List of seven parts of human respiratory system: 1. Nostrils 2. Nasal Cavities 3. Internal Nares 4. Pharynx 5. Larynx 6. Trachea and Primary Bronchi 7. Lungs.
Part # 1. Nostrils (External Nares):
Holes of the nose are called nostrils (external nares). These are paired openings that open into two nasal cavities.
Part # 2. Nasal Cavities:
Two nasal cavities are separated from each other by a thin, cartilaginous median vertical partition called nasal septum. Three bony ridges, the superior, middle and inferior nasal conchae arise from the wall of each nasal cavity. The nasal conchae increase the surface area of the nasal cavities.
Part # 3. Internal Nares:
These are the posterior openings of the nasal cavities that lead into the nasopharynx.
Part # 4. Pharynx:
As it has already been described in the digestive system, it comprises nasopharynx, oropharynx and laryngopharynx. The pharynx provides passage to both air and food.
Part # 5. Larynx (= Voice Box):
Until puberty there is little difference in the size of the larynx in man and woman. Thereafter, it grows larger and becomes prominent in man, therefore, it is called Adam’s apple in man. Human larynx consists of the following structures.
(i) Glottis:
The pharynx opens into the larynx by a slit-like aperture, the glottis.
(ii) Cartilages of the Larynx:
There are nine pieces of cartilages. Three are single and three are paired.
(a) Epiglottis:
It is a leaf shaped cartilage which projects into the pharynx. It is made up of elastic cartilage. During swallowing, the epiglottis closes the glottis to check the entry of food into it.
(b) Thyroid Cartilage:
It is the largest cartilage of the larynx. It has a V-shaped superior notch (depression) called thyroid notch. Thyroid cartilage supports the larynx on the front and sides. It forms a subcutaneous projection named the laryngeal prominence (It is also called Adam’s apple). Thyroid cartilage is made up of hyaline cartilage.
(c) Cricoid Cartilage:
It is like a signet ring and lies below the thyroid cartilage. It is also made up of hyaline cartilage.
(d) Arytenoid Cartilages:
They are small, two in number and are pyramidal in shape. They are situated at the back of the larynx. The greater part of the arytenoid cartilages consists of hyaline cartilage but their apical parts are made up of elastic cartilage.
(e) Corniculate Cartilages (= Cartilages of Santorini):
They are two small conical nodules of elastic cartilage and located at the upper ends of the arytenoid cartilages.
(f) Cuneiform Cartilages:
They are two small rod shaped pieces of elastic cartilage that connect the epiglottis to the arytenoid cartilages.
With advancing age calcification may occur in hyaline cartilage, but not in elastic cartilage.
(iii) Hyoid Bone:
It lies just above the larynx. Embryo logically it is considered a bone of skull.
(iv) Thyrohyoid Membrane:
It is a broad, flat membrane attached to the hyoid bone above and to the thyroid cartilage below.
(v) Laryngeal Ligaments and Muscles:
The laryngeal ligaments connect different cartilages of the larynx. The laryngeal muscles play an important role in the movements of the larynx.
(vi) Vocal Cords:
Inside the larynx there are present the vocal cords. These are two pairs of folds of mucous membrane that extend into the lumen of the larynx from the sides. The upper pair is termed the false vocal cords and the lower pair is called the true vocal cords.
Vocal cords are usually thicker and longer in men than in women and, therefore, they vibrate more slowly. Thus men generally have a lower range of pitch than women. The vocal cords are made up of yellow elastic tissue covered by non-keratinized stratified squamous epithelium. The rima glottidis is the gap between the vocal cords that communicate with the laryngopharynx above.
Voice Production:
Sound is produced by true vocal cords. When expired air is passed through the true vocal cords under pressure from the lungs, the vocal cords are set into vibration, which results in the production of sound.
The pitch of a sound is determined by the tension on the vocal cords— the greater the tension the higher the pitch. The quality of voice depends on the resonators above the larynx, namely the pharynx, mouth and Para nasal sinuses. The quality is controlled by the muscles of the soft palate, tongue, floor of the mouth, cheeks, lips and jaws
Human Respiratory System: The larynx leads into a long tube, the trachea or wind pipe bearing rings of hyaline cartilage which are incomplete posteriorly. These cartilaginous rings support the walls of the trachea and prevent its collapse during inspiration.
The trachea is lined by pseudo stratified ciliated columnar epithelium bearing glandular cells (mucous glands). The secretion of mucous glands keeps the walls of the tube moist and traps dust particles which enter with the air. The vibratile cilia of the epithelium then carry the mucus containing dust particles up to the throat where they can be spitted out.
At its lower end the trachea bifurcates into a pair of primary (= principal) bronchi which enter the right and left lungs. The walls of the bronchi are likewise supported by cartilaginous rings.
Part # 7. Lungs (= Pulmones):
Position:
There are present a pair of lungs (Fig. 17.3) which lie in the thoracic cavity, one on either side of the heart. The thoracic cavity is enclosed behind by the thoracic vertebrae, laterally by the ribs and in-front by the sternum. The thoracic cavity is closed below by the diaphragm.
Pleurae:
Each lung is enclosed in two membranes called the pleurae, The outer membrane is called parietal pleura. The inner membrane is known as visceral pleura, which closely invests the lungs.
A very narrow space exists between the two pleurae. It is called the pleural cavity that contains a pleural fluid secreted by the pleurae. The pleural fluid lubricates the pleurae so that they may slide over each other without friction during breathing.
Mediastinum:
The mediastinum strictly speaking is the partition between the two lungs and therefore, includes the pleura of both sides, but it is generally defined as the interval between two pleural sacs. It contains heart, oesophagus, thoracic duct of lymphatic system, remains of thymus, etc.