60 Respiratory Structures and Functions

Gross Anatomy of the Upper Respiratory Tract

Nose

The external portion of the nose begins at the base of the frontal bone and extends over the maxilla, with the nasal bone providing the bridge of the nose. Extending from the nasal bone is a collection of hyaline cartilages that make up the bulk of the nose. The medial region of the nose consists of a central septal cartilage with two lateral processes. The tip of the nose contains the major alar cartilage. Two minor alar cartilages are found at the sides and base of the lateral septal cartilages. Dense fibrous connective tissue is found under the skin of the sides lateral aspects the nose, away from the cartilage. Variations in the size of a person’s nose, or its form, are due to differences in the various cartilages.

The visible nose is actually the entryway into the nasal cavity, where the major functions of the nose occur. The openings to the nose, the nares, are lined with coarse hairs to aid in filtration of particulate matter. The area immediately inside the nares, the vestibule, contains a large number of sebaceous glands, sweat glands, and hair follicles.

The nasal cavity is divided into right and left sides by the nasal septum. This dividing wall’s anterior portion is made of cartilage; bone contributed by the vomer and part of the ethmoid bones of the skull make up the posterior. The roof of the nasal cavity consists of parts of the ethmoid and sphenoid bones. Its floor, the palate, forms the roof of the mouth. It is separated into the hard and soft palate. The anterior hard palate is formed from the maxillary process of the palatine bone. The posterior soft palate does not contain bone and moves during swallowing to close off the nasal cavity to prevent material from entering it from the mouth.

Extending from the nasal septum are three pairs of C-shaped structures called conchae. The superior, middle, and inferior conchae extend the length of the nasal cavity. They are covered by a mucus membrane that contains a large number of mucus-secreting cells and blood vessels. A bloody nose highlights the richness of the blood supply to the membranes on the conchae. The conchae serve as baffles to increase the surface area of the nasal cavity. The mucus glands and blood vessels aid in humidifying and warming the air coming into the body.

There are two types of epithelial coverings in the nasal cavity: respiratory epithelium and olfactory epithelium. Both types of epithelia appear very similar when viewed through a light microscope. The olfactory mucosa found in the roof of the cavity detects odors. This epithelial layer contains specialized nerve cells that detect odors and transmit impulses to the first cranial nerve. The respiratory epithelium that covers the rest of the nasal cavity is also found through most of the respiratory tract and is pseudostratified, ciliated, columnar epithelia.

Paranasal Sinuses

Within the bones surrounding the nasal cavity are paranasal sinuses (a sinus is a hollow area), which function to make the skull lighter as well as moisten and warm incoming air. These sinuses frequently become filled with excess fluid when a person has a head cold. Since the paranasal sinuses serve as resonators for speech and sound it is not surprising that the sound of the voice becomes altered when they are filled with fluid or swollen. The frontal, sphenoid, ethmoid and maxillary bones all contain sinus cavities.

Pharynx

As the air passes posteriorly through the nasal cavity, it enters the pharynx. This structure encompasses three distinct areas and connects the nasal passage to the larynx in the throat. It extends about 13 centimeters or 5 inches from the base of the skull to the level of the sixth cervical vertebrae. The wall of all three portions contains two layers of skeletal muscle. The inner layer is arranged in a circular pattern, and the outer layer is arranged longitudinally.

Nasopharynx

The superior section of the pharynx is called the nasopharynx. It is posterior to the nasal cavity and inferior to the sphenoid bone. This chamber shares the same epithelia as the nasal cavity and acts only as a conduit for air. The other two portions of the pharynx contain shared passages with the digestive tract and act as conduits for both air and food. The nasopharynx closes off during swallowing by raising the soft palate. Also found in this area are structures associated with immune system. The paired pharyngeal tonsils, also known as the adenoids, lie in the posterior wall of the nasopharynx.

Oropharynx

The second portion of the pharynx is the oropharynx. It runs from the soft palate to the epiglottis and is posterior to the oral cavity. The opening from the oral cavity to the oropharynx is called the oropharyngeal isthmus (Isthmus of Fauces). A difference between the nasopharynx and oropharynx is the epithelial covering of the passages. In comparison to the nasopharynx, which is lined with columnar epithelia, the oropharynx is covered by stratified squamous epithelia, that are often found covering areas which are subject to a great deal of frictional wear. The many layers of epithelial cells serve to protect the underlying tissues from being damaged by the food and material coming into the passage from the mouth.

