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Gas Exchange for EMS Providers

Rusty Gilpin BT, NRP

Introduction

Trillions of cellular metabolic processes happen in the human body every second of every day. Most of these cellular functions require oxygen to effectively and efficiently complete these processes. In most cases, this oxygen consumption results in the creation of carbon dioxide, a potentially dangerous byproduct of cellular metabolism. This large-scale use of oxygen, and the resulting creation of carbon dioxide, requires a highly efficient absorption/elimination and transport system. The body has two routes for the elimination of carbon dioxide: the lungs and the kidneys (the lungs being the primary and far the most efficient route).1 However, the lungs are only one route available for the absorption of oxygen, and the blood (or cardiovascular system) is the only method available for its transportation. Because of this, the lungs must maintain normal function. To do so, they need to work in conjunction with the cardiovascular system and interact with the environment in a highly effective manner. This will allow them to proficiently facilitate the exchange of these vital and dangerous gases.

Gas exchange is the process of absorbing inhaled atmospheric oxygen molecules into the bloodstream and offloading carbon dioxide from the bloodstream into the atmosphere.2 This process is completed in the lungs through the diffusion of gases from areas of high concentration to areas of low concentration. The process also requires that oxygen move from its gaseous environment into a liquid environment and carbon dioxide move from a liquid environment into a gaseous environment. This article will explore the gas exchange process by taking a closer look at the molecular content of atmospheric air, how lungs interact with their environment, the gas carrying capacity of red blood cells, and conditions that can hamper the exchange process.

Gases and Pressures

Atmospheric air generally contains 78% nitrogen, 21% oxygen, and a 1% of a mixture of the following gases:

  • Argon,
  • Carbon dioxide,
  • Neon,
  • Helium,
  • Methane,
  • Krypton,
  • Hydrogen,
  • Nitrous oxide,
  • Xenon,
  • Ozone,
  • Sulfur dioxide, and
  • Nitrogen dioxide.3

Although this ratio mixture remains constant regardless of altitude, this does not mean that the same number of molecules is present in a cubed meter of air at different altitudes. Every molecule of air in the earth’s atmospheric has its own individual weight. The cumulative weight of all of these molecules pressing downward, due to the earth’s gravity, creates what is known as atmospheric pressure.3

Atmospheric air pressures change as altitude changes because of the relative combined molecular weight. At sea level, atmospheric pressure is 14.7 PSI (760 mmHg/Torr or 1 ATM). At 10,000 feet above sea level, atmospheric pressure is 10 PSI (517 mmHg/Torr or 0.68 ATM).4 The difference is due to the fact that there are fewer molecules at 10,000 feet pushing down than there are at sea level. This decreases their combined weight and reduces the pressure that forces the molecules together. As the atmospheric pressure decreases, air molecules spread out and the air becomes thinner (e.g., lower molecule content per cubed meter). This is the basis of Boyle’s Law, which states that volume and pressure are inversely related.5

Every molecule of a given mixture of gas is responsible for the mixture’s overall pressure. Additionally, according to Dalton’s law of partial pressures, each type of gas is partially responsible for the mixture’s overall pressure.3 This means that the percentages of nitrogen (78%), oxygen (21%), and the mix of gases (1%) all partially contribute to the 14.7 PSI (760 mmHg/Torr or 1 ATM) of air pressure at sea level, the 10 PSI (517 mmHg/Torr or 0.68 ATM) at 10,000 feet above sea level and so on. If the percentage concentration of any gas is increased, its partial pressure will increase and vice versa.

Differences in gas pressures, partial or otherwise, create diffusion gradients that facilitate the movement of gas molecules from areas of high concentration to areas of low concentration. This is true even if the concentration gradient involves a gas and a liquid, such as blood. Henry’s law states that the amount of gas that dissolves into a liquid is directly proportional to the partial pressure of that gas.3 It also states that when a liquid is exposed to a gas mixture that does not contain the same gas concentration, a pressure gradient is created by the difference in partial pressures. The partial pressure difference will cause molecules in the air to diffuse into the liquid and unbound gas molecules in the liquid to diffuse into the air. Henry’s law helps explain how oxygen from atmospheric air enters the blood and carbon dioxide leaves the blood into the air.

