Chap 23 The Respiratory System


The lungs provide a warm, moist, protected environment. Under these conditions, diffusion can occur between the air and the blood. The cardiovascular system provides the link between your interstitial fluids and the exchange surfaces of your lungs.


The respiratory system includes the lungs, which are the functional units of this system, and the airways through which air flows as it moves to and from the lungs.


Functions of the Respiratory System

Providing an extensive area for gas exchange between air and circulating blood.


Moving air to and from the exchange surfaces of the lungs.


Protecting respiratory surfaces from dehydration, temperature changes, or other environmental variations and defending the respiratory system and other tissues from invasion by pathogens.


Producing sounds involved in speaking, singing, and nonverbal auditory communication.


Providing olfactory sensations to the central nervous system from the olfactory epithelium in the superior portions of the nasal cavity.


Components of the Respiratory System.

The upper respiratory system consists of the nose, nasal cavity, paranasal sinuses, and pharynx.


The lower respiratory system includes the larynx (voice box), trachea (windpipe), bronchi, bronchioles, and alveoli of the lungs.

Your respiratory tract consists of the airways that carry air to and from the exchange surfaces of your lungs.

The respiratory portion of the tract includes the delicate respiratory bronchioles and the alveoli, air–filled pockets within the lungs where all gas exchange between air and blood occurs.


The total surface area for gas exchange in the lungs of adults is at least 35 times the surface area of the body.



Filtering, warming, and humidification of inhaled air begins at the entrance to the upper respiratory system and continues throughout the rest of the conducting system. By the time air reaches the alveoli, most foreign particles and pathogens have been removed, and the humidity and temperature are within acceptable limits. The success of this "conditioning process" is due primarily to the properties of the respiratory mucosa .

The Respiratory Mucosa  lines the conducting portion of the respiratory system.


The Respiratory Defense System- Contamination is prevented by a series of filtration mechanisms. Goblet cells in the epithelium and mucous glands in the lamina propria produce a sticky mucus that bathes exposed surfaces. In the nasal cavity, cilia sweep that mucus and any trapped debris or microorganisms toward the pharynx, where it will be swallowed and exposed to the acids and enzymes of the stomach. In the lower respiratory system, the cilia also beat toward the pharynx, moving a carpet of mucus in that direction and cleaning the respiratory surfaces. This process is often described as a mucus escalator.


The familiar symptoms of the "common cold" result from the invasion of this respiratory epithelium by any of more than 200 types of viruses (science has cataloged 4000 different viruses, there are probably hundreds of thousands more not yet known).  The irritants produce local inflammation. Airborne irritants are strongly linked to the development of lung cancer.


Cystic fibrosis (CF) is the most common lethal inherited disease affecting Caucasians of Northern European descent, occurring at a frequency of 1 birth in 2500. The condition can result from many different mutations affecting a gene located on chromosome 7. Individuals with CF seldom survive past age 30; death is generally the result of a chronic, massive bacterial infection of the lungs and associated heart failure.


The most serious symptoms appear because the respiratory mucosa in these individuals produces dense, viscous mucus that cannot be transported by the respiratory defense system. The mucus escalator stops working, and mucus blocks the smaller respiratory passageways. This blockage reduces the diameter of the airways, making breathing difficult, and the inactivation of the normal respiratory defenses leads to frequent bacterial infections


The Upper Respiratory System



The Nose, Nasal Cavity, and Pharynx.


The Nose and Nasal Cavity
The nose is the primary passageway for air entering the respiratory system. Air normally enters through the paired external nares, or nostrils, which open into the nasal cavity . The vestibule is the space contained within the flexible tissues of the nose


The nasal septum divides the nasal cavity into left and right portions.  The superior , middle , and inferior nasal conchae project toward the nasal septum from the lateral walls of the nasal cavity.


A bony hard palate , made up of portions of the maxillary and palatine bones, forms the floor of the nasal cavity and separates it from the oral cavity. A fleshy soft palate extends posterior to the hard palate, marking the boundary between the superior nasopharynx and the rest of the pharynx. The nasal cavity opens into the nasopharynx through a connection known as the internal nares.


As cool, dry air passes inward over the exposed surfaces of the nasal cavity, the warm epithelium radiates heat and the water in the mucus evaporates. Air moving from your nasal cavity to your lungs has been heated almost to body temperature, and it is nearly saturated with water vapor. This mechanism protects more delicate respiratory surfaces from chilling or drying out–two potentially disastrous events


The extensive vascularization of the nasal cavity and the relatively vulnerable position of the nose make a nosebleed, or epistaxis, a fairly common event.