Laryngopharynx

The third portion of the pharynx is the laryngopharynx. The shortest of the three parts of the pharynx, it runs inferiorly from epiglottis and ends superior to the esophagus. The laryngopharynx carries both air and food and is lined with stratified sqaumous epithelia.

Tonsils

The paired pharyngeal tonsils, also known as the adenoids, lie in the posterior wall of the nasopharynx. The other tonsils, the tubal tonsils, protect the auditory tubes and middle ear against entering microorganism. They are located at the opening of the tube that connects the middle ear to the nasopharynx. The auditory or Eustachian tube equilibrates air pressures between the environment and the middle ear. The orientation of the auditory tube changes during infancy and early childhood. The tube is oriented horizontally until around age two. Consequently, it frequently serves as a point of entry for bacteria in children less than two years of age. Around age two, the auditory tube changes course and slants down from the ear, allowing fluid to drain more readily. Without the fluid to act as a growth medium and conduit to the middle ear, older children usually experience few ear infections.

The paired palatine tonsils lie along the two sides of the Isthmus of Fauces in the oropharynx. The lingual (lingual meaning tongue) tonsils are not in the oropharynx, but are located at the base of the tongue. Collectively, the tonsils aid in filtering foreign material that could do harm.

Larynx

After air leaves the pharynx, it enters the larynx, a complex structure that extends from the laryngopharynx and the hyoid bone to the trachea. It is about 5 cm (2 in.) long. In addition to providing a passageway for air, the larynx directs air and food to their appropriate tubes. The airway is blocked by closing off the opening of the trachea with the epiglottis upon swallowing. The vocal cords, which are used in making sound and speech, can be found within the larynx. The epithelial lining of the larynx exhibits two different arrangements. Initially stratified squamous epithelia lines from the laryngopharynx to the vocal cords. Inferior to the vocal cords the epithelial lining shifts to pseudostratified, ciliated, columnar epithelia. The nine cartilage structures found in the larynx provide key anatomical landmarks. They function to maintain an open airway. Eight of the cartilages are composed of hyaline cartilage.

Epiglottis

The epiglottis is made of elastic cartilage and covered with stratified squamous epithelia. It connects loosely to the tongue, the hyoid bone, and the rim of the thyroid cartilage. The epiglottis is normally open, allowing air to freely flow into the rest of the larynx and the trachea. When a person swallows, the front of the epiglottis is raised, and the posterior portion descends, covering the glottis, which is the opening to the vocal cords and trachea. This movement directs food and water to the esophagus and prevents it from entering the bottom portion of the larynx and the upper trachea. If someone tries to talk or laugh and swallow at the same time, the air used to talk or laugh forces the epiglottis open, the swallowed material “goes down the wrong tube,” and choking results.

Thyroid cartilage

The thyroid cartilage is the largest of the cartilages of the larynx and is found at the front of the larynx. In the fetus the cartilage originates as two separate plates that fuse before birth. This roughly triangularly shaped cartilage contains the laryngeal prominence, commonly known as the “Adam’s apple”. The laryngeal prominence becomes more prominent in males during puberty as the larynx widens and the voice deepens. The thyroid cartilage connects to the hyoid bone by the thyrohyoid membrane or ligament.

Cricoid cartilage

Inferior to the thyroid cartilage is the cricoid cartilage. It connects superiorly to the thyroid cartilage by the cricothyroid ligament and inferiorly to the trachea by the cricotracheal ligament. When an occlusion of the upper respiratory tract occurs and a tracheostomy is performed to facilitate breathing, the cricothyroid ligament must be punctured.

Arytenoids cartilages, cuneiform cartilages, corniculate cartilages

The next six cartilages are found in three pairs. The first pair, the arytenoids cartilages, anchor the true vocal cords. The second pair are the cuneiform cartilages, and the third pair are the corniculate cartilages. All three pairs of cartilages are found in the lateral and posterior walls of the larynx. Except for the epiglottis, the arrangement of the cartilages of the larynx ensure that the passages through the larynx remain open.