Just because a gas can dissolve into a liquid solution, this does not mean that it will bond and become part of the liquid matrix. Oxygen does not readily dissolve in blood. Therefore, the body relies on an alternate transport system that uses hemoglobin to transport oxygen. Carbon dioxide, on the other hand, will most often bond with a water molecule to form carbonic acid, which can, depending on the body’s needs, then be converted into bicarbonate, freeing a hydrogen atom. The presence or absence of carbon dioxide can affect the amount of free hydrogen molecules in the body. Because of this, a balance between carbonic acid and bicarbonate must be maintained to sustain normal blood and tissue pH levels. The efficient removal of carbon dioxide by the lungs is essential to the maintenance of this balance.

Anatomy of the Lungs

Air enters the lungs and travels through progressively narrowing passages to the alveolus, where gas exchange between the body and the atmosphere takes place. With an average thickness of 0.5 micrometers (0.1 micrometers in some areas) the alveolar wall separates the air in the lungs from the pulmonary capillaries by only a few layers of cells, allowing for the rapid movement of oxygen and carbon dioxide molecules in and out of the bloodstream.6

Alveoli are hollow spherical shaped structures that are clustered in bundles resembling grapes on the vine. Their shape provides a greater surface area for atmospheric air to come into contact with pulmonary capillaries and facilitate gas exchange. The size of an individual alveolus is somewhat consistent, therefore how many alveolus a lung contains depends on the lung’s overall size.7 One cubic millimeter of alveolar tissue contains about 170 individual alveoli, and an average pair of lungs contain about 480 million alveoli.5 This large number of alveoli creates a massive surface area for gas exchange that is around 35 times larger than the surface area of the skin.6

The amount of air that enters and leaves the lungs in one breath is referred to as tidal volume (Vt), which includes air that is unusable by alveoli (air in the dead space) because it remains in the areas of the lung that do not contain alveoli (dead space) and is therefore not useable for gas exchange. A healthy adult has a tidal volume of anywhere from 5 to 7 mL/kg (average of about 500 mL), which includes around 150 mL of air in the dead space (VD).8

A healthy adult has a tidal volume of anywhere from 5 to 7 mL/kg.

Table 1: Tidal Volume by Patient Weight and Gender, by jenberry

Over the course of one minute, the volume of air entering and leaving the lungs is referred to minute volume (Mv), and the volume of air that reaches the alveoli is referred to as alveolar ventilation (VA). Alveolar ventilation can be calculated by subtracting the are in the dead space from the tidal volume and multiplying the result by the number of breaths per minute (R), (Vt – VD) x R = VA. A person breathing 12 times per minute with an average tidal volume of 500 mL and 150 mL of air in the dead space would have 4,200 mL of air reaching the alveolar membranes every minute (500 mL – 150 mL = 350 mL tidal volume, 350 mL x 12 = 4,200 mL minute volume).

Any medical condition, disease or injury of the lungs, brain, chest, abdomen, cardiovascular system, or blood cells can dramatically affect the body’s ability to absorb a sufficient supply of oxygen and eliminate harmful carbon dioxide.

Physiology of Breathing

The physiology of breathing happens in two phases: 1) mechanical (ventilations), and 2) cellular (respirations). The process of mechanical ventilation is regulated by the brain to physically move air in and out of the lungs so that oxygen and carbon dioxide can be exchanged with atmospheric air.9 The movement of air is the result of positive and negative pressure differences created within the thoracic cavity. During inhalation, the diaphragm contracts in a downward motion and the intercostal muscles contract pulling the ribs outward which causes the cavity containing the lungs to expand and enlarge.10 This movement creates a negative pressure environment within the thoracic cavity that draws air into the lungs. The inhalation process is considered an active process because it requires muscle contraction to move the diaphragm and ribs to create negative thoracic pressures.

