Cells utilize oxygen for metabolism and energy production. Carbon dioxide, which is harmful, is also released during these catabolic reactions. Thus, it is evident that oxygen has to be continuously provided to the cells and simultaneously carbon dioxide produced by the cells has to be released out. This process of exchanging oxygen from the atmosphere with the carbon dioxide produced by the cells is known as breathing or respiration. The respiratory system ensures the gaseous exchange of oxygen and carbon dioxide, and thus is vital to life.
The Human Respiratory System
We have a well-developed respiratory system comprising two lungs and associated air passages to perform the function of exchange of respiratory gasses. Air is warmed, moistened and filtered as it moves along the respiratory passages.
The respiratory tract extends from the nose to the lungs. It is mainly divided into two parts, the Upper Respiratory Tract and the Lower Respiratory Tract. The upper respiratory tract consists of the nose and the pharynx, while the lower respiratory tract includes the trachea, bronchi, bronchioles, and lungs.
Upper Respiratory Tract: The upper respiratory tract consists of nostrils, nasal cavities, pharynx, epiglottis, and larynx. Its main function is to receive the air from the external environment. Lungs being very delicate organs, the air is filtered, warmed, and humidified before reaching the lungs. It is the lungs where the actual gaseous exchange takes place. As the air enters nostrils of the nose, it is filtered by the nose hairs to some extent before passing into the nasal cavity. The epithelial tissue of the nasal cavity warms the air and further filters the air by secretion of mucous. At the same time, the endothelial lining of the nasal cavity transports dust and other foreign particles, trapped in mucous, to the back of the nasal cavity. Mucus is then either coughed out or swallowed and digested by the stomach acids. Through the nasal cavity air then passes into the pharynx and larynx.
Lower Respiratory Tract: The lower respiratory tract commences at the larynx. It is inclusive of the trachea, two bronchi that branch from the trachea, and mainly the lungs. The larynx is also known as the voice box. It is responsible for sound production as it houses the vocal cords. It is located in the neck below the point where the tract of the pharynx splits into the trachea and the esophagus. The larynx helps protect the trachea. The larynx also contains the epiglottis, which is a flap of cartilage situated at the opening to the larynx. The larynx closes while swallowing food in order to prevent the entry of swallowed material into the lungs. A strong cough reflex is stimulated by an ingested material in order to protect the lungs.
Vocalization occurs when air passes through the folds of vocal cords making them stretch and vibrate. Thus we need air for the functioning of the larynx and production of sound.
The trachea is a straight tube like structure extending up to the mid-thoracic cavity. It divides at the level of 5th thoracic vertebra into right and left primary bronchi. Each bronchi undergoes repeated divisions to form the secondary and tertiary bronchi and bronchioles ending up in very thin terminal bronchioles. The tracheae, primary, secondary and tertiary bronchi, and initial bronchioles are supported by incomplete cartilaginous ‘C-shaped’ rings. Each terminal bronchiole gives rise to a number of very thin, irregular-walled and vascularised bag-like structures called alveoli. The branching network of bronchi, bronchioles, and alveoli comprise the lungs.
Lungs: We have two lungs which are covered by a double-layered covering known as the pleura. In between the two pleural layers, there is pleural fluid, which helps to reduce friction on the lung surface. The outer pleural membrane is in close contact with the thoracic lining whereas the inner pleural membrane is in contact with the lung surface. The alveoli and their ducts form the respiratory or exchange part of the respiratory system. The upper part of the respiratory system transports the atmospheric air to the alveoli, clears it from foreign particles, humidifies, and also brings the air to body temperature. Actual diffusion of oxygen and carbon dioxide between blood and atmospheric air takes place at the surface of the alveoli. The lungs are situated in the thoracic chamber which is anatomically an air-tight chamber. The thoracic chamber is protected from behind by the vertebral column, from the front by the sternum, laterally by the ribs and on the lower side of the dome-shaped diaphragm. The anatomical setup of lungs in the thorax is such that any change in the volume of the thoracic cavity will be reflected in the lung cavity. Such an arrangement is essential for breathing, as the pulmonary volume cannot be directly altered by us.
Functions of the Respiratory System:
- Breathing / Ventilation: Breathing or respiration is the term used for the exchange of air between the atmosphere and the alveoli. It is also known as ventilation and takes place in two stages, inspiration, and expiration. During inspiration, atmospheric air is taken in while expiration alveolar air is released out. A pressure gradient is created between the lungs and the atmosphere, which causes this movement of air into and out of the lungs. Inspiration occurs when the pressure within the lungs is less than the atmospheric pressure, while expiration occurs when the intrapulmonary pressure is higher than the atmospheric pressure. The strength of inspiration and expiration can be increased with the assistance of additional abdominal muscles. On an average, breathing rate of a healthy human adult is 12-16 times per minute
- External respiration: It is the exchange of gasses (oxygen and carbon dioxide) between inhaled air and the blood. Diffusion of gasses primarily takes place at the alveolar membrane. The rate of diffusion depends on the solubility of gasses and thickness of alveolar membranes. Various factors in our body assist the diffusion of oxygen into the peripheral tissues and diffusion of carbon dioxide from tissues into alveoli for expiration.
