Respiratory SYSTEM

The respiratory system is divided into two tracts: the upper respiratory tract consisting of the nose, pharynx, larynx and trachea, and the lower respiratory tract including bronchi, lungs and alveolar sacs (Fig. 1). While oxygen and carbon dioxide pass through the upper tract during inhalation and exhalation, the actual gas exchange process occurs in the lower respiratory tract. However, the upper respiratory tract mainly functions to warm and moisten air before entering the lungs. 

The nose has several functions. It acts as a filter for the air inhaled picking up the irritants, pathogens and dust through the soft hair inside the nostrils. To prevent the dryness of the respiratory tract, the nose has a mechanism to moisten and warm the air passing through it. Olfaction is one of the main functions of the respiratory system, and it is achieved by the nose (Fig. 2).

The nasopharynx, oropharynx and laryngopharynx are the main components of the pharynx. It starts with the nasopharynx which is sited behind the nose and above the soft palate. Then the oropharynx appears at the back of the mouth. It has the ability to handle food and air through a lining of mucous membrane which cover its walls. It includes the back third of the tongue, the soft palate, the side and back walls of the throat, and the tonsils. There are two types of tonsils: the human palatine tonsils and the nasopharyngeal tonsils. They are composed of lymphoepithelial tissues. The last part of the pharynx is the laryngopharynx. It is considered as an aisle through which food and air can pass to the esophagus (Fig. 3).

It is a tube-shaped part of the respiratory system allowing the air to pass from pharynx to trachea. The vocal cords and the epiglottis are the main divisions of the larynx. During the process of swallowing, the glottal folds of the larynx are shut through the epiglottis to prevent food from entering the trachea, causing choking. The vocal cords in the larynx are supported by cartilaginous walls and advanced muscle system that can facilitate the production of various different sounds (Fig. 4). The larynx is rich with three types of cartilage: thyroid, arytenoid and cricoid. The Adam’s apple refers to the laryngeal prominence of the thyroid cartilage which is bunchier in males. The protection of the upper part of the trachea and the production of sounds are the main functions of the larynx (Fig. 5).

It is a cartilaginous c-shaped tube that connects the larynx to the bronchi and appears parallel to the 4th or 5th thoracic vertebrae with approximately 10 cm in length and 2.5 cm in diameter in adults (Fig. 6).    

The thoracic cavity is divided into two main sections: the mediastinum and pleural cavity. While the mediastinum includes the heart and major vessels, the pleural cavity has two lungs, and it is existed on both sides of the mediastinum (Fig. 7). The lungs are air-filled organ occupying most of the thoracic cavity. It has a spongy formation making its protection with 12 ribs on either side of the cavity a must. Some differences are observed between the two lungs. While the right lung is split into three lobes, the left lung has only two (Fig. 8). It is due to the narrow space in the left of the thoracic cavity mostly occupied by the heart. The two lungs move freely within the thoracic cavity without friction thanks to the fluid lubricating the surface of the layers (Fig. 9). This fluid is produced by the pleura, a membrane covering the thoracic cavity.

At the carina, two similarly structured tubes, known as the right and left bronchi, act as an airway conducting the air to the lungs. The bronchi are composed of rings, and they are further divided to secondary bronchi, tertiary bronchi and bronchioles (Fig. 10). They are covered by hyaline membrane and smooth muscle. The shape of the hyaline membrane changes becoming irregular and smaller with the disappearance of the cartilage. The smooth muscle becomes thicker when the hyaline membrane stops at the level of the bronchioles. The bronchioles are split into smaller and smaller airways ending up with alveoli, tiny air sacs. 

They are tiny sacs taking the shape of bunches of grapes. Alveoli are the units where the lungs and blood vessels exchange oxygen and carbon dioxide. They represent the end of the respiratory tract (Fig. 11). 

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The respiratory system has the mission of providing the body with oxygen and getting rid of carbon dioxide. This work is accomplished through what is called respiration which involves four main events. Breathing the air in and out the lungs to keep the air in the alveoli continuously refreshed. This process is called pulmonary ventilation or breathing. Then gases are exchanged between alveoli and pulmonary blood, and it is called external respiration. The bloodstream is responsible for transferring oxygen and carbon dioxide to and from the different cells and organs of the body in a stage named respiratory gas transport. Through systematic capillaries, different cells of the body exchange gases with the blood, and it is called internal respiration.

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With the contraction of the inspiratory muscles, the diaphragm and external intercostals, the rib cage is lifted, the sternum is thrusted forward and the height of the thoracic cavity increases. The antero-posterior and lateral dimensions of the chest accordingly increase. This is the way how inspiration occurs. As the lungs adhere tightly to the thoracic walls, they are affected by the new, larger size of the chest. It leads to the growing of the volume within the lungs or what is called the intrapulmonary volume (Fig. 12). 

When the inspiratory muscles resume their initial resting length and the diaphragm relaxes, the expiration occurs as the lungs empty their contents. In contrary to the inspiration, which mainly depends on muscle contraction, expiration is an automatic process relying on the natural elasticity of the lungs. Through the process of expiration, various changes in the lung-related volumes happen. The reduction of the intrapulmonary volume compels the gases inside the lungs to get closer. Accordingly, the intrapulmonary pressure gets higher than atmospheric pressure, which forces the gases to go out of the lungs.  In a nutshell, volume changes can be considered as the secret behind this mechanical process (Fig. 12). 

