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What causes inhalation during breathing?

What causes inhalation during breathing?


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I have read here that the two major inhalation muscles are the (1) diaphragm and the (2) external intercostals.

Additionally, inhalation can also be caused by (1) expansion of the abdominal cavity, or (2) movement of the pelvic floor.


I thought that inhalation was caused by muscles that essentially cause the intra-thoracic pressure to drop, thus causing air to flow into the lungs to fill the absence, like a straw. This is what I thought caused us to inhale.

MY QUESTION

If my understanding is correct, then do the muscles cause this drop in pressure? Which spaces within the trunk experience this drop in pressure? If not, how exactly do these muscles facilitate inhalation?


Other than the branching, gas filled spaces within the lungs themselves, there are no other spaces within the trunk and above the diaphragm that contain gas - for a normal healthy lung. All the other space is filled with either tissue or fluid that serve to extend any pressure applied by the muscles you mentioned to the pleural sac that surrounds the lungs - like a hydraulic device. It's only the gas space that expands in volume and this is what causes air to move into that space through the upper airway. Exhalation is caused by elastic recoil - the springing back of this compartment.


Either the diaphragm or the rib lifting muscles dependently or together can enlarge the lungs and cause a vacuum. The vacuum is caused by enlargement of the the chamber like for a bike pump.

The less known muscles raise and lower the ribs: https://en.wikipedia.org/wiki/Intercostal_muscle Here's a video of diaphragm movement: https://www.youtube.com/watch?v=hp-gCvW8PRY

The best is to research the muscle groups on google images and check out graphs of them to see lists of studies.

The pressure drop is actually an expansion of the lung cavity, with the diaphragm being very good at drawing air in like a serynge and the expiration groups being stronger for exhaling, they can both expand, contract and compress for activities like swimming and coughing. You may want to check graphs and studies of air pressure, then it gets complicated with trans-thoracic and trans-pulmonary pressure:


What causes inhalation during breathing? - Biology

The breathing mechanisms of most mammals include two parts: inhalation and exhalation. These mechanisms depend on pressure gradients as well as the muscles in the thoracic cavity.

The mechanism of breathing obeys Boyle’s law which states that that in a closed space, pressure and volume is inversely related as the volume decreases, pressure increases and vice versa. The thoracic cavity always has a slight, negative pressure which aids in keeping the airways of the lungs open. During the process of inhalation, the lung volume expands as a result of the contraction of the diaphragm and intercostal muscles, expanding the thoracic cavity. Due to this increase in volume, the pressure is decreased, based on the principles of Boyle’s Law. This decrease of pressure in the thoracic cavity relative to the environment makes the cavity pressure less than the atmospheric pressure. This pressure gradient between the atmosphere and the thoracic cavity allows air to rush into the lungs inhalation occurs. The resulting increase in volume is primarily attributed to an increase in alveolar space because the bronchioles and bronchi are stiff structures that do not change in size.

During inhalation, the chest wall expands out and away from the lungs. The lungs are elastic therefore, when air fills the lungs, the elastic recoil within the tissues of the lung exerts pressure back toward the interior of the lungs. These outward and inward forces compete to inflate and deflate the lung with every breath. Upon exhalation, the lungs recoil to force the air out of the lungs. The intercostal muscles relax, returning the chest wall to its original position. During exhalation, the diaphragm also relaxes, moving higher into the thoracic cavity—the increases in pressure within the thoracic cavity relative to the environment. Air rushes out of the lungs due to the pressure gradient between the thoracic cavity and the atmosphere.

Surface tension is the force exerted by water molecules on the surface of the lung tissue as those water molecules pull together. Water (H2O) is a highly polar molecule, so it forms intermolecular bonds with other water molecules. The force of these bonds effectively creates an inward force on surfaces in the lungs lowering the surface area as the tissue is pulled together. As the air inside the lungs is moist, there is considerable surface tension within the tissue of the lungs. Because the alveoli of the lungs are highly elastic, they do not resist surface tension on their own, which allows the force of that surface tension to deflate the alveoli as air is forced out during exhalation by the contraction of the pleural cavity.

The force of surface tension in the lungs is so great that without something to reduce the surface tension, the airways would collapse after exhalation, making re-inflation during inhalation much more difficult and less effective. Cells of the alveoli continually secrete a molecule called surfactant that solves this problem. Surfactant is a lipoprotein molecule that reduces the force of surface tension from water molecules on the lung tissue. As a result, the surface tension of the lungs from water is reduced so that the lungs can still inflate and deflate appropriately without the possibility of collapse from surface tension alone.


Practice Questions


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Key Points

• The mechanics of breathing follow Boyle’s Law which states that pressure and volume have an inverse relationship.

• The process of inhalation occurs due to an increase in the lung volume (diaphragm contraction and chest wall expansion) which results in a decrease in lung pressure in comparison to the atmosphere thus, air rushes in the airway.

• The process of exhalation occurs due to elastic recoil of the lung tissue, which causes a decrease in volume, resulting in increased pressure in comparison to the atmosphere thus, air rushes out of the airway.

• There is no contraction of muscles during exhalation it is considered a passive process.

• Surfactant is a phospholipid and lipoprotein substance produced in the lungs that function similarly to a detergent. In essence, it reduces the surface tension between alveoli tissue and air within the alveoli, thereby reducing the work needed for airway inflation.

diaphragm: a dome-shaped muscle present between the thoracic cavity and the abdominal cavity which assist in the breathing of air.

intercostal: between the ribs

surfactant: Surfactant is a complex mixture of phospholipids and lipoproteins that works to reduce the surface tension that exists between the alveoli tissue and the air found within the alveoli.

resiliency: a property to return to its original shape and size

Boyle’s law: states that in a closed space, pressure and volume are inversely related

thoracic cavity: the chamber of the body of vertebrates that contains the lungs protected by the ribs

inhalation: breathing in

bronchiole: branch of bronchi that are 1 mm or less in diameter and terminate at alveolar sacs

elastic recoil: the rebound of the lungs after having been stretched by inhalation

exhalation: breathing out

surface tension: the force exerted by water molecules on the surface of the lung tissue as those water molecules pull together


39.3 Breathing

Mammalian lungs are located in the thoracic cavity where they are surrounded and protected by the rib cage, intercostal muscles, and bound by the chest wall. The bottom of the lungs is contained by the diaphragm, a skeletal muscle that facilitates breathing. Breathing requires the coordination of the lungs, the chest wall, and most importantly, the diaphragm.

