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The respiratory system is responsible for the exchange of gases. When a person takes a breath, the air they take in is composed of several gases. The mixture is mostly made up of oxygen, nitrogen and carbon dioxide. Oxygen is an odorless gas that makes up about 20 percent of the air. It is essential to life because the body uses it for the chemical reactions that occur in the cells of the body. Oxygen is brought in and utilized by the body, while carbon dioxide and nitrogen are expelled from the body.
The respiratory system is divided into the upper respiratory tract (including the nose, nasal cavities, sinuses, pharynx, larynx and trachea), and the lower respiratory tract (the lungs, including the bronchi, bronchioles and alveoli).
Air is brought in primarily through the nose, over the nasal cavities. The nasal cavities are lined with mucous membranes which are covered with tiny hairs known as cilia. The nasal cavities warm, moisten and filter the air of possible contaminants. The sinuses also help warm and humidify the air. Located in the bones of the face and skull, the sinuses are air–filled pockets connected to the nose through small openings. Each sinus is attached to the nasal passages by a continuous mucous membrane lining.
From either the nose or mouth, air then takes the following pathway:
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Pharynx (throat). The passageway at the back of the mouth. Both air and food travel down the pharynx. When food is swallowed, a flap of tissue called the epiglottis, located at the top of the larynx (voice box), presses down to prevent food from going down the trachea (air passage).
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Larynx (voice box). Air passes from the pharynx through the larynx, located at the upper end of the trachea (windpipe). This organ contains the vocal cords and leads to the trachea.
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Trachea (windpipe). A long tube featuring a series of “C”-shaped rings of cartilage that begins at the larynx and then splits into the bronchial tubes that lead to the lungs. It is the largest breathing tube in the body, connecting the upper respiratory system (mouth, nose and throat) to the bronchi leading to the lungs.
The actual exchange of gases takes place in the major organs of the respiratory system – the lungs. These organs have a spongy, porous, highly elastic texture.
Each lung is made up of lobes, three in the right lung and two in the left lung (which is smaller, to make room for the heart). Each lobe contains lung tissue, air sacs, ducts and bronchioles that are used for respiration. The lobes operate relatively independently of each other.
The lungs are located in the thoracic cavity (chest). At the floor of this cavity is a muscle called the diaphragm. The lungs are separated from the chest wall by the pleura, a continuous membrane that covers the lobes and the inner chest wall.
The lungs are separated from each other by the mediastinum, a space that contains the heart, the esophagus (a tube connecting the throat with the stomach) and part of the trachea. When air leaves the trachea, it takes the following path to the lungs:
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Bronchi (bronchial tubes). A pair of tubes that branch off from the trachea, each leading to a lung. Like an upside-down tree, air passageways begin with the trunk (trachea) and divide into large branches (bronchi).
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Bronchioles. Branches of increasingly smaller passageways connecting to the alveoli. In the upside-down tree, they are the smaller branches.
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Alveoli. Tiny sacs in the lungs where gases in air are exchanged for gases in the blood (in tiny blood vessels called capillaries). Alveoli are the endpoint of the respiratory system.
Though physicians often refer to separate upper and lower respiratory systems for the sake of convenience, it is important to note that the two are a continuous airway without any physical division. Much of the medical community has recently begun to support a "one airway" concept of the respiratory system as one continuous airway that should be treated as a whole. A result of this change is that more and more therapies are being prescribed for both bronchial and nasal symptoms together as a treatment for respiratory conditions.
A number of conditions can affect the respiratory system. Among the most common disorders is asthma. Asthma is a chronic inflammation of the body’s bronchial tissues. This causes a narrowing of the airways and an increased sensitivity to allergens or other stimuli. Exposure to these triggers often produces symptoms such as coughing, wheezing and an inability to breathe normally (shortness of breath).
Chronic obstructive pulmonary disease (COPD) is another common disorder of the respiratory system. It is a chronic, progressive disease of the lungs that reduces airflow over time. The condition is characterized by symptoms that include coughing, wheezing and shortness of breath.
The respiratory system may also be involved in allergic conditions. Allergies are exaggerated reactions of the immune system to certain foreign substances that it mistakes as a threat to the body. This misidentification kicks off a series of events known as an allergic cascade that results in uncomfortable allergy symptoms. Symptoms of an allergic reaction can affect many different parts of the body, including the lungs. Respiratory symptoms of an allergic reaction may include shortness of breath or difficulty breathing, wheezing and coughing.
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