In this article we will discuss about the structure of respiratory tract of humans with the help of suitable diagram. Also learn about its functions.
i. Upper respiratory tract extends from the upper nares to the vocal cord. (Fig. 8.1)
ii. Lower respiratory tract extends from the vocal cord to the alveoli.
The trachea (windpipe) is a cartilaginous and membranous tube, about 10 or 11 cm long. It is not quite cylindrical, being flattened posteriorly, its external diameter from side to side is about 2 cm in the adult male and 1.5 cm in the adult female. It is kept patent by incomplete C-shaped rings of cartilage on its anterolateral wall, which keeps air tubes open. There are plenty of seromucous glands in the submucous coat innervated by the vagi. Mucous gland along with goblet cells traps dust in the inspired air and air is moistened by contact with moist lining of serous glands.
The cilia beat spontaneously at the rate of 20 times per minute and by their movement drive out mucus-trapped foreign particles and bacteria towards the mouth from the respiratory passage, the velocity of movement being 24 mm per minute. The ciliary movement is independent of nerves but dependent in the presence of atmospheric oxygen in the respiratory tube.
Functions of the Respiratory Tract:
The infolding of nasal mucous membrane over the turbinates offers an area of contact approximately 160 sq cm for the atmospheric air flowing through the nose.
The inspiratory air is modified during its passage through the nose as described below:
1. Filtering Effect:
It is cleared of dust particles:
i. Large dust particles are caught by the hairs at the nostrils.
ii. Air can flow uninterruptedly through the zigzag passage of the nasal cavity caused by the turbinates but smaller particles suspended in the air due to their momentum are precipitated on the surface of the mucous membrane of the nose and are caught in mucus. This has been described as ‘turbulent precipitation’ and particles upto the size of 6µm are caught in this way and are moved towards pharynx by ciliary action of the nasal epithelium when they are expectorated or swallowed. This size is smaller than the size of red blood cells.
iii. Particles between 1 and 5 µm in diameter get precipitated on the wall of the smaller bronchioles (gravitational precipitation) where they evoke fibrotic reaction (‘coal – miners’ disease).
iv. Particles of size 1 µm or less in diameter diffuse into the alveoli and adhere to the wall of the alveoli.
v. Particles smaller than 0.5 µm remain suspended in the alveolar air and are usually expired out. This size of the particles of cigarette smoke is about 0.3 µm. Most of them diffuse into the alveoli where about 1/3rd get caught in the alveolar fluid the remainder is expired out. Entrapped particles in the alveoli are removed by macrophages or provoke growth of fibrous tissue in the alveolar septa.
An excess of particles provokes fibrous tissue reaction causing permanent disability.
Role of Ciliated Epithelium:
The cilia of trachea and bronchi beat towards the pharynx and propel the mucus with the entangled foreign particles with a velocity of one centimetre per minute towards the pharynx. They thus help materially to keep the respiratory passages free from foreign particles.
2. Air Conditioning Effect:
The inspired air is brought to near about body temperature and is humidified before it is permitted to enter the deeper air passages. Cold and dry air coming in direct contact with the lungs such as after tracheotomy will cause serious lung crusting and infection.
It has been estimated that the inspired air is brought to within 3% of body temperature during its passage through the nose and it is almost completely saturated with water vapour before it is delivered to the lungs.
‘Cough’ is a protective reflex by means of which respiratory passages are kept free from foreign matter. Patients, in whom cough reflex is lost, get drowned in their own secretion. An instrument called ‘Cofulatof has been designed to stimulate cough reflex in these patients.
Sensory stimulus necessary for this reflex is either physical or chemical irritation of the respiratory tract. The larynx and the area of bifurcation of the trachea are most sensitive areas. The afferent fibres run in the vagus to the medulla.
At the commencement of the act a deep inspiration is taken, the epiglottis is then closed and the vocal cord closes the laryngeal opening tightly thus entrapping about 2.5 litres of air into the lungs.
The abdominal muscles then contract forcefully pushing firmly the diaphragm against the distended lungs. Internal intercostal and other expiratory muscles contract forcefully causing the pressure within the lungs to rise to about 100 mm Hg or more. The epiglottis and vocal cord then open suddenly and thus the lung air under pressure comes out with explosive violence with a velocity of about 160 km or 100 miles per hour. This act thus helps in clearing out the obnoxious irritating agents from the respiratory passages.
It is very much like cough reflex but here the zone of afferent stimulus is in the nose and the afferent pathway is in the trigeminal nerve. A large volume of air is drawn into the lungs and the uvula is depressed so that the expired air passes mostly through the nose and partly through the mouth.