Types of Breathing

Amphibians have evolved multiple ways of breathing. Young amphibians, like tadpoles, use gills to breathe, and they don’t leave the water. Some amphibians retain gills for life. As the tadpole grows, the gills disappear and lungs grow. These lungs are primitive and not as evolved as mammalian lungs. Adult amphibians are lacking or have a reduced diaphragm, so breathing via lungs is forced. The other means of breathing for amphibians is diffusion across the skin. To aid this diffusion, amphibian skin must remain moist.

Birds face a unique challenge with respect to breathing: They fly. Flying consumes a great amount of energy therefore, birds require a lot of oxygen to aid their metabolic processes. Birds have evolved a respiratory system that supplies them with the oxygen needed to enable flying. Similar to mammals, birds have lungs, which are organs specialized for gas exchange. Oxygenated air, taken in during inhalation, diffuses across the surface of the lungs into the bloodstream, and carbon dioxide diffuses from the blood into the lungs and expelled during exhalation. The details of breathing between birds and mammals differ substantially.

In addition to lungs, birds have air sacs inside their body. Air flows in one direction from the posterior air sacs to the lungs and out of the anterior air sacs. The flow of air is in the opposite direction from blood flow, and gas exchange takes place much more efficiently. This type of breathing enables birds to obtain the requisite oxygen, even at higher altitudes where the oxygen concentration is low. This directionality of airflow requires two cycles of air intake and exhalation to completely get the air out of the lungs.

Evolution Connection

Avian Respiration

Birds have evolved a respiratory system that enables them to fly. Flying is a high-energy process and requires a lot of oxygen. Furthermore, many birds fly in high altitudes where the concentration of oxygen in low. How did birds evolve a respiratory system that is so unique?

Decades of research by paleontologists have shown that birds evolved from therapods, meat-eating dinosaurs (Figure 39.14). In fact, fossil evidence shows that meat-eating dinosaurs that lived more than 100 million years ago had a similar flow-through respiratory system with lungs and air sacs. Archaeopteryx and Xiaotingia, for example, were flying dinosaurs and are believed to be early precursors of birds.

Most of us consider that dinosaurs are extinct. However, modern birds are descendants of avian dinosaurs. The respiratory system of modern birds has been evolving for hundreds of millions of years.

All mammals have lungs that are the main organs for breathing. Lung capacity has evolved to support the animal’s activities. During inhalation, the lungs expand with air, and oxygen diffuses across the lung’s surface and enters the bloodstream. During exhalation, the lungs expel air and lung volume decreases. In the next few sections, the process of human breathing will be explained.

The Mechanics of Human Breathing

Boyle’s Law is the gas law that states that in a closed space, pressure and volume are inversely related. As volume decreases, pressure increases and vice versa (Figure 39.15). The relationship between gas pressure and volume helps to explain the mechanics of breathing.

There is always a slightly negative pressure within the thoracic cavity, which aids in keeping the airways of the lungs open. During inhalation, volume increases as a result of contraction of the diaphragm, and pressure decreases (according to Boyle’s Law). This decrease of pressure in the thoracic cavity relative to the environment makes the cavity less than the atmosphere (Figure 39.16a). Because of this drop in pressure, air rushes into the respiratory passages. To increase the volume of the lungs, the chest wall expands. This results from the contraction of the intercostal muscles , the muscles that are connected to the rib cage. Lung volume expands because the diaphragm contracts and the intercostals muscles contract, thus expanding the thoracic cavity. This increase in the volume of the thoracic cavity lowers pressure compared to the atmosphere, so air rushes into the lungs, thus increasing its volume. The resulting increase in volume is largely attributed to an increase in alveolar space, because the bronchioles and bronchi are stiff structures that do not change in size.

The chest wall expands out and away from the lungs. The lungs are elastic therefore, when air fills the lungs, the elastic recoil within the tissues of the lung exerts pressure back toward the interior of the lungs. These outward and inward forces compete to inflate and deflate the lung with every breath. Upon exhalation, the lungs recoil to force the air out of the lungs, and the intercostal muscles relax, returning the chest wall back to its original position (Figure 39.16b). The diaphragm also relaxes and moves higher into the thoracic cavity. This increases the pressure within the thoracic cavity relative to the environment, and air rushes out of the lungs. The movement of air out of the lungs is a passive event. No muscles are contracting to expel the air.

Each lung is surrounded by an invaginated sac. The layer of tissue that covers the lung and dips into spaces is called the visceral pleura . A second layer of parietal pleura lines the interior of the thorax (Figure 39.17). The space between these layers, the intrapleural space , contains a small amount of fluid that protects the tissue and reduces the friction generated from rubbing the tissue layers together as the lungs contract and relax. Pleurisy results when these layers of tissue become inflamed it is painful because the inflammation increases the pressure within the thoracic cavity and reduces the volume of the lung.

View how Boyle’s Law is related to breathing and watch a video on Boyle’s Law.

The Work of Breathing

The number of breaths per minute is the respiratory rate . On average, under non-exertion conditions, the human respiratory rate is 12–15 breaths/minute. The respiratory rate contributes to the alveolar ventilation , or how much air moves into and out of the alveoli. Alveolar ventilation prevents carbon dioxide buildup in the alveoli. There are two ways to keep the alveolar ventilation constant: increase the respiratory rate while decreasing the tidal volume of air per breath (shallow breathing), or decrease the respiratory rate while increasing the tidal volume per breath. In either case, the ventilation remains the same, but the work done and type of work needed are quite different. Both tidal volume and respiratory rate are closely regulated when oxygen demand increases.

There are two types of work conducted during respiration, flow-resistive and elastic work. Flow-resistive refers to the work of the alveoli and tissues in the lung, whereas elastic work refers to the work of the intercostal muscles, chest wall, and diaphragm. Increasing the respiration rate increases the flow-resistive work of the airways and decreases the elastic work of the muscles. Decreasing the respiratory rate reverses the type of work required.

Surfactant

The air-tissue/water interface of the alveoli has a high surface tension. This surface tension is similar to the surface tension of water at the liquid-air interface of a water droplet that results in the bonding of the water molecules together. Surfactant is a complex mixture of phospholipids and lipoproteins that works to reduce the surface tension that exists between the alveoli tissue and the air found within the alveoli. By lowering the surface tension of the alveolar fluid, it reduces the tendency of alveoli to collapse.