Vocal cords

Two pairs of folded tissue can be observed in the larynx immediately inferior to the epiglottis. These are the false and true vocal cords respectively. The false vocal cords do not function in making sounds or speech but aid in closing the glottis, which opens to the rest of the larynx and the respiratory system. Below the false vocal cords are the true vocal cords. The true cords run from the arytenoids to the thyroid cartilage. They are reinforced with elastic fibers and vibrate when adequate air is forced through the gap between them, resulting in sound. Pitch control is achieved by adjusting the tension on the cords. Lessening the tension lowers the pitch. During puberty in males the cords are usually lengthened making for a deeper sound. Actual speech is achieved with the coordination of muscles in the pharynx, face, tongue, soft palate, and lips.

 

Gross Anatomy of the Lower Respiratory Tract

Trachea

The flow of air continues from the larynx to the trachea or windpipe. The trachea is about 10 cm (4 in.) long and 2 cm (3/4 in.) wide and extends from the larynx to the level of the fifth thoracic vertebrae. The presence of 16 to 20 C-shaped pieces of hyaline cartilage located along the trachea prevent this airway from closing. The last cartilage in the trachea exhibits a projection from the anterior surface that extends into the lumen. This projection, the carina, is very sensitive to particulate material and causes coughing when stimulated. The openings of the C-shaped cartilages face the posterior of the trachea and contain the trachealis smooth muscle. This muscle constricts when someone coughs, increasing the force of the cough. The trachealis muscle also constricts during an asthmatic reaction, shrinking the airway and making it harder to breathe.

Structurally the trachea consists of three concentric tissue layers: the mucosa, submucosa, and adventitia. This arrangement is continuous throughout the remainder of the upper respiratory system. The layer facing the lumen, the mucosa, is composed of pseudostratified ciliated columnar epitheliia. The submucosa contains blood vessels, nerves, assorted connective tissues, and mucus glands. The outer adventitia is mostly areolar connective tissue that is continuous with the hyaline cartilage and connects the trachea to the surrounding structures and tissues.

The Bronchi and Bronchioles

The bronchial tree is so named because it resembles a tree that has been turned upside-down. The trachea would be the trunk, the progressively smaller bronchi and bronchioles are the branches, and the alveoli are the leaves. Branching of the trachea into the right and left primary bronchi occurs after the last cartilage in the trachea at the level of the seventh thoracic vertebrae. The right primary bronchus is wider, shorter, and more vertical than the left primary bronchus because the left primary bronchus and lung must accommodate the heart. The primary bronchi branch to form the secondary or lobar bronchi. There are three secondary bronchi on the right and two on the left. The secondary bronchi continue to branch into the tertiary or segmented bronchi, which also continue the branching pattern. Approximately 23 successive branches lead to the bronchioles. A bronchiole is a tube with a diameter of less than 1 millimeter (mm). When the measurement reaches less than 0.5 mm, a bronchiole is termed a terminal bronchiole.

The bronchial tree is lined by pseudostratified, ciliated, columnar epithelia with goblet cells dispersed among the columnar cells. Plates of cartilage are found in the walls of the bronchial tree, with the amount of cartilage and the number of plates decreasing as the bronchi and bronchioles become smaller. The terminal bronchioles do not contain cartilage plates, these tubes are small enough to stay open without cartilage. Smooth muscle is found throughout the system, even into the respiratory zone.

The Lungs

The lungs are two of the largest organs of the body, but they are among the lightest. Along with the heart, the lungs take up nearly all of the space in the thorax, superior to the diaphragm. As an organ, the lung is made up of airway tubes and alveoli, giving it little weight. Elastic connective tissues in the stroma of the lungs allow them to expand with incoming air and recoil when expelling air. The lungs contain a large amount of surface area in order to efficiently support the exchange of oxygen and carbon dioxide.