Exhalation, on the other hand, is generally a passive process. In healthy persons when active contraction of the diaphragm and intercostal muscles ceases, the diaphragm and ribs move back to their relaxed positions.10 This passive process creates positive pressure inside the thoracic cavity by reducing the internal thoracic volume. The positive pressure that is created pushes air out of the lungs. This process is typically passive but can be active by recruiting accessory respiratory muscles when necessary, for instance when coughing, blowing out candles, or blowing up a balloon.

At the cellular level, respirations occur as a part of a process or a cycle referred to as the citric acid cycle, which is also known as the Kreb’s cycle. During the citric acid cycle, a series of reactions consume glucose, oxygen, and several other metabolic components to create 30 adenosine triphosphate (ATP) molecules.10 These ATP molecules are then used within the cell as a source of energy for various cellular activities. Only cells containing mitochondria are capable of creating ATP because the citric acid cycle occurs exclusively within cellular mitochondria.10 Mitochondria-containing cells are capable of running thousands of citric acid cycles simultaneously, resulting in the production of tens of thousands of ATP molecules.

Although the citric acid cycle produces essential ATP, it also produces carbon dioxide, a potentially harmful byproduct. The process of converting one glucose molecule into 30 ATP molecules also produces six carbon dioxide molecules.11 Excess carbon dioxide in the presence of water will form carbonic acid, a weak acid capable of adversely affecting tissue pH levels.8 Cellular respiration requires both delivery of large quantities of oxygen and the removal of large quantities of carbon dioxide.

Red Blood Cells

Red blood cells are durable unbound cells designed to carry oxygen and, to a lesser degree, carbon dioxide to and from body tissues while withstanding the forces of bouncing off the walls of the vascular system, collisions with other cells and the high pressure forces of the capillary networks. Externally, their round shape combined with a relatively thick edge and a thinner center increases their surface area while still allowing for them to move freely through the vascular network. Internally, they are predominantly comprised of antioxidant enzymes and structural proteins that protect and support the cell. However, they lack a nucleus and contain only a few organelles, which prevents them from dividing or repairing themselves.13 The remaining third of the cell contains hemoglobin, an iron, and amino acid-containing protein capable of binding with oxygen and carbon dioxide.6,12 The majority of oxygen, which is transported in the blood, is bound to the iron portion of the hemoglobin, while only about 20% of carbon dioxide is transported in this way.12 The remaining carbon dioxide is transported bound in easily reversible molecules of bicarbonate or carbonic acid.14

The average adult has about 25 trillion circulating red blood cells. Each red blood cell contains about 280 million hemoglobin molecules and each hemoglobin molecule can carry up to four oxygen molecules.6 With room for more than 1 billion oxygen molecules per cell, the total circulating oxygen-carrying capacity of the average adult is roughly 2.8 septillion (2.8 x 1024) molecules. This massive oxygen-carrying ability of red blood cells helps maintain some reserve oxygen capacity for the body. As the red blood cells pass through the capillaries at the tissue level, they off-load 25–35% of the oxygen they carry, leaving 65–75% of the oxygen in reserve.15 The brain further uses this reserve capacity by regulating the cerebral vessels to create a cerebral blood/oxygen reserve.16 In other places, myoglobin, a cellular protein similar to hemoglobin, is the primary source of oxygen reserves, myoglobin is predominantly found in cardiac and skeletal muscles.17 These reserves provide the body the necessary oxygen needed to rapidly respond to stressful events.

Cardiovascular System

The cardiovascular system is specially designed to move blood throughout the body. This system consists of two primary components, the heart (pump) and the vessels (arteries, veins and capillaries). The heart, which weighs 250–300 grams, is a relatively small organ with a large and unending job.8 This four-chambered hollow organ is the sole means of moving blood throughout the entire body. The amount of blood pumped out of the heart in one beat is called stroke volume (SV), which is on average 60–100 mL. The amount of blood pumped by the heart in one minute is called cardiac output (CO). Cardiac output can be calculated with the following formula: CO = SV x HR).8 Over the course of an average day, the heart will circulate between 7,000–9,000 L of blood.8