Surfactant works like a detergent to reduce the surface tension and allows for easier inflation of the airways. When a balloon is first inflated, it takes a large amount of effort to stretch the plastic and start to inflate the balloon. If a little bit of detergent was applied to the interior of the balloon, then the amount of effort or work needed to begin to inflate the balloon would decrease, and it would become much easier to start blowing up the balloon. This same principle applies to the airways. A small amount of surfactant to the airway tissues reduces the effort or work needed to inflate those airways. Babies born prematurely sometimes do not produce enough surfactant. As a result, they suffer from respiratory distress syndrome , because it requires more effort to inflate their lungs. Surfactant is also important for preventing collapse of small alveoli relative to large alveoli.

Lung Resistance and Compliance

Pulmonary diseases reduce the rate of gas exchange into and out of the lungs. Two main causes of decreased gas exchange are compliance (how elastic the lung is) and resistance (how much obstruction exists in the airways). A change in either can dramatically alter breathing and the ability to take in oxygen and release carbon dioxide.

Examples of restrictive diseases are respiratory distress syndrome and pulmonary fibrosis. In both diseases, the airways are less compliant and they are stiff or fibrotic. There is a decrease in compliance because the lung tissue cannot bend and move. In these types of restrictive diseases, the intrapleural pressure is more positive and the airways collapse upon exhalation, which traps air in the lungs. Forced or functional vital capacity (FVC) , which is the amount of air that can be forcibly exhaled after taking the deepest breath possible, is much lower than in normal patients, and the time it takes to exhale most of the air is greatly prolonged (Figure 39.18). A patient suffering from these diseases cannot exhale the normal amount of air.

Obstructive diseases and conditions include emphysema, asthma, and pulmonary edema. In emphysema, which mostly arises from smoking tobacco, the walls of the alveoli are destroyed, decreasing the surface area for gas exchange. The overall compliance of the lungs is increased, because as the alveolar walls are damaged, lung elastic recoil decreases due to a loss of elastic fibers, and more air is trapped in the lungs at the end of exhalation. Asthma is a disease in which inflammation is triggered by environmental factors. Inflammation obstructs the airways. The obstruction may be due to edema (fluid accumulation), smooth muscle spasms in the walls of the bronchioles, increased mucus secretion, damage to the epithelia of the airways, or a combination of these events. Those with asthma or edema experience increased occlusion from increased inflammation of the airways. This tends to block the airways, preventing the proper movement of gases (Figure 39.18). Those with obstructive diseases have large volumes of air trapped after exhalation and breathe at a very high lung volume to compensate for the lack of airway recruitment.

Dead Space: V/Q Mismatch

Pulmonary circulation pressure is very low compared to that of the systemic circulation. It is also independent of cardiac output. This is because of a phenomenon called recruitment , which is the process of opening airways that normally remain closed when cardiac output increases. As cardiac output increases, the number of capillaries and arteries that are perfused (filled with blood) increases. These capillaries and arteries are not always in use but are ready if needed. At times, however, there is a mismatch between the amount of air (ventilation, V) and the amount of blood (perfusion, Q) in the lungs. This is referred to as ventilation/perfusion (V/Q) mismatch .

There are two types of V/Q mismatch. Both produce dead space , regions of broken down or blocked lung tissue. Dead spaces can severely impact breathing, because they reduce the surface area available for gas diffusion. As a result, the amount of oxygen in the blood decreases, whereas the carbon dioxide level increases. Dead space is created when no ventilation and/or perfusion takes place. Anatomical dead space or anatomical shunt, arises from an anatomical failure, while physiological dead space or physiological shunt, arises from a functional impairment of the lung or arteries.

An example of an anatomical shunt is the effect of gravity on the lungs. The lung is particularly susceptible to changes in the magnitude and direction of gravitational forces. When someone is standing or sitting upright, the pleural pressure gradient leads to increased ventilation further down in the lung. As a result, the intrapleural pressure is more negative at the base of the lung than at the top, and more air fills the bottom of the lung than the top. Likewise, it takes less energy to pump blood to the bottom of the lung than to the top when in a prone position. Perfusion of the lung is not uniform while standing or sitting. This is a result of hydrostatic forces combined with the effect of airway pressure. An anatomical shunt develops because the ventilation of the airways does not match the perfusion of the arteries surrounding those airways. As a result, the rate of gas exchange is reduced. Note that this does not occur when lying down, because in this position, gravity does not preferentially pull the bottom of the lung down.

A physiological shunt can develop if there is infection or edema in the lung that obstructs an area. This will decrease ventilation but not affect perfusion therefore, the V/Q ratio changes and gas exchange is affected.

The lung can compensate for these mismatches in ventilation and perfusion. If ventilation is greater than perfusion, the arterioles dilate and the bronchioles constrict. This increases perfusion and reduces ventilation. Likewise, if ventilation is less than perfusion, the arterioles constrict and the bronchioles dilate to correct the imbalance.


The Mechanics of Human Breathing

Figure 2. This graph shows data from Boyle’s original 1662 experiment, which shows that pressure and volume are inversely related. No units are given as Boyle used arbitrary units in his experiments.

Boyle’s Law is the gas law that states that in a closed space, pressure and volume are inversely related. As volume decreases, pressure increases and vice versa (Figure 2). The relationship between gas pressure and volume helps to explain the mechanics of breathing.

There is always a slightly negative pressure within the thoracic cavity, which aids in keeping the airways of the lungs open. During inhalation, volume increases as a result of contraction of the diaphragm, and pressure decreases (according to Boyle’s Law). This decrease of pressure in the thoracic cavity relative to the environment makes the cavity less than the atmosphere (Figure 3a). Because of this drop in pressure, air rushes into the respiratory passages. To increase the volume of the lungs, the chest wall expands. This results from the contraction of the intercostal muscles, the muscles that are connected to the rib cage. Lung volume expands because the diaphragm contracts and the intercostals muscles contract, thus expanding the thoracic cavity. This increase in the volume of the thoracic cavity lowers pressure compared to the atmosphere, so air rushes into the lungs, thus increasing its volume. The resulting increase in volume is largely attributed to an increase in alveolar space, because the bronchioles and bronchi are stiff structures that do not change in size.

Figure 3. The lungs, chest wall, and diaphragm are all involved in respiration, both (a) inhalation and (b) expiration. (credit: modification of work by Mariana Ruiz Villareal)

Figure 4. A tissue layer called pleura surrounds the lung and interior of the thoracic cavity. (credit: modification of work by NCI)

The chest wall expands out and away from the lungs. The lungs are elastic therefore, when air fills the lungs, the elastic recoil within the tissues of the lung exerts pressure back toward the interior of the lungs. These outward and inward forces compete to inflate and deflate the lung with every breath. Upon exhalation, the lungs recoil to force the air out of the lungs, and the intercostal muscles relax, returning the chest wall back to its original position (Figure 3b).