The hilus (meaning depression or pit) of the lungs is an indentation on the medial side of the lungs and the point of entry of blood vessels, primary bronchi, nerves, and lymphatics. This collection of vessels and nerves makes up the root of the lung. The tip or apex of the lungs is a blunted point found just above the clavicles. The posterior, lateral, and anterior sides of the lungs are surrounded by the ribs. These areas are called the costal surfaces of the lungs referring to the costal cartilage surrounding them. The flat, inferior surface of the lungs is found superior to the diaphragm and referred to as the base of the lung. Since, the liver is found on the right side of the body and inferior to the diaphragm, the insertion of the diaphragm is slightly raised on the right. Consequently the right lung is usually slightly shorter than the left. The lungs extend from the first costal cartilage to the tenth thoracic vertebrae.

The lungs consist of a right lung and a left lung. Even though the right lung is slightly shorter than the left, the left lung has about 10 percent less mass than the right due to the cardiac notch on the medial side of the left lung. The heart is tucked into this notch. The heart, the right lung, and the left lung, are each located in their own anatomical compartment in the upper thorax. The right lung is divided into three lobes. A horizontal fissure separates the superior and middle lobes, and an oblique fissure separates the middle and inferior lobes. The smaller left lung contains only two lobes. An oblique fissure separates the superior and inferior lobes on this side.

Each lobe is divided into bronchopulmonary segments separated by connective tissue septa. There are a total of 10 of these segments and each contains a tertiary bronchiole, a pulmonary and bronchial artery, and a lymphatic branch. The presence of these segments aids in further isolating parts of the lungs to prevent the spread of infection or disease. Connective tissue further divides the segments into lung lobules, the smallest anatomical unit in the lungs. A lobule is hexagonal in shape and less than a centimeter in diameter. Each lobule contains a terminal bronchiole and its associated alveoli. The connective tissue associated with lobules may be blackened by tobacco smoke or pollution from the environment.

The image below compares the lungs from a smoker with those from a non-smoker.

Usually, lungs are a pink with a relatively smooth surface. The lungs on the right are blackened and irregularly shaped from the accumulation of material inhaled with tobacco smoke. This material can be cleared from the lungs, but only over a long period of time and only if the person stops smoking. Cigarette smoking causes the deaths of over 440,000 people in the United States each year, including people exposed to second-hand smoke. The financial losses associated with these deaths are in excess of 200 billion dollars a year.

There are over 4,000 chemical compounds that are created when the more than 600 chemical ingredients in cigarettes are burned and inhaled. More than 50 of the chemicals produced by the burning of a cigarette are classified as carcinogens. A partial list of these chemicals includes: acetone, acetic acid, ammonia, arsenic, benzene, butane, cadmium, carbon monoxide, formaldehyde, lead, naphthalene, methanol, nicotine, tar, and toluene.

The direct effect of many of the chemicals in the smoke is to paralyze and destroy cilia. When the cilia cannot function, mucus accumulates in the lung tubules. Opportunistic bacterial and other microorganisms utilize the mucus to grow and colonize the lungs. Many of these organisms create pathologic conditions inside the lung.

Smoker’s cough is a condition that exists because the mucus and other debris accumulate in the lung tissue. These accumulations stimulate the cough reflex as means to clear the lungs.

The inhaled chemicals can also contribute to the breakdown of connective tissue fibers, especially elastin. The degeneration of the connective tissue can lead to emphysema, a form of chronic obstructive pulmonary disease (COPD).

Blood and Nerve Supply

The lungs have a dual blood supply. The pulmonary artery brings oxygen-poor blood from the right ventricle of the heart. This blood passes through the pulmonary capillaries, where some carbon dioxide will leave the blood and a large amount of oxygen will be acquired. The newly oxygenated blood enters the pulmonary veins and returns back to the left side of heart. The pulmonary circulation holds about 500 milliliters (ml) of blood, or about 10 percent of the body’s supply. About 75 ml of blood is in the pulmonary capillaries for gas exchange at any one time. The blood supply that nourishes the tissues of the lungs arrives through the bronchial artery, which branches off of the aorta and carries oxygen-rich blood to support the lung tissues. The bronchial supply anastomoses with the pulmonary vessels, and a mixture of blood leaves through the bronchial and pulmonary veins. Blood passes through the lungs at a rate equal to cardiac output, or about five liters per minute.

