The respiratory system is in place to extract Oxygen (O2) from the atmosphere and get rid of Carbon Dioxide (CO2). O2 is used in chemical reactions by the cells to create energy for use in their metabolic activities, CO2 is usually a waste product created from these reactions.
The respiratory system can often be segmented into the ‘upper respiratory tract’ and the ‘lower respiratory tract’. Upper consists of the nose, the pharynx, and the larynx.
The nose and nasal cavity are used as a main source of air entry and are lined with columnar cells (ciliated columnar epithelium) which contain mucus secreting goblet cells. At the anterior of the nose there are also nasal hairs. Within the nasal cavity, conchae exist to increase surface area and cause a turbulence effect for air passing through. The respiratory function of the nose is to warm the air (which is done by the increased vascularity of the nose which explains the potential large blood loss during epistaxis), and to clean and humidify it (which is done through mucus coated nasal hairs trapping dirt and moist mucosa humidifying the air).
The pharynx follows in the next step of the upper respiratory tract. The pharynx begins at the end of the nasal cavity (nasopharynx), extends past the back of the mouth and tongue (oropharynx), and finishes at the larynx and oesophagus (laryngopharynx). The walls of the pharynx contain mucous membrane lining, submucosa, and smooth muscle. The respiratory functions for the pharynx include passage of air, warming and humidifying of air (done through the vascularity of the walls and moist mucosa), protection from pathogens (bacteria etc. through the placement of the tonsils and lymphatic tissue), and assistance in speech (through acting as a resonating chamber for sound).
The last of the upper respiratory tract ends with the larynx (voice box). The larynx is mainly made up from cartilage, dividing into 4 main structures: the thyroid cartilage, the cricoid cartilage, the arytenoid cartilage, and the epiglottis. The thyroid, cricoid and arytenoid are made up from hyaline cartilage, and the epiglottis is made up from elastic fibrocartilage. In-between the arytenoid cartilage are the vocal cords and entrance to the trachea. The respiratory functions of the larynx are to protect the lower respiratory tract (through the epiglottis blocking the passage of food into the trachea whilst eating and swallowing), to humidify and warm the air (through the moist mucosa and vascularity of the walls), and to produce speech and sound (through the use of the vocal cords).
The lower respiratory tract consists of the trachea, the lungs, the bronchi and bronchioles, the respiratory bronchioles and alveoli.
The trachea begins the lower respiratory tract anatomy, consisting of 16 to 20 incomplete ‘C’ shaped hyaline cartilage rings atop of each other. There are three layers of tissue which enclose the trachea and cartilaginous rings; the outer consists of fibrous and elastic tissue which enclose the cartilage. The middle consists of the cartilage and smooth muscle that wraps around the trachea. The inner layer is lined with columnar cells (epithelium) which contain mucus secreting goblet cells. The respiratory functions of the trachea are to warm and humidify air, to expel mucus with unwanted particles and dust (through cilia on the inner lining cells wafting mucus upwards towards the larynx where it can be either swallowed or coughed up), to provide a cough reflex (nerve endings in the larynx, trachea, and bronchi stimulate a cough reflex when irritated), and provide support and patency to the airway (through the C-shaped cartilage providing support to keep the airway open but also allow flexibility for movement and swallowing food).
The lungs are the organs which contain the further lower respiratory tract, and assist in the inspiration and expiration of air. The left lung consists of a superior and inferior lobe, the right consists of a superior, middle, and inferior lobe. The pulmonary artery, veins, and left and right bronchus enter the lungs through the hilum. Surrounding the lungs are the pleura and pleural cavity; visceral pleura coats the outside of the lungs, parietal pleura coats the inside of the chest wall and thoracic surface of the diaphragm, and the pleural cavity is the space between which contains no air, only a thin film of serous fluid. With the pleura in place the surfaces can glide over each other easily and can only be pulled apart with great difficulty. With the expansion of the chest wall and diaphragm during inspiration, atmospheric pressure within the lungs drop and air floods in to stabilize the level to normal. On relaxation of the chest the pressure increases and air is forced out.
The bronchi stem from the trachea acting as openings for air into the lungs. The left bronchus is about 5cm long and narrow, whereas the right is 2.5cm long and wider. The bronchi (more cartilaginous in structure) further expand and branch off into bronchioles (which are less cartilaginous and more smooth muscle and epithelial). The bronchioles further lead on to the end of the lower respiratory tract where the gaseous exchange can take place. Further down the bronchioles, ciliated epithelium are replaced with non-ciliated epithelium and mucus secreting goblet cells disappear. The respiratory functions of the bronchioles are to regulate the control of air entry (through contraction or relaxation of smooth muscle in the walls), and provide warming and humidifying to the air.
The bronchioles divide further into smaller respiratory bronchioles and end with the alveoli. These are thin membrane pockets (or sacks) which are surrounded by capillaries and supported by elastic fibres. Gaseous exchange happens between the membrane wall of the alveoli and capillary, which are fused firmly together and known as the respiratory membrane. Septal cells lying between the squamous cells of the alveoli secrete surfactant which prevents the alveoli from drying out. There are about 150 million alveoli in the adult lung which provide gaseous exchange. The respiratory functions of the alveoli are to provide the centre of gaseous exchange between O2 and CO2.
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