The transport system for blood throughout the body is the vascular system, which is divided into two circuits: the pulmonary and systemic circuits.8 The pulmonary circuit circulates blood to and from the lungs via the right side of the heart. The systemic circuit circulates blood to the entire body via the left side of the heart. The right side of the heart (right atrium and right ventricle) receives oxygen-depleted, carbon-dioxide rich blood from the body via the superior and inferior venae cavae. From there, blood passes through the right atrium into the right ventricle, where it is then pumped into the pulmonary circuit via the pulmonary artery. Once the blood is in the pulmonary arteries, it is pushed through the capillaries surrounding the alveoli of the lungs and collected in the pulmonary veins. The pulmonary veins return the oxygen-rich blood, which has been depleted of carbon dioxide, to the left side of the heart.

The left side of the heart (left atrium and left ventricle) receives the freshly oxygenated blood from the pulmonary circuit. The blood passes through the left atrium into the left ventricle, where it is pumped to the body via the ascending and descending aorta. Blood is pushed through the arterial network and into the capillaries at the tissue level and collected in the veins. The venous system collects the oxygen-depleted, carbon-dioxide rich blood from the body via the superior and inferior venae cavae, and the cycle repeats.

Under normal conditions, blood pressure in the pulmonary circuit is much lower than the blood pressure in the systemic circuit.21 This difference results from the lower vascular resistance attributed to the much smaller size of the pulmonary circuit.21 The dramatic difference in vascular resistance also helps explain the significant differences in the muscle thicknesses of the left and right sides of the heart.

The cardiovascular system and the lungs play equally vital roles in the gas exchange process. An inefficiency of one system will compromise both. The coloration between the amount of air reaching the alveoli and the amount of blood reaching the capillaries surrounding the alveoli can be measured using what is known as the ventilation-perfusion or V/Q ratio (V/Q = Mv / CO).22 This number is important in determining the combined efficiency of the cardiovascular system and the lungs. A ventilation-perfusion ratio of 0.8 (4 L minute volume /5 L cardiac output) would typically yield normal blood gas levels.19 A decrease in V/Q ratio, whether caused by a decrease Mv or increased CO, will result in a buildup of carbon dioxide levels.

Gas Exchange Process

The purpose of mechanical ventilation is to bring oxygen molecules into contact with alveolar capillaries of the lungs. The functionality of the lungs completely depends on the number of oxygen molecules that not only reach the alveolus but are able to pass through the alveolar membrane and reach the hemoglobin of the red blood cells. At sea level, a cubic meter of atmospheric air (21% oxygen concentration) contains approximately 5.2 septillion oxygen molecules (5.2 x 1024), roughly two times the oxygen-carrying capacity of all the body’s hemoglobin.4

Atmospheric air contains 21% oxygen, which means that the pressure that oxygen alone exerts at sea level is slightly more than three PSI (155 mmHg) [14.7 PSI x 21%]. Conversely, atmospheric air contains only 0.04% carbon dioxide, which exerts a paltry 0.006 PSI (0.31 mmHg/Torr or 0.0004 ATM) [14.7 PSI x 0.04%].3,6 According to Henry’s law, as long as there is a pressure difference, gases will diffuse through air or liquid in an effort to balance out the partial pressures of each medium. Because the body is 70% water, even at the relatively low concentration of 21%, the alveolus and surrounding tissues will become saturated with oxygen when they come in contact with atmospheric air. However, carbon dioxide will readily leave the tissues because the atmospheric concentrations are so low. At sea level (21% oxygen concentration), approximately 2.6 sextillion (2.6 x 1021) oxygen molecules are inhaled with each breath and 1.9 sextillion (1.9 x 1021) are exhaled (15.3% oxygen concentration).6 The lung tissues and blood vessels absorb roughly 7 quintillion (7 x 1020) oxygen molecules (5.7% of the oxygen inhaled) or 0.025% of the total oxygen-carrying capacity of the blood with each breath. Of course, this absorption/elimination rate depends greatly on bodily metabolic demands, and such activities as exercise or sleeping will have a significant effect on the exchange rates.