Blood pressure measured in the pulmonary circulation is less than it is in corresponding vessels of the systemic circulation. To some extent this decrease results from the decreased resistance found in this shorter pulmonary pathway. Under normal conditions, this filtrate is readily removed by the lymphatic vessels. Under conditions where left atrial pressure rises dramatically, such as in mitral valve stenosis or congestive heart failure, pulmonary capillary pressure will also rise, increasing the rate of capillary filtration. If the lymphatic system cannot keep up with the higher filtration rate, fluid will accumulate in the alveoli as pulmonary edema. This can interfere with the exchange of oxygen, leading to cyanosis, decreased activity tolerance, etc.

The pulmonary circulation responds to hypoxia differently than the systemic circulation. The systemic circulation dilates under hypoxic conditions causing an increased blood flow through the tissues. The arterioles of pulmonary circulation constrict selectively in cases of alveolar hypoxia. This constriction diverts blood to areas in the lungs that are better ventilated. This helps ensure adequate gas exchange.

Nerves from the pulmonary plexus enter the lungs at the hilus. These nerves contain a mixture of visceral sensory and autonomic nerve fibers that follow the bronchial tree and blood vessels. Parasympathetic nerve stimulation results in bronchoconstriction, constriction of the bronchioles, while sympathetic nerve stimulation results in bronchodilation, dilation of the bronchioles.

Pleura

Each lung is found in a pleural cavity bounded by the pleural membrane, a double sided membrane that contains a thin layer of pleural fluid. The visceral pleural is a mucus membrane that covers the lungs and folds over at the hilus. The folded membrane continues and becomes the parietal pleura, which lines the inner wall of the thoracic cavity. The space between the two membranes is called the pleural cavity or space. From 1 to 15 ml of pleural fluid is found on the facing surfaces of the pleural membranes. This fluid helps to lubricate the membrane surfaces so that the movement of the lungs during inhaling and exhaling does not cause frictional damage to the tissues. The fluid also lightly holds the two membranes together so that they move together as the chest wall expands and contracts.

Since pleural fluid is mainly a filtrate of plasma, the factors that affect the amount of pleural fluid production and removal are the same factors that govern interstitial fluid volumes in most regions of the body. The main factors include capillary hydrostatic pressure, capillary colloid osmotic pressure, the permeability (“leakiness”) of the capillaries, and the rate of fluid removal by the lymphatics. Under normal conditions, there is a slow but steady turnover of pleural fluid, but under certain conditions excess fluid can build up, and impede lung expansion. This condition is known as a pleural effusion and may be secondary to blockade of venous drainage by tumors, increased capillary permeability because of infections, etc.

Respiratory Functions

The movement of gases from the atmosphere into the lungs and the cells of the body and back out again is called external respiration. External respiration includes ventilation, the bulk flow of air from outside the body into the repiratory system (inhalation) and the bulk flow of air from the respiratory system back to the outside (exhalation). Also included under the processes described as external respiration are the exchange of oxygen and carbon dioxide between the lungs and the blood, the transport of oxygen and carbon dioxide by the blood, and the exchange of gasses between the cells and the blood. The conduit for moving air into the lungs and then returning it to the external environment is referred to as the conducting zone (division). Gas exchange does not take place in the structures that make up the conducting zone. The conducting zone consists of a system of tubes that connect the nose and mouth to the alveoli. Air movement starts at the nose or mouth and continues through the pharynx, larynx, trachea, and bronchial tree, terminating at the structures of the respiratory zone (division) where gas exchange will occur.

The respiratory zone includes the very small respiratory bronchioles, the alveoli and the blood capillaries. It is critical that both the conducting and respiratory zones function adequately for the body to maintain homeostasis. Organization of conducting and respiratory zones will be the focus of this discussion.

The Conducting Zone

The conducting zone of the respiratory system brings gases into and out of the respiratory system. In addition, the organs and tissues along this path warm the incoming air to approximately body temperature, moisten the air to about 100 percent humidity, and begin filtering out any harmful microorganisms or particles that may be suspended in the inhaled air.