Respiratory volume refers to the volume of gases in the lungs at a given time, and it is defined as the tidal volume (TV). TV is affected by several factors including the physical condition, sex, age and size. The normal quiet rate of breathing is around 4,800 ml in healthy young men and 3,100 ml in healthy young women. It is called the vital capacity (VC), and it is measured through the following equation:

Tidal Volume + Inspiratory Reserve Volume + Expiratory Reserve Volume

Spirometer is very useful in reading the respiratory capacities. It is used to follow up the course of some pulmonary diseases and to evaluate the efficiency of respiratory functions. It works through monitoring the volumes of air exhaled (Fig. 13).

During the processes of inhalation and exhalation, two sounds are produced, and they can be recognized by a stethoscope. These sounds are either bronchial or vesicular. When air rushes through trachea and bronchi, bronchial sounds are heard. When air fills the alveoli, vesicular sounds are produced.  

The regulation of the work of the respiratory muscles, the diaphragm and external intercostals, is conducted by the brain through sending nerve impulses carried by the phrenic nerves and intercostal nerves. The rhythm of breathing is organized by the impulses sent by the inspiratory and expiratory neurons, existed with the ventral respiratory group (VRG) in the medulla.   

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It is a chronic inflammatory condition affecting the air passages in the lungs. It is due to tightening and inflammation of the muscles around the small airways (Fig. 14). The symptoms include shortness of breath, bronchoconstriction, chest tightness, wheezing, coughing and increased mucus production. Patients should do their best to avoid exposure to allergens as inhaling mould spores, animal dander, dust mites, pollens as well as tobacco smoke; certain chemicals can worsen the condition. Patients with asthma may have some difficulties in daily life due to their inability to exercise or perform daily activities. In addition, they may suffer a persistent cough and permanent lung damage. Also, it can cause death.

It is an acute respiratory infection affecting the air sacs in one or both lungs (Fig. 15). Various microorganisms such as viruses, bacteria, fungi or parasites, can cause this acute inflammation. Also, pneumonia can occur as a result of a severe exposure to smoke, dust or chemicals. There are three types of pneumonia: aspiration pneumonia, lobar pneumonia and bronchopneumonia. It can be diagnosed easily through sputum culture and chest x-ray. Antibiotics are the optimal treatment for pneumonia in addition to oxygen and physiotherapy.

The enlargement of some alveoli because of the destruction of the walls of others is referred to as Emphysema (Fig. 16). In this pulmonary condition, patients exert incredible efforts to exhale as the fibrosis of the lungs, from the chronic inflammation, reduces the elasticity of the lungs. The outflow of air is obstacle through the collapse of the airways. Because air is retained in the lungs, oxygen exchange is surprisingly efficient, and cyanosis does not usually appear until late in the disease. Consequently, emphysema sufferers are sometimes referred to as “pink puffers.” However, overinflation of the lungs leads to a permanently expanded barrel chest.

It refers to the infection of the main airways of the lungs, or it is an irritation and inflammation of the bronchi. Bronchitis can be acute or chronic. Acute bronchitis occurs as a result of a bacterial or viral infection. In addition, it is caused through an exposure to fumes, dust, air pollution or tobacco smoke (Fig. 17). Patients with acute bronchitis suffer from chest tightness, severe cough, shortness of breath and wheezing. It can be treated through bronchodilator, rest and fluids. It will take just few days for patients to recover. In chronic bronchitis, patients suffer from a persistent productive cough lasting for at least three months per year for two consecutive years. Chronic obstructive pulmonary disease is the umbrella disease from which bronchitis is derived.

It is a viral infection attacking the respiratory tract. Influenza is contagious. During sneezing or coughing, the virus moves from one person to another through droplets (Fig. 18). Influenza tends to occur mostly in the colder months. Patients with influenza suffer from a high fever, severe malaise, headache and myalgia, and these symptoms can recover without medication within one to two weeks. However, influenza in some ages such as babies and old people can develop to pneumonia and even death. Each year, different strains of influenza with new vaccinations appear.

Lung cancer or bronchogenic carcinoma is a disease affecting the pulmonary tissues and the cells lining the lungs. It mainly occurs because of tobacco smoking. Also, the presence of other lung diseases such as COPD and the inhalation of other irritants such as asbestos can cause lung cancer (Fig. 19). Four main types of lung cancer are found: small cell carcinoma (20% of all lung cancers), large cell anaplastic carcinoma (10%), squamous cell carcinoma (50%) and adenocarcinoma (20%). Signs and symptoms include haemoptysis, a persistent cough, dyspnoea and weight loss. Lung cancer can be diagnosed through MRI, CT scan, chest x-ray, bronchoscopy, sputum culture and lung biopsy.

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  a. Tertiary bronchi

  b. Primary bronchi 

  c. Alveoli 

  d. Secondary bronchi

  a. Respiration

  b. Cellular respiration

  c. Pulmonary semilunar valve

  d. Expiration

  a. Total lung capacity

  b. Vital capacity 

  c. Tidal volume

  d. Ventilation rate

  a. Exchange of gases between the bloodstream and tissue cells

  b. Breathing between the atmosphere and the alveoli

  c. Exchange of gases between alveoli and the bloodstream

  d. Production of ATP

  a. Influenza 

  b. Bronchitis 

  c. Emphysema 

  d. Pneumonia