RESPIRATORY SYSTEM
Author: Dr.R.Menaka
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The primary function of respiratory system is to provide the exchange of respiratory gases i.e. oyxgen and carbondioxide. The conducting airways also serve a protective function by conditioning incoming air( respired air). The mucociliary blanket, which covers the mucosal surface of conducting airways, serves to trap inhaled particles and conveys them and cellular debris out of the system.
Nasal cavity:
This area divides in to three region
1. Cutaneous region - nasal vestibule
2. Respiratory region- lined by respiratory epithelium (Pseudo stratified ciliated columnar)
3. Olfactory region - neurosensory cells, sustentacular cells and basal cells.
The respiratory epithelium consists of
1. Goblet cells - secretes mucus
2. Basal cells- polyhydral cells and located along the basal lamina
3. Ciliated cells- columnar type
4. Neuroendocrine cells
5. Brush cells- have long , thick villi and cytoplasm containing mitochondrai and many filaments.
Vomeronasal organ- Organ of Jacob:
Located in the mucosa of the ventral portion of the nasal septum.
It consists of an internal epithelial duct (vomeronasal duct), propria submucosa (middle layer) and an external cartilaginous support.
Vomeronasal glands are located in the lamina propria submucosa which is highly vascularised.
Significance:
The vomeronasal organ functions in the chemoreception of liquidborne compounds of low volatility.
Sensing these compounds is believed to function in sexual behaviour of both the male and the female.
It is associate dwith the lip-curl type of facial grimace (Flehmen) action used by some male animals top sample substances in the urine of the female.
Nasopharynx:
The nasopharynx is, the portion of the pharynx and located dorsal to the soft palate, extending from the nasal cavity to the laryngopharynx. It is lined by respiratory epithelium (Pseudo stratified ciliated columnar epithelium) but is stratified squamous epithelium over the caudodorsal portion of the soft palate.
Trachea:
The trachea provides the air passage way between the larynx and the bronchi.
Tunica mucosa:
Epithelium- Pseudo stratified ciliated columnar epithelium (respiratory epithelium). The respiratory epithelium of the tracheobronchial tree is comprised of ciliated cells, brush cells, secretory cells, clara cells (bronchiolar exocrine cells) and neuroendocrine cells(APUD).
Lamina propria- Submucosa- consists of loose connective tissue. seromucous glands are present.
The most distinct feature of the trachea is hyaline cartilage. The hyaline cartilage forms C or U shape seperate pieces in most species. The dorsal free ends of the cartilages are bridged by the trachealis muscle. The hyaline cartilage is surrounded by connective tissue called the perichondrium. The external perichondrium is surrounded by the loose connective tissue of the adventitia. The chondrocytes are arranged in the lacunae and forms cell nest or isogenous cell groups. The amorphous ground substance of hyaline cartilage is a firm gel laced with type II collagen fibers.
Note: In most species, the trachealis muscle attaches to the perichondrium on the internal side of the cartilage. But in carnivores, this attachment is on the external surface of the cartilage.
Lung:
The lung is encapsulated by a layer of connective tissue and mesothelial cells termed as visceral pleura. It is divided in to
1. Intra pulmonar conducting airways
2. Gas exchange area
3. Pleura
1. Intra pulmonar conducting airways:
Bronchus and Bronchi - Pseudo stratified ciliated columnar epithelium. Submucosal glands are present in the lamina propria-submucosa. Amount of cartilages decreases and smooth muscle fibers relatively increases.
Bronchioles- lined by simple columnar or cuboidal type of epithelium. The epithelium comprised of ciliated cells and clara cells (bronchiolar exocrine cells). Clara cells have characteristics of both secretory cells and cells capable of metabolizing xenobiotic compound. Lamina propria - submucosal glands and cartilages are absent.
2. Gas exchange area-
It includes, respiratory bronchioles, alveolar ducts, alveolar sacs and alveoli.
Respiratory bronchioles- Simple columnar epithelium. respiratory bronchioles branch into tubular structures termed alveolar ducts. The alveolar ducts termiantes into alveolar sacs. The alveolar sac open in alveoli which is basic unit of gas exchange in the pulmonary paranchyma.
Alveoli is lined by simple squamous epithelium. There are two kind of epithelial cells. i.e. Alveolar epithelial cell type I and Alveolar epithelial cell type II (forms blood -air barrier). Septal and dust cells are present in the alveolar septa.