Overview of Respiratory System

The respiratory system serves one of the vital functions of the animal body by delivering oxygen from the surroundings to each cell of the body and removing carbon dioxide to be

7.1.1 Components of Respiratory Apparatus

Mammals’ respiratory systems are divided into two main functional sections—(1) the respiratory component, which is responsible for exchanging oxygen for carbon dioxide in the lungs, and (2) the conducting and conditioning part, which includes respiratory airways.

The lungs are present in chest or thoracic cavity, more or less symmetrically about the heart. Each lung is surrounded by a double layer of pleura, internal and external forming a pleural cavity. Around and below the root of the lung, the pleural layers are continuous. It is not a real cavity because there is no physical space between the two pleural walls. Instead, a tiny layer of fluid that is sandwiched between the two membranes lubricates them when breathing. The lung is held to the surface of the pleural membrane by surface ten­sion. The bronchus, pulmonary artery, pulmonary vein, bron­chial arteries and veins, pulmonary nerves, lymphatic vessels, and bronchial lymph nodes make up the root of the lung. Each lung is further divided into a number of separate lobes, depending on the species. In cattle, the right lung has four lobes, while the left lung has two.

7.1.1.1 Airways to Lung

Air is taken into the respiratory tract through external nares (nostrils) and passed forward through nasal cavities. The quantum of air entering the nasal cavity is determined by the degree of flexibility, termed “pliability” of nares, which differs between species.

The epithelium of nasal cavities is pseudostratified- ciliated-columnar epithelium containing aggregates of lymphoreticular tissue, lymphocytes and tubuloalveolar glands. The nasal cavities are endowed with complex struc­ture for carrying out its functions. One such structure is conchae, which are spiral bone laminas coated with mucosa present inside the nasal cavity. The mouse has two nasal conchae—dorsal and ventral conchae. In some species (e.g. small laboratory animals), a distinct nose-associated lymphoid tissue (NALT) is present in the nasal cavity. Bidi­rectional air movement in nasal cavities keeps the surface cool that allows further cooling of nasal venous blood tem­perature as compared to the general body temperature. The internal carotid artery passes through the cavernous sinuses, where the cool venous blood decreases blood temperature of the internal carotid artery in the countercurrent transfer mech­anism. This arrangement keeps the temperature of brain 1-3 ° C lesser as compared to general body temperature.

Air passes the nasopharyngeal tube to reach the pharynx, which is the intersection point with the digestive tract. The mucosa of nasopharynx has pseudostratified-ciliated-colum- nar epithelium and goblet cells for secretion of mucosa. Waldeyer’s tonsillar or lymphatic ring, a ringed arrangement of lymphoid organs, is also present. This organ is vital for inducing immunity at the mucosal layer. The larynx links the nasopharynx with the lower respiratory tract and forms the landmark of its beginning. The larynx also contains epiglottis and vocal cords (plica vocalis). The mucosa of larynx (except epiglottis and vocal cords) is lined by a pseudostratified- columnar epithelium and isolated goblet cells; in the submu­cosa of the epiglottis, plica aryepiglottica and vestibulum laryngis, lymphatic nodules are present.

The trachea is comprised of 32-36 “C”-shaped rings of hyaline cartilage with strong fibroelastic membranes bridging the gaps between the rings.

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Vitamin A plays a vital role in developing the lungs in the foetal stage. Studies have shown that specific retinoic acid (RA) receptor proteins are organised in a particular way during the formation of branches and growth of airways in the foetus. This retinoic acid can regulate other genes affecting lung development, such as homeobox genes, certain growth factors and matrix molecules. A deficiency of retinol can result in similar pathology as bronchopulmonary dysplasia in prema­ture babies.

The large bronchial tubes subdivide each lung like a tree branch. Functionally, the airway system in lungs can be categorised into a conducting zone (trachea, bronchi, bronchioles and terminal bronchioles), a transition zone (respiratory bronchioles) and a gas-exchange region (alveolar ducts, sacs and walls). This structural arrangement enhances respiratory efficiency by influencing the distribution of air and blood. The bronchial tree ends are called the terminal bronchioles, and the parts lying after the terminal bronchioles are involved in gaseous exchange with the blood and are also called the respiratory zone (Fig. 7.1a). These parts are the respiratory bronchioles, alveolar ducts, sacs and pulmonary alveoli (Fig. 7.1b). As the branching increases in the bron­chial tree, the diameter decreases, but the cross-sectional area increases exponentially. This aids in drastically reducing airflow resistance along the airway path following Hagen- Poiseuille’s law.

The respiratory bronchioles are an extension of terminal bronchioles and have the same diameter as terminal bronchioles, i.e. about 0.24 mm. About 5 or 6 alveolar ducts (alveolar duct diameter is about 0.19 mm) are attached to the respiratory bronchioles. About 3-6 alveolar sacs are attached to each alveolar duct. The pulmonary alveoli are hemispherical out-pouching of the alveolar sacs. Their diam­eter ranges from 0.075 to 0.125 mm, and their total number is estimated to be about 750 million. The inner lining of pulmo­nary alveoli is made up of a single layer of epithelial cells.

Fig. 7.1 “Respiratory zone” demonstrating its various parts where gaseous exchange occurs (a). The zone beyond terminal bronchiole comprises the site of gaseous exchanges (b)

The alveoli walls also contain elastic fibre and a massive capillary network that aids in gaseous exchange. The entire set of dichotomous branching is called respiratory tree.

7.2