Temperatures too far below 37°C (98.6°F) can harm the delicate alveoli and inhibit enzyme function. Damaged aveoli, or improperly functioning enzymes could lead to decreased gas diffusion and subsequent homeostatic imbalances. For example, the inter-conversion of carbon dioxide and carbonic acid is catalyzed by the enzyme carbonic anhydrase. Similar to most other body enzymes, it loses efficiency at lower temperatures. If carbonic anhydrase is not functioning properly there could be an increase in carbonic acid levels. Since carbonic acid breaks down into bicarbonate ions and free hydrogen ions, there would be resulting decrease in pH leading to acidosis. The temperature in the respiratory zone is so important that a person who runs in cold weather is advised to put a scarf across their nose and mouth to help pre-warm the air. Otherwise, the tissues in the conducting zone may not be able to warm the air sufficiently to prevent damage to the system.

If the air in the respiratory zone is not near 100 percent humidity, the thin-walled alveoli can become dehydrated and may deteriorate. The alveoli are lined with a very thin layer of water and chemicals that prevent damage to the tissues and help to keep the alveoli open. If the air taken to the respiratory zone doesn’t have sufficient humidity (water vapor), the water layer lining the aveoli will evaporate resulting in damage to the structures and interference with respiration.

The hairs found at the entrance to the nose, along with the mucus and cilia found inside the nose help to trap and remove pathogens and particles taken in with air. Large particles can build up at the entrance of the nose where the hairs trap them. Coal miners, for example, will accumulate coal dust at the opening of their nostrils due to the action of these hairs. Smaller particles, such as some viruses and bacteria will be trapped in the mucus in the nose and the cilia on the epithelial cells will sweep them to the back of the pharynx. The combined action of the mucus and the cilia is referred to as the mucociliary escalator. Ultimately the mixture of mucus and particles will be swallowed and destroyed by the acid and digestive enzymes in the stomach. This process of trapping and removal of particles and microorganisms occurs throughout the conducting zone and through the first part of the respiratory zone.

The Respiratory Zone

A transition occurs in the composition of the epithelial lining at the end of the terminal bronchioles. The epithelial lining changes to simple cuboidal cells without any intermixed goblet cells. The walls of the tubes at this point become very thin and are called respiratory bronchioles. The loss of the goblet cells means that no mucus is secreted into the bronchioles at this point, but the cilia are still present to sweep away any mucus that should come down into them. Some portions of the respiratory bronchioles are not completely covered by cilia, and a number of phagocytic macrophages are found in this area to compensate for the loss of the cilia. Some gas exchange can occur through the walls of the respiratory bronchioles.

Respiratory bronchioles lead into the alveolar ducts, which still have smooth muscle and some elastic fibers in the walls. Alveolar sacs occur as clusters of aveoli (singular alveolus) sharing a common chamber along the alveolar ducts. It has been estimated that there are 300 million alveoli in the lungs. These alveoli provide a surface area of 27 square meters, or an area 25 feet by 30 feet, which is roughly the size of a three-car garage. Adjacent alveoli are interconnected through alveolar pores.

Pulmonary Ventilation

Pulmonary ventilation is the act of breathing and the first step in the respiratory process. Pulmonary ventilation brings in air with a new supply of oxygen and a very small amount of carbon dioxide from the atmosphere into the alveoli. This mixture then participates in external respiration, the exchange of oxygen and carbon dioxide between the alveoli and pulmonary capillary blood across the respiratory membrane. Internal respiration is the exchange of gasses between the tissues of the body and the blood, which provides oxygen for aerobic cellular respiration and removes carbon dioxide. Aerobic Cellular respiration refers to the intracellular use of oxygen and the generation of carbon dioxide waste through metabolic pathways.

Pressure

Breathing or ventilation, involves changes in pressure as a result of mechanical work.