- Internal respiration: Internal respiration involves the transport of gasses by the blood and exchange of these gasses between the blood and tissue fluids. Blood acts as a medium of transport for oxygen and carbon dioxide. The majority of the oxygen (97%) is transported by the red blood cells and a very small amount is transported in a dissolved state through the plasma. Oxygen binds to hemoglobin which is a pigment present in the RBCs. As for the carbon dioxide, a major fraction is carried as bicarbonate while only 20-25% is transported by the red blood cells. Carbon dioxide that is transported by the RBCs is also bound to the hemoglobin and is carried as carbamino-hemoglobin.
- Cellular respiration: Diffusion of gasses between blood and tissues and utilization of oxygen by the cells for catabolic reactions and resultant release of carbon dioxide.
- Regulation of the blood pH in coordination with the kidneys.
Regulation of Respiration
The nervous system has the ability to moderate the respiratory rhythm. The respiratory rhythm needs to be moderated depending upon the demands of the body tissues. This is done by a specialized center in the brain by reducing the duration of inspiration which eventually alters the respiratory rate. The increase in carbon dioxide and hydrogen ions in the blood leads to the activation of a chemosensitive area that is present adjacent to the brain rhythm center. This area is highly sensitive to even a slight increase in the levels of carbon dioxide and hydrogen ions. Activation of this area sends a signal to the rhythm center in order to moderate the respiratory process thereby eliminating the harmful substances.
Disorders of the Respiratory System
Lungs are located in a very moist environment. This, however, acts as a shortcoming as it provides a favorable environment for bacterial and viral growth which further leads to many respiratory illnesses. Our respiratory health can be adversely affected because of constant exposure to the harmful bacteria and viruses in our environment. Some of these infections are simple and can be treated easily, while others can be quite serious.
- Upper Respiratory Tract Infections (URI): Nasal cavities, pharynx, and larynx make up the upper respiratory tract. URI can spread from nasal cavities to sinuses, ears as well as larynx. Most common bacterial infection of the upper respiratory tract is the streptococcal infection also known as ‘strep throat’. Common symptoms of strep throat are high-grade fever, severe sore throat, white patches on a dark red throat, and stomach ache. Antibiotics are successful in complete resolution in most cases.
- Tonsillitis: It is the infection and inflammation of the tonsils which are present in the posterior wall of the nasopharynx. They are sometimes called as adenoids. Frequent bouts of tonsillitis can make breathing difficult. In such cases, tonsillectomy or removal of tonsils is advised.
- Laryngitis: It is the infection of the larynx that leads to hoarseness of voice. Normally it responds to treatment. However, if it is persistent for a long period of time and is not responsive to any medications, it could be a warning sign of cancer and requires further detailed evaluation by a physician.
- Acute bronchitis: Whenever there is an infection of the bronchi it is known as bronchitis. Most of the times it is a bacterial infection that has occurred secondary to upper respiratory tract infection. Signs include a cough that expectorates and is rich in mucus and sometimes pus.
- Pneumonia: It is a serious infection wherein the lungs are filled with fluid. It can be a bacterial or viral infection. High fever with chills, headache, and chest pain are the commonly seen symptoms. The extent of severity depends on the amount of fluid in the lungs.
- Emphysema: It is a chronic disorder wherein alveolar walls are collapsed due to damage. Cigarette smoking is an important cause of damage. It effectively leads to a reduced respiratory surface area or the area where the exchange of gasses takes place, thus reducing the respiratory capacity.
- Lung Fibrosis: It is seen as an occupational disorder in workers of certain industries, especially those involving grinding or stone-breaking. Long term inhalation of particles such as sand, asbestos, coal dust, or fiberglass can give rise to inflammation leading to fibrosis (proliferation of fibrous tissues), causing serious lung damage. Here the elasticity of lungs is reduced thus reducing the respiratory efficiency. Thus lungs cannot inflate properly and keep on deflating. Workers in such industries should wear protective masks.
- Asthma: It is a difficulty in breathing causing wheezing due to inflammation of bronchi and bronchioles. These patients have bronchial inflammation which reduces the airway diameter. Wheezing and shortness of breath are classic symptoms of asthma.