The respiratory system delivers air to the lungs, brings oxygen into the body, and expels the carbon dioxide back out into the air. Understanding the structure and functions of the respiratory system is vital. It is made up of more than just the lungs…it also includes the nose, throat, larynx, windpipe, bronchi, alveolar ducts, and respiratory membrane. The respiratory system is divided into a respiratory zone and a conducting zone. The respiratory system is responsible for pulmonary ventilation, external respiration, gas transport, and internal respiration.



Structure of the Respiratory System

There are many components of the respiratory system, and to understand how the respiratory system works, you must first understand its structure. The nose consists of the visible external nose and the internal nasal cavity. The nasal septum divides the nasal cavity into right and left sides. The pharynx consists of three regions. The first is the nasopharynx, which receives the incoming air. The two Eustachian tubes that equalize air pressure in the middle ear also enter here. The pharyngeal tonsil (adenoid) is at the back of the nasopharynx. The oropharynx receives air from the nasopharynx and food from the oral cavity. The palatine and lingual tonsils are located here. The laryngopharynx passes food to the esophaguRespiratory_system2.jpgs and air to the larynx.

The larynx receives air from the laryngopharynx. It consists of nine pieces of cartilage that are joined by membranes and ligaments. The epiglottis, thyroid cartilage, arytenoid cartilages, and cricoid cartilage are just a few of the nine pieces.

The trachea or windpipe is a flexible tube that is approximately 4 inches long and 1 inch in diameter. Its walls consist of four layers: the mucosa, sub mucosa, hyaline cartilage (the C-shaped rings that prevent the trachea from collapsing during inspiration), and adventitia.

The primary bronchi are two tubes that branch from the trachea to the left and right lungs.
Inside the lungs, each primary bronchus divides repeatedly into branches of smaller diameters, forming secondary bronchi, tertiary bronchi, and numerous bronchioles. The walls of the primary bronchi are constructed like the trachea, but as the branches get smaller, the cartilaginous rings and the mucosa are replaced by smooth muscle.

Alveolar ducts are the final branches of the bronchial tree. Each alveolar duct has enlarged, bubble-like swellings. Each of these swellings is called an alveolus, and a cluster of adjoining alveolar is called an alveolar sac.

The respiratory membrane consists of the alveolar and capillary walls. Gas exchange occurs across this membrane. Type I cells are thin, squamous epithelial cells where oxygen diffusion occurs. Type II cells are cubical epithelial cells that are interspersed among the type I cells. They secrete pulmonary surfactant that reduces the surface tension of the alveolar walls. This permits oxygen to diffuse more easily. It also prevents the airway from collapsing. Alveolar macrophages wander among the other cells of the alveolar wall removing debris and microorganisms. A thin epithelial basement membrane forms the outer layer of the alveolar wall. A dense network of capillaries surrounds each alveolus. The capillary walls consist of endothelial cells surrounded by a thin basement membrane. The basement membranes of the alveolus and the capillary are often so close that they fuse.

Mechanics of Breathing

Boyle’s law sates that the pressure of a given quantity of gas is inversely proportional to its volume. If the lung volume increases, then the intrapulmonary pressure must decrease (or vice versa). This is often expresses as a formula: P1V1 = P2V2.
Breathing occurs when the contraction or relaxation of muscle around the lungs changes the total volbreathing.gifume of air within the air passages (bronchi, bronchioles) inside the lungs. When the volume of the lungs changes, the pressure of the air in the lungs changes in accordance to Boyle’s law. If the pressure is greater in the lungs than outside the lungs, then air rushes out. If the opposite occurs, then air rushes in.

Inspiration is what occurs when the inspiratory muscles (the diaphragm and external intercostal muscles) contract. Contraction of the diaphragm causes an increase in the size of the thoracic cavity, while contraction of the external intercostals elevates the ribs and sternum. Both muscles cause the lungs to expand, increasing the volume of their internal air passages. The air pressure inside the lungs then decreases below that of the air outside the body. Because gases move from areas of high pressure to areas of low pressure, air rushes into the lungs.

Expiration occurs when the diaphragm and external intercostal muscles relax. The elastic fibers in lung tissue cause the lungs to recoil to their original volume. The pressure of the air inside the lungs then increases above the air pressure outside the body, and air rushes out. During high rates of ventilation, expiration is facilitated by contraction of the expiratory muscles (the intercostal muscles and abdominal muscles).

Lung compliance is a measure of the ability of the lungs and thoracic cavity to expand. Due to the elasticity of lung tissue and the low surface tension, the lungs normally have high compliance.

Regulation of Breathing

Respiration is controlled by the areas of the brain that stimulate the contraction of the diaphragm and the intercostal muscles. These areas (the medullary inspiratory center, pheumotaxic area, and the apneustic area) are collectively called respiratory centers. The medullary inspiratory center is located in the medulla oblongata. It generates rhythmic nerve impulses that stimulate contraction of the inspiratory muscles (diaphragm and external intercostals). Normally, expiration occurs when these muscle relax, but when breathing is rapid, the inspiratory center facilitates expiration by stimulating the expiratory muscles (the internal intercostal muscles and abdominal muscles). The pheumotaxic area is located in the pons. It inhibits the inspiratory center, limiting the contraction of the inspiratory muscles, anMedulla_Respiratory_Center.gifd preventing the lungs from overinflating. The apneustic area is also located in the pons, and it stimulates the inspiratory center, prolonging the contraction of inspiratory muscles.