The Pharynx  is a chamber shared by the digestive and respiratory systems. It extends between the internal nares and the entrances to the larynx and esophagus.  The pharynx is divided into the nasopharynx, the oropharynx, and the laryngopharynx.


The nasopharynx is the superior portion of the pharynx. It is connected to the posterior portion of the nasal cavity through the internal nares and is separated from the oral cavity by the soft palate.

The oropharynx extends between the soft palate and the base of the tongue at the level of the hyoid bone.


The narrow laryngopharynx, the inferior part of the pharynx, includes that portion of the pharynx between the hyoid bone and the entrance to the larynx and esophagus.


The Larynx  is a cartilaginous structure that surrounds and protects the glottis.  Essentially a cylinder, the larynx has incomplete cartilaginous walls that are stabilized by ligaments and skeletal muscles



Cartilages and Ligaments of the Larynx
Three large, unpaired cartilages form the larynx: (1) the thyroid cartilage, (2) the cricoid cartilage, and (3) the epiglottis. The thyroid cartilage is the largest laryngeal cartilage. Consisting of hyaline cartilage, it forms most of the anterior and lateral walls of the larynx.


The thyroid cartilage sits superior to the cricoid  cartilage , another hyaline cartilage


The shoehorn–shaped epiglottis projects superior to the glottis and forms a lid over it.


The vestibular ligaments and the vocal ligaments extend between the thyroid cartilage and the arytenoid cartilages.  The vestibular ligaments lie within the superior pair of folds, known as the vestibular folds . These folds, which are relatively inelastic, help prevent foreign objects from entering the glottis and protect the more delicate vocal folds.  The vocal folds, inferior to the vestibular folds, guard the entrance to the glottis. The vocal folds are highly elastic, because the vocal ligaments consist of elastic tissue. The vocal folds are involved with the production of sound, and for this reason they are known as the vocal cords .


Sound Production

Air passing through the glottis vibrates the vocal folds and produces sound waves. The pitch of the sound produced depends on the diameter, length, and tension in the vocal folds.


At puberty, the larynx of males enlarges much more than does that of females. The vocal cords of an adult male are thicker and longer, and produce lower tones, than those of an adult female.
Sound production at the larynx is called phonation. Phonation is one component of speech production. However, clear speech also requires articulation , the modification of those sounds by other structures.  The final production of distinct words depends further on voluntary movements of the tongue, lips, and cheeks.


When you swallow, sets of muscles cooperate to prevent food or drink from entering the glottis.  Food or liquids that touch the vestibular or vocal folds trigger the coughing reflex . In a cough, the glottis is kept closed while the chest and abdominal muscles contract, compressing the lungs.


An infection or inflammation of the larynx is known as laryngitis. It commonly affects the vibrational qualities of the vocal folds; hoarseness is the most familiar symptom. Mild cases are temporary and seldom serious. However, bacterial or viral infection of the epiglottis can be very dangerous; the resulting swelling may close the glottis and cause suffocation. This condition, acute epiglottitis, can develop rapidly after a bacterial infection of the throat.












The Trachea and Primary Bronchi



The Primary Bronchi
The trachea branches within the mediastinum, giving rise to the right and left primary bronchi. An internal ridge called the carina separates the two bronchi. 

Before branching further, each primary bronchus travels to a groove along the medial surface of its lung. This groove, the hilus of the lung, also provides access for entry to pulmonary vessels and nerves








The Lungs

Each lung is a blunt cone, the tip, or apex, of which points superiorly. The apex on each side extends into the base of the neck superior to the first rib. The broad concave inferior portion, or base, of each lung rests on the superior surface of the diaphragm.


Tuberculosis, or TB, results from an infection of the lungs by Mycobacterium tuberculosis ; other organs may be invaded as well. This bacterium may colonize the respiratory passageways, the interstitial spaces, the alveoli, or a combination of the three. Symptoms are variable, but generally include coughing and chest pain, with fever, night sweats, fatigue, and weight loss. In 1900, TB, then known as "consumption," was the leading cause of death.


Lobes and Surfaces of the Lungs
The lungs have distinct lobes that are separated by deep fissures. The right lung has three lobes: superior , middle , and inferior , separated by the horizontal and oblique fissures . The left lung has only two lobes: superior and inferior , separated by the oblique fissure . The right lung is broader than the left, because most of the heart and great vessels project into the left thoracic cavity.


The Bronchi
The primary bronchi and their branches form the bronchial tree.   Because the left and right primary bronchi are outside the lungs, they are called extrapulmonary bronchi . As the primary bronchi enter the lungs, they divide to form smaller passageways.


The branches are collectively called the intrapulmonary bronchi .   Each primary bronchus divides to form secondary bronchi , also known as lobar bronchi . In each lung, one secondary bronchus goes to each lobe, so the right lung has three secondary bronchi and the left lung has two.


