Each day we breathe about 20,000 times. All of this breathing couldn’t happen without help from the respiratory system. The major function of the respiratory system is gas exchange. When we breathe, we inhale oxygen from the environment needed by body cells for energy and growth and exhale waste carbon dioxide. The process of taking air into the lungs is called inhalation or inspiration, and the process of breathing it out is called exhalation or expiration. The external or mechanical respiration is the process of gas exchange between the alveoli and adjacent capillaries within the lungs and the internal or cellular respiration – between the bloodstream and body cells. As gas exchange occurs in the humans, the acid-base balance of the body is maintained as a component of homeostasis. In the absence of proper ventilation, two conditions could occur: respiratory acidosis, a life threatening condition caused by a deficiency of ventilation, or respiratory alkalosis, caused by an excess of ventilation, or hyperventilation. In an average resting adult, the lungs take in about 250 mL of oxygen and excrete about 200 mL of carbon dioxide every minute. During an average breath, an adult will exchange from 500 to 700 mL of air. This average breath capacity is called the tidal volume.
The respiratory system also helps maintain body temperature and eliminate excess water from the body. The movement of gas through the larynx, pharynx, and mouth allows humans to speak. Because of this, gas movement is extremely vital for communication purposes, including speaking, singing, or whistling.
The upper respiratory tract begins with the nostrils (nares) of the nose, which open into the nasopharynx (nasal cavity). In the nasal passages the air is warmed, moistened and filtered out from foreign matter and organisms which can be very harmful to the body. Tiny hairs called cilia protect the nasal passageways and other parts of the respiratory tract. Pollutants (like dust) are blown out or coughed out, destroyed by digestive juices, or eaten by macrophages, a type of blood cell that patrols the body looking for germs. The nasopharynx opens into the oropharynx (behind the oral cavity). During inhalation, air entering the oropharynx passes into the laryngopharynx and empties into the larynx (voicebox), which contains the vocal cords. Air then moves through the glottis down into the trachea (windpipe) which is a tube that enters the chest cavity. The walls of the trachea are strengthened by stiff rings of cartilage and also lined with cilia, which sweep fluids and foreign particles out of the airway.
In the lower respiratory tract the trachea splits into two smaller tubes called the bronchi. Each bronchus then divides 16 times forming the bronchioles. The bronchioles lead to the respiratory zone of the lungs, which consists of respiratory bronchioles, alveolar ducts, and the tiny sacs called alveoli. The average adult’s lungs contain about 600 million of these thin-walled, spongy, air-filled sacs surrounded by a capillary bed. The alveoli are where the exchange of oxygen and carbon dioxide occurs. The inhaled oxygen passes into the alveoli and then diffuses through the capillaries into the arterial blood. Meanwhile, the waste-rich blood from the veins releases its carbon dioxide into the alveoli. The carbon dioxide follows the same path out of the lungs when the human exhales. The network of alveoli, bronchioles, and bronchi is known as the bronchial tree.
The lungs are large, lobed and paired organs in the thoracic cavity. They contain elastic tissues that allow them to inflate and deflate without losing shape and are encased by a thin, moist and shiny membrane, the pleura. The lungs are not equal in size: the right lung is divided into three lobes and the left – into two lobes. The lungs are separated from one another by a median dividing wall and an area containing the heart and other thoracic organs (the thymus, part of the oesophagus, and several large blood vessels) embedded in connective tissue. This area is called the mediastinum. The lungs have a somewhat conical shape and an elastic, spongy texture derived from the nature of the alveoli. The bottom of the thoracic cavity is formed by the diaphragm which plays a leading role in breathing. The diaphragm is the main muscle of respiration. When we inhale, muscles in the chest wall contract, lifting the ribs and pulling them, outward. The diaphragm at this time moves downward enlarging the chest cavity. Reduced air pressure in the lungs causes air to enter the lungs. As we exhale, the diaphragm moves upward and the chest wall muscles relax, causing the chest cavity to contract. Air pressure in the lungs rises, so air flows from the lungs out of respiratory system through the nose or mouth.
Each lung is surrounded by a two-layered membrane called the pleura. The inner layer of the pleura is called the visceral pleura. This layer covers the surface of each lung and reaches into the fissures between the lobes of the lung and encloses the mediastinum. The outer layer of the pleura, called the parietal pleura, lines the inner surface of the thoracic cavity. The visceral and parietal pleura are continuous with one another at a point where the primary bronchus, blood vessels, and nerves enter each lung. Therefore, the two layers of the pleura form a collapsed sac. The area within the sac (between the visceral and parietal pleura) is called the pleural cavity. Fluid in the cavity keeps the two pleural membranes in close contact with each other and allows them to glide smoothly over each other. The fluid also adheres the two layers of the pleura to one another.
Many factors, including genetics, pollutants, irritants, and infectious diseases, can affect the health of our lungs and respiratory system and cause respiratory problems which can be classified into four general areas:
Infectious, environmental and other diseases (pneumonia, tuberculosis, asbestosis, particulate pollutants). It is in this category that coughing is of major importance, as it is the body’s main method to remove dust, mucus, saliva, and other debris from the lungs.
Due to its extensive surface area, the respiratory tract is constantly exposed to microbes, which explains why the respiratory system includes many mechanisms to defend itself and prevent pathogens from entering the body. Disorders of the respiratory system are usually treated by a pulmonologist.