The physical movement of air into the lungs is a result of the production of differences in total pressure between the interior (alveolar pressure) and exterior (atmospheric pressure) of the respiratory zone. Atmospheric pressure at sea level is typically 1 atmosphere or 760 mmHg, and this value will be used as the reference atmospheric pressure here. One atmosphere is the air pressure that would push a column of the mercury up in a thin tube a distance of 760 mm. Mercury is used because it is a liquid at room temperature, and its density changes very little over pressure and temperature ranges. In the process of ventilation, air moves down pressure gradients going from an area of higher pressure to an area of lower pressure. Assuming a person is at sea level, if his or her intrapulmonary pressure, the pressure in the alveoli, is less than 1 atm, air enters the lungs and fills the alveoli. If his or her intrapulmonary pressure is greater than 1 atm, then air moves out of the lungs into the environment.

There is another pressure often discussed in the mechanics of ventilation, intrapleural (sometimes just pleural) pressure. This is the pressure within the pleural sac (space). Intrapleural pressure is maintained slightly less than the pressure in the alveoli. This pressure difference aids in keeping the lungs slightly inflated at all times and ensures that they do not collapse when exhaling.

Inspiration and Expiration

Gases have a property of distributing to fill whatever size and shape container they occupy. If a closed container is made larger (an increase in volume), the total number of gas molecules will stay the same, but they will redistribute to fill the larger space. In doing so, they decrease their concentration (remember that concentration is a property of mass and volume). This increase in volume but not number of molecules of the gas leads to a corresponding decrease in the pressure exerted by that gas on the walls of the container. If the same closed container is made smaller, the concentration of gases increases( even though the actual number of gas molecules has not changed) and the pressure increases. This is an example of a physical law called Boyle’s Law.

The law is expressed mathematically as:

P₁V₁ = P₂V₂

where P = pressure, V = volume, 1 is the initial pressure and volume, and 2 is the resulting pressure and volume. Boyle’s Law states that, at a constant temperature, the pressure of a gas is inversely proportional with the volume of its container. Very simply, as volume goes up, pressure goes down, and as volume goes down, pressure goes up.

These properties become important for ventilation because the passage of air into and out of the lungs is controlled by the size of the lungs’ container, the thorax. The difference between the example above and the lungs is that the lungs are not a closed system (unless you have a closed glottis), with the opening to the lungs maintained through the conducting zone. But over very short periods of time Boyle’s law still applies. In this case, when we inhale, the movement of the diaphragm and the ribs increases the volume of the thorax. This immediately decreases the air pressure within the thorax, creating a pressure gradient between the atmosphere and the alveoli (the latter is called intrapulmonary pressure), and drawing air into the lungs. Air will enter until the atmospheric and intrapulmonary pressures are equal. However, the volume of the thorax remains larger than before the start of inhalation. To exhale, the thorax is allowed to decrease in volume (the ribs and diaphragm return to their original positions), increasing the intrapulmonary pressure and creating a gradient in the opposite direction resulting in the flow of air out to the atmosphere. Changing the size of the thoracic cavity would be impossible if the ribs were solidly attached to the sternum. If they were solidly attached, it would be like trying to move the sides of a birdcage. The costal cartilage allows the ribs to move and expand or contract the chest wall.

Inspiration

Inspiration is the act of inhaling. As stated above, inspiration occurs by increasing the volume of the thorax. This active process involves the use of chest and neck muscles. Resting inspiration is achieved mostly by movement of the diaphragm. The relaxed shape of the diaphragm resembles a shallow dome with the apex pointing toward the lungs, similar to the shape of an open umbrella. When the diaphragm contracts, it tends to flatten out, expanding the volume of the thorax in an inferior direction. Consequently, the intrapleural and intrapulmonary pressures decrease below atmospheric pressure resulting in air being pulled through the conducting zone and into the lungs. The external intercostal muscles work in conjunction with the diaphragm. The normal orientation of the ribs is around the side of the thorax and angled inferiorly to the sternum. When the external intercostal muscles contract, the ribs are pulled up, also expanding the thoracic cavity in a horizontal direction. In adults, ventilation occurs about 12 times a minute and moves roughly 500 ml of air during each breath.

A deep breath involves a greater shortening of the diaphragm and the external intercostal muscles plus additional contractions of other neck and chest muscles. The scalene muscles elevate the first two ribs. The sternocleidomastoid muscles elevate the sternum, and the pectoralis minor muscles help to elevate the third, fourth, and fifth ribs. Two physical conditions that interfere with inspiration are obesity and advanced pregnancy. In both cases, the abdominal organs are pushed against the diaphragm, restricting its downward movement and hindering the expansion of the thoracic cavity.

