Prof. J.P.N. Mishra
The various organs that are directly concerned with the passage of air to and from the lungs constitute the respiratory system (fig. 1.13).
The Nose: The external nose is only a small part of nose. The most important part of it is situated inside the head, behind the two holes and above the hard palate that forms a part of the roof of his mouth. The uppermost part of nose is used for smelling and the lower parts are used for breathing. During the normal and calm breathing only the lower parts are used while in forced breathing the upper part is also utilized.
The mucous membrane: which covers the respiratory tract, is thick and spongy whereas that which lines the olfactory region is softer and more delicate. The other distinguishing feature is the very great vascularity of the respiratory tract. In fact it is marked by the presence of a rich venous plexus.
The Pharynx: The mucous membrane covering the nose is continues with the pharynx. The nasal chambers posteriorly open into the throat. These openings are situated above the soft palate and below the base of the cranium. This portion marks the beginning of the pharynx. Below the tongue the wall descends in the form of a sack that ends in two openings, one of which leads to the esophagus and the other to the larynx. From the backward openings of the nasal chambers to the lower openings leading to the esophagus and the larynx, the same canal stretches continuously and is known as Pharynx. The pharynx is used for the passage of the air.
The Larynx: The larynx portion is located in front of the third cervical vertebra and it extends across the fourth, fifth and the sixth. Larynx is a box like structure made of different cartilage and measures less than two inches. These cartilages are the thyroid cartilage and the cricoid cartilage. These and other cartilage including the epiglottis, which is also a cartilage, are, held together by muscles which move them according to the needs of the situation. The mucous membrane, which covers the pharynx continuing to cover the larynx also.
The trachea: The tube like structure measuring approximate 4 inches in length starts at the bottom of the larynx from behind the cricoid cartilage and extends down into the chest just behind the breastbone. Sixteen to twenty Cartilaginous rings form this tube. These rings are arranged in such a way that the circular part is placed in front and deficient part is placed behind. The deficient part of the tube is covered by fibrous membrane, which clothes the whole trachea. The membrane part of the trachea and the esophagus run together.
The Bronchi and Bronchioles: The trachea, when descends into the chest to the level of the fifth thoracic vertebra, is divided into two tubes called the bronchi. The tube on the left side enters the left lung and is known as the left bronchus. On the right side, entering the right lung being called the right bronchus. After entering into the substance of the lungs, each bronchus divides into several branches, these branches again dividing themselves into smaller tubes. These divisions and subdivisions continue to penetrate the substance of the lungs through and through till at last the tubes become smaller and smaller. Like trachea similar cartilaginous portions are present throughout the bronchial tubes except in the last finest branches. The mucous glands embedded in the mucous membrane keep their passage moist under healthy circumstances.
The Lungs: - There are air-cells (Alveoli) (Fig 1-13) matted together by means of fibrous tissues. Blood -vessels running through these cells divide and subdivide themselves into myriad of capillaries covering these air-cells, again there is a network of nerves spreading throughout these structures. All these parts, the cells, the blood vessels and the nerves form themselves into two masses of spongy substance each of which is called a lung. The walls of the alveoli are extremely thin and elastic so that the air, which flows into the cells through the air, tubes is very freely diffused through these walls, although its direct passage is blocked up here. The lungs are short in front and longer behind. In front they descend to the sixth ribs whereas behind they stretch up to the eleventh ribs. This position of the lungs is determined by the position of the diaphragm, which forms the floor of the thorax (Tortora at. el., 2006).