The respiratory centers are influenced by stimuli received from three groups of sensory neurons: central chemoreceptors, peripheral chemoreceptors, and stretch receptors. Central chemoreceptors are the nerves of the central nervous system. They are located in the medulla oblongata, and they monitor the chemistry of cerebrospinal fluid. When CO2 from the plasma enters the cerebrospinal fluid, it forms HCO3- and H+, and the pH of the fluid drops (becomes more acidic). Because of the decrease in pH, the central chemoreceptors stimulate the respiratory center to increase the inspiratory rate. Peripheral chemoreceptors are nerves of the peripheral nervous system. They are located in aortic bodies in the wall of the aortic arch and in carotid bodies in the walls of the carotid arteries. They monitor the chemistry of the blood. An increase in pH can cause these receptors to stimulate the respiratory center. Stretch receptors in the walls of bronchi and bronchioles are activated when the lungs expand to their physical limit. These receptors signal the respiratory center to discontinue stimulation of the inspiratory muscles, allowing expiration to begin. This response is called the inflation or Hering-Breur reflex.


In health care facilities, we see many patients with chronic obstructive pulmonary disease (COPD). This includes asthma, bronchitis, and emphysema where recurrent obstruction of airflow is a prominent feature. Any factor that leads to chronic alveolar inflammation can lead to the development of emphysema. COPD is the fourth most common cause of death in the US. More than 80% of the people that suffer from COPD have a history of smoking. Some patients have developed the disease because of chemical inhalation exposure, air pollution, or occupational settings as well. Patients that suffer from this disease always appear to be short of breath. Often, it is hard for them to even walk from the waiting room to the exam room. The weather has a big affect on patients also. The cold temperatures and humidity can make it extremely hard to breath. These patients are usually on inhalers/bronchodilators to help with mucus clearance. Some are on oxygen all the time and others just at night. Patients come in to be seen or request antibiotics when they are getting sick. These patients are always examined by the doctor. They usually are found to have decreased breath sounds, wheezes, crackles in the lungs, and distant heart sounds. Their diaphragm is limited in movement, so they are unable to take in deep breaths. In some severe cases, the chest size is increased. Patients often lean forward to use all the muscles of the chest and neck to help to bring air in. Pulmonary function tests are used to determine the severity of the disease. These tests measure how fast a patient is able to fully exhale air, how much air they can exhale in one second, and how much air they can inhale. X-rays and CT scans can also be used to assess damage to the lungs. The arterial blood gases are a series of blood tests to determine how much oxygen and carbon dioxide is in the blood. As the disease progresses, oxygenation goes down and carbon dioxide levels rise. COPD can lead to heart failure, as the right side of the heart cannot pump blood into the damaged lungs. Pneumonia is also a common and severe problem for sufferers as well. Chest x-rays are used to determine if pneumonia is present. Unfortunately, poor lung function often interferes with treatment of the other diseases.


How is ventilation accomplished? Incorporate the role of Boyle’s Law, as well as the action of muscles, volume change and pressure change in the thoracic cavity during inhalation and exhalation.

Ventilation is the same thing as breathing. It is accomplished by expansion of the chest and contraction of the diaphragm. The intrapleural and intrapulmonary pressures vary during ventilation. The intrapulmonary pressure is subatmospheric during inspiration and greater than the atmospheric pressure during expiration. The inverse relationship of these pressure changes in the lungs is in accordance with Boyle’s Law. Boyle’s Law describes the relationship between the pressure (P) and the volume (V) of a gas. It states that if the volume increases, then the pressure must decrease (or vice versa). Inspiration and expiration are accomplished by the contraction and relaxation of striated muscles...the contraction of the diaphragm and expansion of the chest. When the diaphragm and the chest relax, reducing the volume inside the chest, expiration occurs. During quiet inspiration, the diaphragm and external intercostal muscles contract and thus increase the volume of the thorax. During quiet expiration, these muscles relax, and the elastic recoil of the lungs and thorax causes a decrease in thoracic volume. Expiration can be forced by the contraction of abdominal muscles that increase intra-abdominal pressure, forcing the diaphragm up.

The following video gives a brief, but great explaination of how breathing occurs.

All information was obtained from Human Physiology; Stuart Ira Fox; 12th Edition, Burton’s Microbiology For The Health Sciences; Paul G. Engelkirk and Janet Duben-Engelkirk; 9th Edition, http://www.wikipedia.org/_, http://www.howstuffworks.com/,_ http://www.physioweb.org/, and http://www.webmd.com/.
All sources for photos can be accessed directly by clicking on the photos.