The diaphragm also relaxes and moves higher into the thoracic cavity. This increases the pressure within the thoracic cavity relative to the environment, and air rushes out of the lungs. The movement of air out of the lungs is a passive event. No muscles are contracting to expel the air.

Each lung is surrounded by an invaginated sac. The layer of tissue that covers the lung and dips into spaces is called the visceral pleura. A second layer of parietal pleura lines the interior of the thorax (Figure 4). The space between these layers, the intrapleural space, contains a small amount of fluid that protects the tissue and reduces the friction generated from rubbing the tissue layers together as the lungs contract and relax. Pleurisy results when these layers of tissue become inflamed it is painful because the inflammation increases the pressure within the thoracic cavity and reduces the volume of the lung.

Link to Learning

View how Boyle’s Law is related to breathing and watch these videos on Boyle’s Law:


What causes inhalation during breathing? - Biology

Introduction

Coughing. Fever. Shortness of breath. Hypoxia. All are symptoms of a number of pulmonary diseases, from a chronic obstructive pulmonary disease (COPD) flare to Streptococcus pneumoniae (pneumococcal) pneumonia to a type of hypersensitivity pneumonitis also known as extrinsic allergic alveolitis (EAA). This last example is a bit more esoteric, and can be brought on by hypersensitivity to anything from dried grass to rat urine to mold that grows in hot tubs&mdashwhat is sometimes called hot tub lung. Not all cases of hot tub lung are severe, but certainly none are enjoyable. They’re often misdiagnosed as asthma or bronchitis, and may be treated with steroids, which quell the immune system and reduce the inflammation associated with this illness. Because hot tub lung can potentially go away by itself, antibiotic therapy is not always recommended. As a physician, you may end up simply having to tell your patients that the best way to avoid hot tub lung is to make sure that the tub is cleaned properly and routinely before use.

The lesson here isn’t to avoid hot tubs. It’s that the lungs are essential, sensitive organs with delicate membranes that must be protected. Many types of stressors (pathogens, particles, or chemicals) can irritate them and cause respiratory distress. In this chapter, we’ll look at the structure of the lungs and the microanatomy of respiration. We’ll also talk about the mechanics of breathing as well as the overall function of the lungs.

6.1 Anatomy and Mechanism of Breathing

The lungs are located in the thoracic cavity, the structure of which is specially designed to perform breathing.

The anatomy of the respiratory system is summarized in Figure 6.1. Gas exchange occurs in the lungs. Air enters the respiratory tract through the external nares of the nose and then passes through the nasal cavity, where it is filtered through mucous membranes and nasal hairs (vibrissae).

KEY CONCEPT

The nose and mouth serve several important purposes in breathing by removing dirt and particulate matter from the air and warming and humidifying it before it reaches the lungs.

Next, air passes into the pharynx and the larynx. The pharynx resides behind the nasal cavity and at the back of the mouth it is a common pathway for both air destined for the lungs and food destined for the esophagus. In contrast, the larynx lies below the pharynx and is only a pathway for air. To keep food out of the respiratory tract, the opening of the larynx (glottis) is covered by the epiglottis during swallowing. The larynx contains two vocal cords that are maneuvered using skeletal muscle and cartilage. From the larynx, air passes into the cartilaginous trachea and then into one of the two mainstem bronchi. The bronchi and trachea contain ciliated epithelial cells to catch material that has made it past the mucous membranes in the nose and mouth.

In the lungs, the bronchi continue to divide into smaller structures known as bronchioles, which divide further until they end in the tiny balloon-like structures in which gas exchange occurs (alveoli). Each alveolus is coated with surfactant, a detergent that lowers surface tension and prevents the alveolus from collapsing on itself. A network of capillaries surrounds each alveolus to carry oxygen and carbon dioxide. The branching and minute size of the alveoli allow for an exceptionally large surface area for gas exchange&mdashapproximately 100 m 2 .

Figure 6.1. Anatomy of the Respiratory System

The left lung has a small indentation, making it slightly smaller than the right lung. It also contains only two lobes, while the right lung contains three. Why might this be? It is due to the position of the heart in the thoracic cavity.

The lungs themselves are contained in the thoracic cavity, which also contains the heart. The chest wall forms the outside of the thoracic cavity. Membranes known as pleurae surround each lung, as shown in Figure 6.2. The pleura forms a closed sac against which the lung expands. The surface adjacent to the lung is the visceral pleura, and the outer part is the parietal pleura.

Figure 6.2. Lung Membranes

The lungs do not fill passively and require skeletal muscle to generate the negative pressure for expansion. The most important of these muscles is the diaphragm, a thin, muscular structure that divides the thoracic (chest) cavity from the abdominal cavity. The diaphragm is under somatic control, even though breathing itself is under autonomic control. In addition, muscles of the chest wall, abdomen, and neck may also participate in breathing, especially when breathing is labored due to a pathologic condition.

Before we discuss breathing itself, it is worth taking a closer look at the relationship between the pleura and the lungs. Imagine that you have a large, partially deflated balloon. Now, imagine taking your fist and pushing it against the balloon so that the balloon comes up and surrounds your hand. This is analogous to a lung and its pleura. Our fist is the lung, and the balloon represents both pleural layers. The side directly touching our fist is the visceral pleura, and the outer layer is the parietal pleura, which is associated with the chest wall in real life. The space within the sac is referred to as the intrapleural space, which contains a thin layer of fluid. This pleural fluid helps lubricate the two pleural surfaces. The pressure differentials that can be created across the pleura ultimately drive breathing, as we explore in the next section.

The intrapleural space is an example of a potential space&mdasha space that is normally empty or collapsed. In some pathologic states, that potential space can be expanded by fluid or air that accumulates within the space. For example, in a pleural effusion, fluid accumulates in the intrapleural space. In a pneumothorax, air collects here. Each of these states disturbs the normal mechanics of the breathing apparatus and can cause atelectasis, or lung collapse.

Let’s turn to the mechanics of ventilation, which are grounded in physics. As discussed in Chapters 2 and 3 of MCAT Physics and Math Review, we can use pressure to do useful work in a system. Here, we use pressure differentials between the lungs and intrapleural space to drive air into the lungs.

Inhalation is an active process. We use our diaphragm as well as the external intercostal muscles (one of the layers of muscles between the ribs) to expand the thoracic cavity, as shown in Figure 6.3. As the diaphragm flattens and the chest wall expands outward, the intrathoracic volume(the volume of the chest cavity) increases. Specifically, because the intrapleural space most closely abuts the chest wall, its volume increases first. Can we predict what will happen to intrapleural pressure? From our understanding of Boyle’s law, an increase in intrapleural volume leads to a decrease in intrapleural pressure.