In each lung, the secondary bronchi branch to form tertiary bronchi , or segmental bronchi . The branching pattern differs between the two lungs, but each tertiary bronchus ultimately supplies air to a single bronchopulmonary segment , a specific region of one lung. The right lung has 10 bronchopulmonary segments. During development, the left lung also has 10 segments, but subsequent fusion of adjacent tertiary bronchi generally reduces that number to eight or nine.


The walls of the primary, secondary, and tertiary bronchi contain progressively lesser amounts of cartilage. In the secondary and tertiary bronchi, the cartilages form plates arranged around the lumen. These cartilages serve the same purpose as the rings of cartilage in the trachea and primary bronchi. As the amount of cartilage decreases, the relative amount of smooth muscle increases.


During a respiratory infection, the bronchi and bronchioles can become inflamed. In this condition, called bronchitis , the smaller passageways may become greatly constricted, leading to difficulties in breathing. One method of investigating the status of the respiratory passageways is the use of a bronchoscope , a fiber–optic bundle small enough to be inserted into the trachea and steered along the conducting passageways to the level of the smaller bronchioles. This procedure is called bronchoscopy. In addition to permitting the direct visualization of bronchial structures, the bronchoscope can collect tissue or mucus samples from the respiratory tract. In bronchography, a bronchoscope or catheter introduces a radiopaque material into the bronchi. This technique can permit detailed X–ray analysis of bronchial masses, such as tumors, or other obstructions.


The Bronchioles
Each tertiary bronchus branches several times within the bronchopulmonary segment, giving rise to multiple bronchioles . These passageways branch further into the finest conducting branches, called terminal bronchioles . Roughly 6500 terminal bronchioles are supplied by each tertiary bronchus. The lumen of each terminal bronchiole has a diameter of 0.3–0.5 mm.


In functional terms, the bronchioles are to the respiratory system what the arterioles are to the cardiovascular system.



Sympathetic activation leads to bronchodilation , the enlargement of the airway diameter. Parasympathetic stimulation leads to bronchoconstriction , a reduction in the diameter of the airway. Bronchoconstriction also occurs during allergic reactions such as anaphylaxis, in response to histamine released by activated mast cells and basophils.

Bronchodilation and bronchoconstriction adjust the resistance to airflow toward or away from the respiratory exchange surfaces. Tension in the smooth muscles commonly throws the bronchiolar mucosa into a series of folds, limiting airflow; excessive stimulation, as in asthma, can almost completely prevent airflow along the terminal bronchioles.


The respiratory bronchioles deliver air to the gas exchange surfaces of the lungs.







The Bronchioles.
(a) The distribution of a respiratory bronchiole supplying a portion of a lobule. (b) Alveolar sacs and alveoli. (c) An SEM of the lung. Notice the open, spongy appearance of the lung tissue





Alveolar Ducts and Alveoli
Respiratory bronchioles are connected to individual alveoli and to multiple alveoli along regions called alveolar ducts


Alveolar ducts end at alveolar sacs , common chambers connected to multiple individual alveoli. Each lung contains about 150 million alveoli, and their abundance gives the lung an open, spongy appearance. An extensive network of capillaries is associated with each alveolus.


The alveolar epithelium consists primarily of simple squamous epithelium . The squamous epithelial cells, called Type I cells , are unusually thin and delicate. Roaming alveolar macrophages ( dust cells ) patrol the epithelial surface, phagocytizing any particulate matter that has eluded other respiratory defenses and reached the alveolar surfaces. Septal cells , also called Type II cells , are scattered among the squamous cells. The large septal cells produce surfactant, an oily secretion containing a mixture of phospholipids and proteins.



At the respiratory membrane, the total distance separating alveolar air from blood can be as little as 0.1 um. It averages about 0.5 um.  Diffusion across the respiratory membrane proceeds very rapidly, because (1) the distance is small and (2) both oxygen and carbon dioxide are lipid soluble.




Alveolar walls are very delicate–rather like air bubbles; without surfactant, the surface tension would be so high that the alveoli would collapse. Surfactant forms a thin surface layer that interacts with the water molecules, reducing the surface tension and keeping the alveoli open.

A person who does not produce enough surfactant is soon exhausted by the effort required to keep inflating and deflating the lungs. This condition is called respiratory distress syndrome .
Gas exchange occurs across the respiratory membrane of the alveoli.


The Pleural Cavities and Pleural Membranes


Each lung occupies a single pleural cavity, which is lined by a serous membrane called the pleura. The pleura consists of two layers: the parietal pleura and the visceral pleura. The parietal pleura covers the inner surface of the thoracic wall and extends over the diaphragm and mediastinum. The visceral pleura covers the outer surfaces of the lungs, extending into the fissures between the lobes.