Changing the atmospheric concentrations of either oxygen or carbon dioxide will affect their partial pressures and thus their absorption and elimination ratios. If oxygen levels are low, the partial pressure of oxygen will be low and less oxygen will diffuse into the lung tissues. If carbon dioxide levels are high, the partial pressure of carbon dioxide will be high, causing less carbon dioxide to diffuse out of the lung tissues.

The oxygen cascade

Figure 1: Transfer of Gases, by jenberry

Even at normal concentrations (21%), oxygen will saturate deep into the lung tissues through the alveolar and capillary membranes, and connective tissue and into the plasma of the blood. As the blood plasma becomes saturated with oxygen the hemoglobin of the red blood cells will begin to bind with the excess oxygen molecules. Hemoglobin’s affinity for oxygen will increase as more and more oxygen molecules become bound until a saturation point is reached.23 The partial pressure of oxygen needs to be at or above 1.16 PSI or 60 mmHg to maintain this saturation. Once the partial pressure of oxygen drops below this level, oxygen molecules will begin to offload from the hemoglobin. This is what is known as the oxyhemoglobin dissociation curve. So when the red blood cells reach the capillaries of the body tissues, which contain a lower partial pressure of oxygen and higher partial pressure of carbon dioxide, the oxygen will leave the plasma and enter the tissues through the capillary walls. This decreases the oxygen plasma concentrations and the partial pressure of oxygen, causing oxygen to unbind from the hemoglobin of the red blood cells and enter the plasma. As carbon dioxide is released by the cells metabolizing oxygen, much of it is quickly bound to either a water molecule as carbonic acid or a hydrogen molecule and an oxygen molecule as bicarbonate. Some carbon dioxide molecules, however, stay unbound. This creates a higher partial pressure of carbon dioxide in the tissues, which diffuses into the blood plasma and then binds to the hemoglobin of red blood cells.

When the red blood cells return to the capillaries of the lungs, the partial pressure of carbon dioxide in the blood plasma is higher than that of the capillaries and surrounding tissues. This pressure gradient causes carbon dioxide to diffuse out of the plasma, off the hemoglobin, through the capillary and alveolar walls and into the air space of the lungs to be exhaled. Additionally, some of the carbonic acid and bicarbonate will disassociate, freeing carbon dioxide molecules into the plasma and then the tissues of the lungs to be exhaled as well. At the same time, the high partial pressure of oxygen in the alveolar tissues causes oxygen to diffuse back on to the hemoglobin of the red blood cells, and the cycle repeats.

At sea level, higher percentages of oxygen (above 21%) create higher partial pressures, which improves hemoglobin oxygen loading by creating a much steeper concentration gradient and a higher diffusion rate. However, attempting to increase the pressure of oxygen by forcing a higher volume of air into the lungs will not necessarily increase the net movement of oxygen if the concentration of the forced air stays the same. Pressure changes because of altitude can dramatically affect oxygen diffusion rates. At sea level, the partial pressure of oxygen is a little more than 3 PSI (155 mmHg) [14.7 PSI x 0.21%]. However, at 10,000 feet elevation, partial oxygen pressure drops to 2.1 PSI (108 mmHg).4 The percentage of atmospheric oxygen remains the same at 21%, but the partial pressure exerted by oxygen will continue to drop as altitude increases. This drop in pressure will reduce the oxygen molecule content of the air and reduce the oxygen diffusion rate. At 13,435 feet, the molecular oxygen content of a cubed meter of atmospheric air is nearly half that of the air at sea level (even though the percentage of oxygen is still 21%). At 18,044 feet, the molecular oxygen content is so low that humans cannot survive without supplemental oxygen.24