Expiration

Expiration is the act of exhaling. Resting expiration is a passive process. The muscles used during inspiration relax, and allow the chest wall and the diaphragm to move back to their original position, thus decreasing the volume of the thorax and forcing air from the lungs. The compression of the chest wall also aids in moving blood and lymph through the vessels that drain the lungs. Expiration becomes an active process when a more forceful exhale is required. The internal intercostal muscles pull the ribs down, helping to compress the chest. The external and internal oblique and transverse abdominal muscles press on the abdominal organs, which move them up against the diaphragm and force the diaphragm higher than it would normally go on relaxation, further decreasing the volume of the thoracic cavity.

 

Airway Resistance, Alveolar Surface Tension, and Lung Compliance

One thing that works against the pressure gradient created by the expansion of the thorax is the resistance found within the conducting zone. Under normal conditions this resistance is quite low, and the airways have little trouble passing air between the atmosphere and the alveoli. The small amount of resistance found in the system stems mainly from the bronchioles, which are analogous to the arterioles of the vascular system. Both the bronchioles and the arterioles contribute a large portion of the resistance of their entire system, and both have smooth muscles in their walls that allow them to constrict or dilate. Under conditions of bronchiolar constriction (bronchoconstriction), resistance to airflow can increase dramatically. When this happens rapidly, it is classified as an acute asthma attack. The afflicted individual will need to generate much larger changes in intrapulmonary pressure to maintain a normal flow rate of air during breathing. Recall that if resistance increases an increased pressure gradient is necessary to maintain the same flow. To achieve this, the person uses the accessory muscles to breathe, and will appear to be “straining” as he or she does so. Treatment usually involves an inhaler containing a bronchodilator. Resistance will also be affected if the airways become narrowed by mucus (chronic asthma) or aspirated substances.

Along with the resistance of the airways, the compliance of the lung tissue also determines how much effort breathing requires. Compliance is a property that explains the relationship between a volume change and pressure change. Typically it takes a 2-3 cm of water change in pleural pressure to change volume in the lungs by 500 ml. This is a compliance of about 200 ml/cm H₂O. The compliance of the lungs is dependent on the elasticity of the connective tissues of the lung as well as the alveolar surface tension. Remember that a thin liquid layer containing water as well as other molecules lines the interior lung surface. Because of its polar nature, water exhibits cohesion, such that it takes some effort to separate water molecules (you may have experienced the force of this cohesion if you ever belly flopped into the water). This surface tension is actually enough to collapse the alveoli each time a person exhales. In order to prevent this from happening, the type II alveolar cells make a combination of lipids and proteins that serves as a surfactant in the alveoli. A surfactant is a chemical that acts like a detergent breaking the surface tension of the water lining the alveoli. This action allows the alveoli to remain open when exhaling.

Compliance of the lung can be decreased either by fibrosis of the lung tissue creating a stiffening of the tissue (this can occur with conditions such as tuberculosis) or lack of surfactant (common in premature infants). Either way breathing becomes much more difficult, requiring more effort and sometimes mechanical ventilation.

 

Respiratory Volumes, Capacities, and Function Tests

Various lung volumes, capacities (capacities are combinations of lung volumes) and flow rates can be evaluated through a process called spirometry. The patient breathes in and out under controlled conditions, and the amount of air passing through the system, as well as the time it takes for air passage is measured. The values for these measurements depend on lung function and the size of the patient.

Table 1: Respiratory Volumes, Descriptions, and Normal Adult Values

Respiratory Volumes	Description	Normal adult values
Tidal Volume (TV)	Volume of a resting breath	500 mL/breath
Inspiratory Reserve Volume (IRV)	Maximum volume that can be inhaled after a normal inhale	1900 – 3300 mL
Expiratory Reserve Volume (ERV)	Maximum volume that can be exhaled after a normal exhale	700 – 1200 mL
Residual Volume (RV)	Air left in the lungs after exhaling completely (i.e. after an ERV)	1200 mL
Dead Space	Air inhaled during breathing that stays in the conducting zone	150 mL