An Overview of Respiratory Physiology


External respiration includes all the processes involved in the exchange of oxygen and carbon dioxide between the body's interstitial fluids and the external environment. The goal of external respiration, and the primary function of the respiratory system, is to meet the respiratory demands of cells.


Internal respiration is the absorption of oxygen and the release of carbon dioxide by cells.





Pulmonary ventilation , or breathing, which involves the physical movement of air into and out of the lungs.


Gas diffusion across the respiratory membrane between alveolar air spaces and alveolar capillaries and across capillary walls between blood and other tissues.


 The transport of oxygen and carbon dioxide between alveolar capillaries and capillary beds in other tissues.


Hypoxia , or low tissue oxygen levels, places severe limits on the metabolic activities of the affected area. For example, the effects of coronary ischemia result from chronic hypoxia affecting cardiac muscle cells. If the supply of oxygen is cut off completely, the condition of anoxia results. Anoxia kills cells very quickly ( 3 minutes). Much of the damage caused by strokes and heart attacks is the result of localized anoxia.


Pulmonary Ventilation


Pulmonary ventilation is the physical movement of air into and out of the respiratory tract.


Although we are seldom reminded of the fact, this atmospheric pressure has several important physiological effects.


Gas Pressure and Volume (Boyle's Law)


An inverse relationship thus exists between the pressure ( P ) and volume ( V ) of a gas in a closed container: Gas pressure is inversely proportional to volume. That is, if you decrease the volume of a gas, its pressure will rise; if you increase the volume of a gas, its pressure will fall .




 (a) As the ribs are elevated or the diaphragm is depressed, the volume of the thoracic cavity increases. (b) An anterior view with the diaphragm at rest, with no air movement. (c) Inhalation : Elevation of the rib cage and contraction of the diaphragm increase the size of the thoracic cavity. Pressure decreases, and air flows into the lungs. (d) Exhalation : When the rib cage returns to its original position, the volume of the thoracic cavity decreases. Pressure rises, and air moves out of the lungs


The diaphragm forms the floor of the thoracic cavity. The relaxed diaphragm has the shape of a dome that projects superiorly into the thoracic cavity. When the diaphragm contracts, it tenses and moves inferiorly. This movement increases the volume of the thoracic cavity and exerts pressure on the contents of the abdominopelvic cavity. When the diaphragm relaxes, it returns to its original position, and the volume of the thoracic cavity decreases.


Owing to the nature of the articulations between the ribs and the vertebrae, superior movement of the rib cage increases the depth and width of the thoracic cavity. Inferior movement of the rib cage reverses the process and reduces the volume of the thoracic cavity.


At the start of a breath, pressures inside and outside the thoracic cavity are identical, and no air moves into or out of the lungs. When the thoracic cavity enlarges, the pleural cavities and lungs expand to fill the additional space. This increase in volume lowers the pressure inside the lungs. Air now enters the respiratory passageways, because the pressure inside the lungs is lower than atmospheric pressure Air continues to enter the lungs until their volume stops increasing and the internal pressure is the same as that outside. When the thoracic cavity decreases in volume, pressures rise inside the lungs, forcing air out of the respiratory tract


The compliance of the lungs is an indication of their expandability. The lower the compliance, the greater is the force required to fill and empty the lungs. Factors affecting compliance include the following:


The Connective–Tissue Structure of the Lungs. The loss of supporting tissues resulting from alveolar damage, as in emphysema , increases compliance.


The Level of Surfactant Production. On exhalation, the collapse of alveoli as a result of inadequate surfactant, as in respiratory distress syndrome, reduces the lungs' compliance.


The Mobility of the Thoracic Cage. Arthritis or other skeletal disorders that affect the articulations of the ribs or spinal column will also reduce compliance.


Pressure Changes during Inhalation and Exhalation.
These graphs follow changes in the (a) intrapulmonary and (b) intrapleural pressures during a single respiratory cycle and relate the changes to (c) the tidal volume.


The Respiratory Cycle
A respiratory cycle is a single cycle of inhalation and exhalation. The tidal volume is the amount of air you move into or out of your lungs during a single respiratory cycle.


An injury to the chest wall that penetrates the parietal pleura or a rupture of the alveoli that breaks through the visceral pleura can allow air into the pleural cavity. This pneumothorax  breaks the fluid bond between the pleurae and allows the elastic fibers to recoil, resulting in a "collapsed lung," or atelectasis. The treatment for a collapsed lung involves the removal of as much of the air as possible from the affected pleural cavity before the opening is sealed. This treatment lowers the intrapleural pressure and reinflates the lung.