Conversely, scuba diving increases atmospheric pressure by adding the weight of water to the equation. Because water weighs considerably more than air, 33 feet of water depth is the equivalent of one atmosphere, or 14.7 PSI (760 mmHg).25 Therefore, Boyle’s law tells us that a volume of air in the lungs will decrease in size by half at a water depth of 33 feet. However, its pressure will double to 29.4 PSI. The increases in partial the pressures of oxygen and carbon dioxide rarely cause issues because they are metabolized in the body.5 However, partial oxygen pressures at or above 20.58 PSI (1478 mmHg) [6.86 times higher than the partial oxygen pressure at sea level or a water depth of 193 feet are capable of producing acute neurotoxicity.5 Nitrogen, on the other hand, accounts for most of the dissolved gas in the body at water depth. This is because it not only accounts for a larger percentage of the partial pressure, typically 78%, but also because it is not used by any of the body’s processes. Nitrogen partial pressures can build up within the tissues of the body while diving because more nitrogen molecules are absorbed as its partial pressure increases.25

Disease Effects

Healthy, properly functioning lungs are needed for efficient gas exchange between the organism and the environment to take place. Unfortunately, many diseases, illnesses and injuries can compromise the lungs’ ability to interact with the environment. The following are some of the more common of such diseases as well as a brief description of how each disease can affect gas transport and exchange.

Asthma: This is a chronic illness that causes episodic bronchospasams, airway inflammation and mucus plugging that is closely linked to allergen insult which results in reoccurring episodes of coughing, wheezing and breathlessness.25 With asthma, alveolar function remains normal, but the bronchioles become swollen and inflamed. This most often affects the patient’s ability to effectively move air out of the alveolus, resulting in air trapping, a buildup of carbon dioxide and a decrease in blood-oxygen saturation.

Emphysema: Emphysema is a permanent enlargement of the air spaces within the lungs. It’s usually accompanied by damage to the alveolar walls.24 This increase in air spaces causes air trapping to occur within the lungs, which impedes the ability of fresh air to reach the alveolus. Additionally, the alveolus is often damaged, which results in loss of elasticity of the alveolar sac. This inability to completely exchange carbon dioxide-filled air for fresh oxygen rich air results in a buildup of carbon dioxide and a decrease of oxygen in the blood. The body becomes acclimated to a higher internal partial pressure of carbon dioxide over time. This can be adversely affected if a high level of oxygen is administered for long periods of time.

Bronchitis: In this condition, the mucus-secreting cells in the bronchial walls become overactive in response to an environmental insult and secrete large amounts of mucus.10 The mucus invades the bronchial tree, restricting air flow and hampering gas exchange by reducing the amount of air delivered to the alveoli. The result is ineffective offloading of carbon dioxide and reduced absorption of oxygen by the pulmonary capillaries.

Pneumonia: This infection can affect any area of the lungs.13 As the inflammation from the infection progresses, the following happens:

  • Mucus production increases;
  • Fluids leak into the alveoli and thicken, and
  • The air passages begin to swell.

This limits the movement of air into the alveoli.6 The area of lung infected becomes less able to perform gas exchange due to the thick fluid barrier and the poor movement of air. If left unchecked, the infection will spread to the point of severe hypoxia and ultimately death.

Pulmonary fibrosis: Pulmonary fibrosis is an excessive accumulation of extracellular matrix and remodeling of the lung tissues.25 This excessive accumulation and remodeling of the extracellular matrix crowds out the space needed for healthy lung cells and replaces the connective tissues between the cells with a thick, scar-like substance. Diffusion through the heavy extracellular matrix becomes less possible over time, which results in increasingly poor gas exchange. The condition is typically progressive and leads to a poor prognosis.25

Blood disorders: Such blood disorders as anemia, leukemia and hemorrhage affect the oxygen-carrying ability of blood. Anemia is characterized with a greatly diminished number of hemoglobin or red blood cells in the body, which results in less hemoglobin to carry oxygen and carbon dioxide.13 Sickle cell anemia is a mutation of the hemoglobin that causes the cells to change to abnormal shapes when they offload their oxygen.6 Hemorrhage is an internal or external loss of blood. Leukemia causes a reduction in red blood cell production that results in anemia.13 Because each of these disorders affects the body’s oxygen-carrying abilities, they all can hamper the gas-exchange process.