Boyle’s law states that the pressure and volume of gases are inversely related. This is the principle underlying negative-pressure breathing: as the chest wall expands, the pressure in the lungs drops, and air is drawn into the lungs.

Now we have low pressure in the intrapleural space. What about inside the lungs? The gas in the lungs is initially at atmospheric pressure, which is now higher than the pressure in the intrapleural space. The lungs will therefore expand into the intrapleural space, and the pressure in the lungs will drop. Air will then be sucked in from a higher-pressure environment&mdashthe outside world. This mechanism is referred to as negative-pressure breathing because the driving force is the lower (relatively negative) pressure in the intrapleural space compared with the lungs.

Figure 6.3. Stages of Ventilation The diaphragm contracts during inhalation and relaxes during exhalation.

Unlike inhalation, exhalation does not have to be an active process. Simple relaxation of the external intercostal muscles will reverse the processes we discussed in the last paragraph. As the diaphragm and external intercostals relax, the chest cavity decreases in volume. What will happen to pressure in the intrapleural space? It will go up, again explained by Boyle’s law. Now pressure in the intrapleural space is higher than in the lungs, which is still at atmospheric pressure. Thus, air will be pushed out, resulting in exhalation. During active tasks, we can speed this process up by using the internal intercostal muscles and abdominal muscles, which oppose the external intercostals and pull the rib cage down. This actively decreases the volume of the thoracic cavity. Finally, recall that surfactant prevents the complete collapse of the alveoli during exhalation by reducing surface tension at the alveolar surface.

KEY CONCEPT

Inhalation and exhalation require different amounts of energy expenditure. Muscle contraction is required to create the negative pressure in the thoracic cavity that forces air into the lungs during inspiration. Expiration during calm states is entirely due to elastic recoil of the lungs and the musculature. During more active states, the muscles can be used to force air out and speed up the process of ventilation.

Remember the balloon analogy from before. The lungs have an elastic quality and are attached via the pleurae to the chest wall. The chest wall expands on inhalation, pulling the lungs with it and creating the pressure differential required for inhalation. As the chest wall relaxes, the lungs recoil and accentuate the relaxation process. When the lungs recoil, their volume becomes smaller, and the pressure increases. Now the pressure inside the lungs is higher than the outside pressure, and exhalation occurs. Note that the indirect connection of the lungs to the chest wall also prevents them from collapsing completely on recoil, like surfactant.

Emphysema is a disease characterized by the destruction of the alveolar walls. This results in reduced elastic recoil of the lungs, making the process of exhalation extremely difficult. Most cases of emphysema are caused by cigarette smoking.

LUNG CAPACITIES AND VOLUMES

In pulmonology (the medical field associated with the lungs and breathing), we frequently must assess lung capacities and volumes&mdashbut we don’t have the luxury of removing an individual’s lungs to do so. One instrument used to measure these quantities is a spirometer. While a spirometer cannot measure the amount of air remaining in the lung after complete exhalation (residual volume), it provides a number of measures that are useful in clinical medicine.

Commonly tested lung volumes include:

·&emspTotal lung capacity (TLC): The maximum volume of air in the lungs when one inhales completely usually around 6 to 7 liters

·&emspResidual volume (RV): The minimum volume of air in the lungs when one exhales completely

·&emspVital capacity (VC): The difference between the minimum and maximum volume of air in the lungs (TLC – RV)

·&emspTidal volume (TV): The volume of air inhaled or exhaled in a normal breath

·&emspExpiratory reserve volume (ERV): The volume of additional air that can be forcibly exhaled after a normal exhalation

·&emspInspiratory reserve volume (IRV): The volume of additional air that can be forcibly inhaled after a normal inhalation

These different lung volumes and capacities can be seen in Figure 6.4.

Figure 6.4. Lung Volumes

Breathing requires input from our nervous control center. Ventilation is primarily regulated by a collection of neurons in the medulla oblongata called the ventilation center that fire rhythmically to cause regular contraction of respiratory muscles. These neurons contain chemoreceptors that are primarily sensitive to carbon dioxide concentration. As the partial pressure of carbon dioxide in the blood rises (hypercarbia or hypercapnia), the respiratory rate will increase so that more carbon dioxide is exhaled, and carbon dioxide levels in the blood will fall. These cells also respond to changes in oxygen concentration, although this tends to have significance only during periods of significant hypoxia (low oxygen concentration in the blood).

We can, to a limited extent, control our breathing through the cerebrum. We can choose to breathe more rapidly or slowly however, extended periods of hypoventilation would lead to increased carbon dioxide levels and an override by the medulla oblongata (which would jump-start breathing). The opposite process (hyperventilation) would blow off too much carbon dioxide and ultimately inhibit ventilation.

MCAT Concept Check 6.1:

Before you move on, assess your understanding of the material with these questions.

1. List the structures in the respiratory pathway, from where air enters the nares to the alveoli.

2. Which muscle(s) are involved in inhalation? Exhalation?

3. What is the purpose of surfactant?

4. What is the mathematical relationship between vital capacity (VC), inspiratory reserve volume (IRV), expiratory reserve volume (ERV), and tidal volume (TV)?

5. If blood levels of CO2 become too low, how does the brain alter the respiratory rate to maintain homeostasis?

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Human Breathing System

Structure of the human breathing system

Nasal and buccal cavities:

  • Mouth and internal areas of the nose
  • Function in warming and moistening air entering lungs
  • Mucus and small hairs filter the air and then transport the dirt-loaded mucus to the pharynx where it is swallowed

Pharynx (throat):

  • Area between oesophagus and windpipe (trachea)
  • Pharynx has a sphincter (epiglottis) that closes over the opening to the trachea (glottis) that prevents food travelling into the trachea

Glottis:

Epiglottis:

  • Sphincter that closes over the glottis to prevent food getting into the trachea during swallowing
  • Swallowing causes the vocal cords to pull on the glottis and the larynx to be pulled upwards thereby closing the epiglottis over the glottis

Larynx (voice box):

  • Made of cartilage and sits on top of the trachea
  • Three functions:
    • Produces sound
    • Controls air flowing into and out of the trachea
    • Directs food into the oesophagus

    Trachea (windpipe):

    • Directs inhaled air into the lungs
    • Contains c-shaped rings of cartilage that keeps the trachea open
    • Cilia of trachea carry dirt-laden mucus up the pharynx

    Bronchi:

    • Two divisions of the trachea
    • Directs air into each lung
    • Supported by cartilage

    Bronchioles:

    • Tiny divisions of the bronchi
    • Air passages that are less then 1 mm in diameter
    • Not supported by cartilage

    Lungs:

    • Composed of spongy, elastic tissue that expands easily during inhalation and recoils rapidly as exhalation occurs

    Pleural membranes:

    • Thin pair of membranes covering and separating the lungs from other organs, such as the heart
    • The lungs are stuck to the rib cage and diaphragm by the pleural fluid (think of a layer of water between a table and a piece of glass and how difficult it is to lift it off the table)

    Rib cage:

    • Composed of 12 thoracic vertebrae, 12 ribs, and the sternum
      • First 7 pairs are called ‘true’ ribs (because they attach directly to the sternum)
      • Next 3 pairs are called ‘false’ ribs (because they are only attached to the sternum by cartilage)
      • Final 2 pairs are called ‘floating’ ribs (because they do not attach to the sternum at all)

      Alveoli:

      • Tiny air sacs at the end of the bronchioles where gas exchange occurs
      • Walls of alveoli are only 1 cell thick to maximise diffusion
      • Each alveolus has rich blood capillary network surrounding it
      • There are

      Essential Features of Alveoli and Capillaries

      • Alveoli are numerous
      • Alveoli have rich blood capillary network nearby
      • Alveoli have walls only one-cell thick
      • Alveoli surface is moist
      • Alveoli walls are elastic
      • Capillaries that surround each alveolus have walls that are only one-cell thick

      Respiratory muscles

      The lungs have no skeletal muscles of their own. The work of breathing is done by the diaphragm, the muscles between the ribs (intercostal muscles), the muscles in the neck, and the abdominal muscles.

      The diaphragm, a dome-shaped sheet of muscle that separates the chest cavity from the abdomen, is the most important muscle used for breathing in (called inhalation or inspiration). The diaphragm is attached to the base of the sternum, the lower parts of the rib cage, and the spine. As the diaphragm contracts, it increases the length and diameter of the chest cavity and thus expands the lungs. The intercostal muscles help move the rib cage and thus assist in breathing.

      The process of breathing out (called exhalation or expiration) is usually passive when a person is not exercising. The elasticity of the lungs and chest wall, which are actively stretched during inhalation, causes them to return to their resting shape and to expel air out of the lungs when inspiratory muscles are relaxed. Therefore, when a person is at rest, no effort is needed to breathe out. During vigorous exercise, however, a number of muscles participate in exhalation. The abdominal muscles are the most important of these. Abdominal muscles contract, raise abdominal pressure, and push a relaxed diaphragm against the lungs, causing air to be pushed out.

      The muscles used in breathing can contract only if the nerves connecting them to the brain are intact. In some neck and back injuries, the spinal cord can be severed, which breaks the nervous system connection between the brain and the muscles, and the person will die unless artificially ventilated.

      Diaphragm’s Role in Breathing

      When the diaphragm contracts and moves lower, the chest cavity enlarges, reducing the pressure inside the lungs. To equalize the pressure, air enters the lungs. When the diaphragm relaxes and moves back up, the elasticity of the lungs and chest wall pushes air out of the lungs.


      Inhalation of air, as part of the cycle of breathing, is a vital process for all human life. The process is autonomic (though there are exceptions in some disease states) and does not need conscious control or effort. However, breathing can be consciously controlled or interrupted (within limits).

      Breathing allows oxygen (which humans and a lot of other species need for survival) to enter the lungs, from where it can be absorbed into the bloodstream.

      Examples of accidental inhalation includes inhalation of water (e.g. in drowning), smoke, food, vomitus and less common foreign substances [1] (e.g. tooth fragments, coins, batteries, small toy parts, needles).

      Recreational use Edit

      Legal – helium, nitrous oxide ("laughing gas")

      Illegal – various gaseous, vaporised or aerosolized recreational drugs

      Medical use Edit

      Diagnostic Edit

      Various specialized investigations use the inhalation of known substances for diagnostic purposes. Examples include pulmonary function testing (e.g. nitrogen washout test, diffusion capacity testing (carbon monoxide, helium, methane)) and diagnostic radiology (e.g. radioactive xenon isotopes).

      Therapeutic Edit

      Gases and other drugs used in anaesthesia include oxygen, nitrous oxide, helium, xenon, volatile anaesthetic agents. Medication for asthma, croup, cystic fibrosis and some other conditions.

      Inhalation begins with the contraction of the muscles attached to the rib cage this causes an expansion in the chest cavity. Then takes place the onset of contraction of the diaphragm, which results in expansion of the intrapleural space and an increase in negative pressure according to Boyle's law. This negative pressure generates airflow because of the pressure difference between the atmosphere and alveolus.

      The inflow of air into the lungs occurs via the respiratory airways. In health, these airways begin with the nose. [2] [3] It is possible to begin with the mouth, which is the backup breathing system. However, chronic mouth breathing leads to, or is a sign of, illness. [4] [5] [6] They end in the microscopic dead-end sacs called alveoli) are always open, though the diameters of the various sections can be changed by the sympathetic and parasympathetic nervous systems. The alveolar air pressure is therefore always close to atmospheric air pressure (about 100 kPa at sea level) at rest, with the pressure gradients that cause air to move in and out of the lungs during breathing rarely exceeding 2–3 kPa. [7] [8]

      Other muscles that can be involved in inhalation include: [9]

      Hyperinflation or hyperaeration is where the lung volume is abnormally increased, with increased filling of the alveoli. This results in an increased radiolucency on X-ray, a reduction in lung markings and depression of the diaphragm. It may occur in partial obstruction of a large airway, as in e.g. congenital lobar emphysema, bronchial atresia and mucous plugs in asthma. [10]

      Yogis such as B. K. S. Iyengar advocate both inhaling and exhaling through the nose in the practice of yoga, rather than inhaling through the nose and exhaling through the mouth. [11] [12] [13] They tell their students that the "nose is for breathing, the mouth is for eating." [12] [14] [15] [11]


      30.3 Breathing

      In this section, you will explore the following questions:

      • How do the structure of the lungs and thoracic cavity control the mechanics of breathing?
      • What is the importance of compliance and resistance in the lungs?

      Connection for AP ® Courses

      The information in this section is not within the scope for AP ® and does not align to the Curriculum Framework. However, understanding how breathing occurs enhances ones understanding of the structure and function of the respiratory system.

      Mammalian lungs are located in the thoracic cavity where they are surrounded and protected by the rib cage, intercostal muscles, and bound by the chest wall. The bottom of the lungs is contained by the diaphragm, a skeletal muscle that facilitates breathing. Breathing requires the coordination of the lungs, the chest wall, and most importantly, the diaphragm.

