesicular and located centrally or basally.
• the olfactory epithelium is always kept moist by a thin, watery secretion produced by the serous tubuloacinar olfactory (Bowman’s) glands
• Cell types of the nonolfactory epithelium:

  • Basal cells
  • Ciliated cells
  • Secretory cells
  • Brush cells
• Basal cells - are electron dense columnar cells attached at the base of the epithelium to the basal lamina of the hemidesmosomes.
• Ciliated cells - are anchored to the apical cytoplasm by the basal bodies and the microvilli; sometimes branched, contain microfilaments and extend down into the cytoplasm.
• Secretory cells - occur either as goblet or as slender mucus-secreting cells, varies with both secretory phase and location within the nasal cavity. Goblet cells of most species are thought to secrete primarily a sulfated acid glycoprotein as a major component of mucus.
• Brush cells - have long, thick microvilli containing bundles of microfilaments, and a cytoplasm containing mitochondria, granular ER, and a large number of filaments.
• VOMERONASAL ORGAN - is a tubular, blind-ending organ located in the mucosa of the ventral portion of the nasal septum. It consists of an external cartilaginous support, a middle lamina propria that is both highly glandular and vascular, and an internal epithelial duct.
• Right and left vomeronasal ducts - open into ipsilateral incisive ducts.
• hyaline vomeronasal cartilage - is J-shaped, enclosing all the dorsolateral portion of the organ.
• Lining epithelium - passes through a transition from a stratified cuboidal lining rostrally near the incisive duct to a ciliated pseudostratified columnar over much of the caudal portion of the vomeronasal duct.
• medial epithelium - is a pseudodtratified columnar with basal, sustentacular and neurosecretory cells.