The respiratory system
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Unit 7 THE RESPIRATORY SYSTEM Key words: breathing, internal respiration, external respiration, oxygen, carbon dioxide, the upper respiratory system, the lower respiratory system, lungs, bronchi, bronchioles, alveoli, larynx, acute bronchitis, chronic bronchitis, acute and chronic asthma, pulmonary fibrosis, breathing exercises, pneumonia, tuberculosis, lung carcinoma. The term respiration refers to three separate but related functions: 1. ventilation (breathing) 2. gas exchange, which occurs between the air and blood in the lungs and between the blood and other tissues of the body, and 3. oxygen utilization by the tissues in the energy-liberating reactions of cell respiration. Ventilation and the exchange of gases (oxygen and carbon dioxide) between the air and blood are called external respiration. Gas exchange between the blood and other tissues and oxygen utilization by the tissues are known as internal respiration. Ventilation is a mechanical process that moves air into and out of the lungs. The respiratory system is formed by the nasal cavity, pharynx, larynx, trachea, the bronchi, bronchioles and alveoli within the lungs. The structures of the upper respiratory system include the nose, pharynx and associated structures. The structures of the lower respiratory system include the larynx, trachea, bronchial tree, alveoli, and the lungs. The nose is the primary passageway for air entering. The pharynx is a common passageway for the respiratory and digestive systems. It receives air from the nasal cavity, and air, food and fluid from the oral cavity. The nasal cavity is divided into right and left halves by a nasal septum. The pharynx is a passageway that connects the nasal and oral cavities to the larynx. The larynx contains the vocal cords, which are responsible for sound production. The vocal cords do not vibrate when whispering. The trachea, commonly called the windpipe, is a passageway, approximately 12 cm long and 2.5 cm in diameter, connecting the larynx to the primary bronchi. The cartilages of the trachea ensure that the airway will always remain open. Air from the trachea passes through a bronchial tree to alveoli, where gas exchange occurs. The alveoli are the functional units of the respiratory system and account for most of the mass of the lungs. The trachea splits to form a right and left primary bronchus. The bronchus divides deeper in the lungs to form secondary bronchi and tertiary bronchi. The bronchial tree continues to branch into even smaller tubules called bronchioles. At the end of each bronchiole there are microscopic alveoli, or air sacs. The lungs are paired organs situated within the thoracic cavity. Their surfaces are bordered by the ribs to the front and back. The right lung is divided into three lobes and the left one into two lobes. As we breathe, most of the work is done by diaphragm, a sheet of muscle and fibrous tissue that forms a complete wall between the chest and the abdomen. The ribs provide the upper part of the cage that encloses the heart and lungs and the diaphragm forms the bottom. The muscular fibres of the diaphragm contract when we breathe in. Like any other muscle, the diaphragm receives instructions to contract or relax from the nervous system. The rate of breathing is controlled by the respiratory centre of the brain, the medulla oblongata, and is regulated according to the levels of carbon dioxide in the blood, rather than the amount of oxygen present. The brain will respond to an increased production of carbon dioxide, such as when the body undergoes physical exercise, and adjust the breathing rate accordingly. We breathe an average of 15 times per minute during quiet breathing and more quickly during exertion or in the course of various illnesses. A normal person exchanges approximately 6 litres of air in each minute of quiet breathing. Cessation of breathing during sleep, or sleep apnoea, is caused by a variety of disease processes. Victims of sudden infant death syndrome as a tragic form of sleep apnoea are apparently healthy twoto five-month-old babies who die in their sleep without apparent reason. These deaths seem to be caused, for instance, by failure of the respiratory control mechanism in the brain stem. COMMON RESPIRATORY DISORDERS A cough is the most common symptom of respiratory disorders. The common cold is the most widespread of all respiratory diseases. Colds occur repeatedly because acquired immunity to one virus does not protect against other viruses that cause colds. Cold viruses cause acute inflammation of the respiratory mucosa, resulting in a flow of mucus, a fever, and often a headache. Influenza is a viral disease that can become epidemic, but fortunately vaccines are available. Acute bronchitis is an inflammation of the trachea and bronchial tubes. Acute bronchitis results from living in a dusty, damp and foggy atmosphere and smoking. It is characterized by a persistent dry cough that may last several weeks. Chronic bronchitis results from recurrent attacks of acute bronchitis or prolonged exposure to chemical irritation from cigarettes, exposure to smoke and dust. It is incurable, but early treatment prevents progression and lung damage. Chronic bronchitis is a disorder characterized by increased mucus secretion in the bronchi. Asthma is a chronic disorder manifested by attacks of dyspnoea in which the air in the alveoli becomes trapped and entrance of fresh air is prevented. The main reason of asthma is allergy, such as hay fever, hypersensitivity to certain drugs or food or substances inhaled. In a condition of pulmonary fibrosis the normal structure of the lungs is altered by the accumulation of fibrous connective tissue proteins. The type of fibrosis anthracosis, or black lung, is produced by the inhalation of carbon particles from coal dust. Pneumonia is an acute infection and inflammation of lung tissue. It is usually caused by bacteria, most commonly the pneumococcus bacterium. Bacterial disease has a