      Types of Breathing

      Amphibians have evolved multiple ways of breathing. Young amphibians, like tadpoles, use gills to breathe, and they don’t leave the water. Some amphibians retain gills for life. As the tadpole grows, the gills disappear and lungs grow. These lungs are primitive and not as evolved as mammalian lungs. Adult amphibians are lacking or have a reduced diaphragm, so breathing via lungs is forced. The other means of breathing for amphibians is diffusion across the skin. To aid this diffusion, amphibian skin must remain moist.

      Birds face a unique challenge with respect to breathing: They fly. Flying consumes a great amount of energy therefore, birds require a lot of oxygen to aid their metabolic processes. Birds have evolved a respiratory system that supplies them with the oxygen needed to enable flying. Similar to mammals, birds have lungs, which are organs specialized for gas exchange. Oxygenated air, taken in during inhalation, diffuses across the surface of the lungs into the bloodstream, and carbon dioxide diffuses from the blood into the lungs and expelled during exhalation. The details of breathing between birds and mammals differ substantially.

      In addition to lungs, birds have air sacs inside their body. Air flows in one direction from the posterior air sacs to the lungs and out of the anterior air sacs. The flow of air is in the opposite direction from blood flow, and gas exchange takes place much more efficiently. This type of breathing enables birds to obtain the requisite oxygen, even at higher altitudes where the oxygen concentration is low. This directionality of airflow requires two cycles of air intake and exhalation to completely get the air out of the lungs.

      Evolution Connection

      Avian Respiration

      Birds have evolved a respiratory system that enables them to fly. Flying is a high-energy process and requires a lot of oxygen. Furthermore, many birds fly in high altitudes where the concentration of oxygen in low. How did birds evolve a respiratory system that is so unique?

      Decades of research by paleontologists have shown that birds evolved from therapods, meat-eating dinosaurs (Figure 30.14). In fact, fossil evidence shows that meat-eating dinosaurs that lived more than 100 million years ago had a similar flow-through respiratory system with lungs and air sacs. Archaeopteryx and Xiaotingia, for example, were flying dinosaurs and are believed to be early precursors of birds.

      Most of us consider that dinosaurs are extinct. However, modern birds are descendants of avian dinosaurs. The respiratory system of modern birds has been evolving for hundreds of millions of years.

      1. The “flow-through respiratory system” allows air to move only entirely in one direction, without retracing its pathway at all, allowing the animal to breathe faster while in flight.
      2. The “flow-through respiratory system” contains several air sacs that make the animal lighter compared to similar-sized animals with no air sacs.
      3. The “flow-through respiratory system” contains many lungs, which allow the animals to take in the large quantities of oxygen needed for flight.
      4. The “flow-through respiratory system” allows air to move in an almost unidirectional pathway to reach a series of air sacs that provide air to hollow bones, making the animals lighter and providing efficient respiration.

      All mammals have lungs that are the main organs for breathing. Lung capacity has evolved to support the animal’s activities. During inhalation, the lungs expand with air, and oxygen diffuses across the lung’s surface and enters the bloodstream. During exhalation, the lungs expel air and lung volume decreases. In the next few sections, the process of human breathing will be explained.

      The Mechanics of Human Breathing

      Boyle’s Law is the gas law that states that in a closed space, pressure and volume are inversely related. As volume decreases, pressure increases and vice versa (Figure 30.15). The relationship between gas pressure and volume helps to explain the mechanics of breathing.

      There is always a slightly negative pressure within the thoracic cavity, which aids in keeping the airways of the lungs open. During inhalation, volume increases as a result of contraction of the diaphragm, and pressure decreases (according to Boyle’s Law). This decrease of pressure in the thoracic cavity relative to the environment makes the cavity less than the atmosphere (Figure 30.16a). Because of this drop in pressure, air rushes into the respiratory passages. To increase the volume of the lungs, the chest wall expands. This results from the contraction of the intercostal muscles, the muscles that are connected to the rib cage. Lung volume expands because the diaphragm contracts and the intercostals muscles contract, thus expanding the thoracic cavity. This increase in the volume of the thoracic cavity lowers pressure compared to the atmosphere, so air rushes into the lungs, thus increasing its volume. The resulting increase in volume is largely attributed to an increase in alveolar space, because the bronchioles and bronchi are stiff structures that do not change in size.

      The chest wall expands out and away from the lungs. The lungs are elastic therefore, when air fills the lungs, the elastic recoil within the tissues of the lung exerts pressure back toward the interior of the lungs. These outward and inward forces compete to inflate and deflate the lung with every breath. Upon exhalation, the lungs recoil to force the air out of the lungs, and the intercostal muscles relax, returning the chest wall back to its original position (Figure 30.16b). The diaphragm also relaxes and moves higher into the thoracic cavity. This increases the pressure within the thoracic cavity relative to the environment, and air rushes out of the lungs. The movement of air out of the lungs is a passive event. No muscles are contracting to expel the air.

      Each lung is surrounded by an invaginated sac. The layer of tissue that covers the lung and dips into spaces is called the visceral pleura. A second layer of parietal pleura lines the interior of the thorax (Figure 30.17). The space between these layers, the intrapleural space, contains a small amount of fluid that protects the tissue and reduces the friction generated from rubbing the tissue layers together as the lungs contract and relax. Pleurisy results when these layers of tissue become inflamed it is painful because the inflammation increases the pressure within the thoracic cavity and reduces the volume of the lung.

      Link to Learning

      View how Boyle’s Law is related to breathing and watch this video on Boyle’s Law.

      1. Boyle’s law states that the pressure of a gas is directly proportional to its volume. Therefore, the lungs control the volume of air by altering the air pressure of the thoracic cavity.
      2. Boyle’s law states that the pressure of a gas is inversely proportional to its volume. Therefore, the lungs control air pressure by altering the volume of the thoracic cavity.
      3. Boyle’s law states that the pressure of a gas is inversely proportional to its volume. Therefore, the lungs control air pressure by expanding and pushing out the muscles of the thoracic cavity.
      4. Boyle’s law states that the pressure of a gas is directly proportional to its volume. Therefore, the lungs control air pressure by controlling the volume of the thoracic cavity, not vice versa.

      The Work of Breathing

      The number of breaths per minute is the respiratory rate. On average, under non-exertion conditions, the human respiratory rate is 12–15 breaths/minute. The respiratory rate contributes to the alveolar ventilation, or how much air moves into and out of the alveoli. Alveolar ventilation prevents carbon dioxide buildup in the alveoli. There are two ways to keep the alveolar ventilation constant: increase the respiratory rate while decreasing the tidal volume of air per breath (shallow breathing), or decrease the respiratory rate while increasing the tidal volume per breath. In either case, the ventilation remains the same, but the work done and type of work needed are quite different. Both tidal volume and respiratory rate are closely regulated when oxygen demand increases.

      There are two types of work conducted during respiration, flow-resistive and elastic work. Flow-resistive refers to the work of the alveoli and tissues in the lung, whereas elastic work refers to the work of the intercostal muscles, chest wall, and diaphragm. Increasing the respiration rate increases the flow-resistive work of the airways and decreases the elastic work of the muscles. Decreasing the respiratory rate reverses the type of work required.

      Surfactant

      The air-tissue/water interface of the alveoli has a high surface tension. This surface tension is similar to the surface tension of water at the liquid-air interface of a water droplet that results in the bonding of the water molecules together. Surfactant is a complex mixture of phospholipids and lipoproteins that works to reduce the surface tension that exists between the alveoli tissue and the air found within the alveoli. By lowering the surface tension of the alveolar fluid, it reduces the tendency of alveoli to collapse.

      Surfactant works like a detergent to reduce the surface tension and allows for easier inflation of the airways. When a balloon is first inflated, it takes a large amount of effort to stretch the plastic and start to inflate the balloon. If a little bit of detergent was applied to the interior of the balloon, then the amount of effort or work needed to begin to inflate the balloon would decrease, and it would become much easier to start blowing up the balloon. This same principle applies to the airways. A small amount of surfactant to the airway tissues reduces the effort or work needed to inflate those airways. Babies born prematurely sometimes do not produce enough surfactant. As a result, they suffer from respiratory distress syndrome, because it requires more effort to inflate their lungs. Surfactant is also important for preventing collapse of small alveoli relative to large alveoli.

      Lung Resistance and Compliance

      Pulmonary diseases reduce the rate of gas exchange into and out of the lungs. Two main causes of decreased gas exchange are compliance (how elastic the lung is) and resistance (how much obstruction exists in the airways). A change in either can dramatically alter breathing and the ability to take in oxygen and release carbon dioxide.

      Examples of restrictive diseases are respiratory distress syndrome and pulmonary fibrosis. In both diseases, the airways are less compliant and they are stiff or fibrotic. There is a decrease in compliance because the lung tissue cannot bend and move. In these types of restrictive diseases, the intrapleural pressure is more positive and the airways collapse upon exhalation, which traps air in the lungs. Forced or functional vital capacity (FVC), which is the amount of air that can be forcibly exhaled after taking the deepest breath possible, is much lower than in normal patients, and the time it takes to exhale most of the air is greatly prolonged (Figure 30.18). A patient suffering from these diseases cannot exhale the normal amount of air.

      Obstructive diseases and conditions include emphysema, asthma, and pulmonary edema. In emphysema, which mostly arises from smoking tobacco, the walls of the alveoli are destroyed, decreasing the surface area for gas exchange. The overall compliance of the lungs is increased, because as the alveolar walls are damaged, lung elastic recoil decreases due to a loss of elastic fibers, and more air is trapped in the lungs at the end of exhalation. Asthma is a disease in which inflammation is triggered by environmental factors. Inflammation obstructs the airways. The obstruction may be due to edema (fluid accumulation), smooth muscle spasms in the walls of the bronchioles, increased mucus secretion, damage to the epithelia of the airways, or a combination of these events. Those with asthma or edema experience increased occlusion from increased inflammation of the airways. This tends to block the airways, preventing the proper movement of gases (Figure 30.18). Those with obstructive diseases have large volumes of air trapped after exhalation and breathe at a very high lung volume to compensate for the lack of airway recruitment.

      Dead Space: V/Q Mismatch

      Pulmonary circulation pressure is very low compared to that of the systemic circulation. It is also independent of cardiac output. This is because of a phenomenon called recruitment, which is the process of opening airways that normally remain closed when cardiac output increases. As cardiac output increases, the number of capillaries and arteries that are perfused (filled with blood) increases. These capillaries and arteries are not always in use but are ready if needed. At times, however, there is a mismatch between the amount of air (ventilation, V) and the amount of blood (perfusion, Q) in the lungs. This is referred to as ventilation/perfusion (V/Q) mismatch.

      There are two types of V/Q mismatch. Both produce dead space, regions of broken down or blocked lung tissue. Dead spaces can severely impact breathing, because they reduce the surface area available for gas diffusion. As a result, the amount of oxygen in the blood decreases, whereas the carbon dioxide level increases. Dead space is created when no ventilation and/or perfusion takes place. Anatomical dead space or anatomical shunt, arises from an anatomical failure, while physiological dead space or physiological shunt, arises from a functional impairment of the lung or arteries.

      An example of an anatomical shunt is the effect of gravity on the lungs. The lung is particularly susceptible to changes in the magnitude and direction of gravitational forces. When someone is standing or sitting upright, the pleural pressure gradient leads to increased ventilation further down in the lung. As a result, the intrapleural pressure is more negative at the base of the lung than at the top, and more air fills the bottom of the lung than the top. Likewise, it takes less energy to pump blood to the bottom of the lung than to the top when in a prone position. Perfusion of the lung is not uniform while standing or sitting. This is a result of hydrostatic forces combined with the effect of airway pressure. An anatomical shunt develops because the ventilation of the airways does not match the perfusion of the arteries surrounding those airways. As a result, the rate of gas exchange is reduced. Note that this does not occur when lying down, because in this position, gravity does not preferentially pull the bottom of the lung down.

      A physiological shunt can develop if there is infection or edema in the lung that obstructs an area. This will decrease ventilation but not affect perfusion therefore, the V/Q ratio changes and gas exchange is affected.

      The lung can compensate for these mismatches in ventilation and perfusion. If ventilation is greater than perfusion, the arterioles dilate and the bronchioles constrict. This increases perfusion and reduces ventilation. Likewise, if ventilation is less than perfusion, the arterioles constrict and the bronchioles dilate to correct the imbalance.


      Feature: Myth vs. Reality

      Drowning is defined as respiratory impairment from being in or under a liquid. It is further classified according to its outcome into: death, ongoing health problems, or no ongoing health problems (full recovery). Four hundred Canadians die annually from drowning, and drowning is one of the leading causes of death in children under the age of five. There are some potentially dangerous myths about drowning, and knowing what they are might save your life or the life of a loved one, especially a child.


      Watch the video: Mechanism of Breathing, Animation (December 2022).