Lung cancer: Lung cancer, like most other cancers, replaces functioning cells with abnormal, poorly functioning cells. This diminishes overall lung function.13 Since fewer healthy cells are facilitating gas exchange, less oxygen is absorbed and less carbon dioxide is eliminated. Lung cancer leads all other cancer related deaths among both women and men.26,26

Pulmonary edema: This condition is characterized by a buildup of fluids in the air spaces and tissues of the lungs.8 The fluid buildup creates an additional barrier through which gases must diffuse. Even though gases readily diffuse into liquids, pulmonary edema can create fluid barriers that make diffusion difficult or impossible.

Pump and vascular failures: The movement of blood through the body relies on a working pump and intact vascular system. If the heart is damaged, its pumping ability is impeded or if the vascular system is significantly damaged, blood will not pass the capillaries of the lungs and/or body tissues in sufficient enough volumes to be effective. Correcting the pump or vascular problem corrects the gas exchange issues associated with poor circulation.

Pneumothorax: When air enters and becomes trapped the space between the lung and the chest wall, either by traumatic or medical causes, the lung can gradually lose its ability to expand with enough volume to facilitate proper gas exchange. As the volume of trapped air enlarges, the pressure created within the chest cavity increases, causing ever-increasing amounts of blood to be shunted from the lungs, blood return to the heart is impaired and the ventilation-perfusion ratio is adversely affected. This limits the amount of blood available for gas exchange.28 Additionally, the amount of oxygen reaching the alveoli is reduced as lung volume progressively decreases. These two conditions cause a significant reduction in arterial oxygen concentrations.28 Once the trapped air is removed; however, it can still take several hours for oxygen concentrations to return to normal.28

Pertussis: Pertussis, also known as whooping cough, is a preventable viral infection of the lungs that affects people of all ages. Although the symptoms of pertussis in teens and adults range from mild to moderate, the illness can be serious in children less than 1 year old. Approximately half of all children less than age 1 who present with pertussis will need hospital care, and one in four will develop pneumonia.29 Although pertussis does not adversely affect gas exchange, complications, such as pneumonia, can create a fluid barrier that hampers the diffusion process.

Pulmonary embolism: When a large blood clot becomes trapped in the pulmonary circuit and obstructs blood flow through the lungs, gas exchange no longer occurs in the affected area of the lung(s). Although the lungs are still capable of gas exchange, they are no longer receiving an adequate flow of blood to exchange oxygen and carbon dioxide. If the obstruction becomes large enough, the right ventricle will not be able to generate enough pressure to overcome the obstruction. Hypoxia and hypotension followed by cardiac arrest will result.30

Pulmonary contusion: Blunt chest trauma resulting in pulmonary contusion causes an accumulation of blood and fluids in the alveolus that disrupts normal alveolar function.31 The addition of blood and fluids creates a fluid barrier similar to the barrier created by pulmonary edema. The condition typically develops over a 24-hour period following the injury, and as many as 60% of those suffering from a pulmonary contusion will develop acute respiratory distress syndrome.31

Adult Respiratory Distress Syndrome (ARDS): Adult respiratory distress syndrome can follow a variety of severe direct or indirect lung insults.32 ARDS is comprised of a group of lung conditions that often follow some other lung related insult. These conditions include severe respiratory distress, pulmonary edema, pneumonia, reduced lung compliance, and pulmonary shunting.32 ARDS can reduce gas exchange by either creating additional barriers for gasses to pass through, limiting blood flow to the alveolar capillaries, or reducing lung volume.

Viral respiratory infections: Most viral respiratory infections are self-limiting, meaning once the virus runs its course the illness is over and the infected person returns to normalcy.33 However, some viral infections can be much more serious. Such viral respiratory infections as the respiratory syncytial virus (RSV) and the human rhinovirus (HRV) can have a significant impact on infants, the elderly, and individuals with underlying respiratory disease. Viral infections of the respiratory tract can cause small airway inflammation similar to bronchitis, which can limit the amount of air that reaches the alveoli and decrease diffusion rates.33

Pleural effusion: Pleural effusion is the buildup of excessive fluid in the pleural space surrounding the lung.34 Several medical conditions can lead to pleural effusion, including congestive heart failure, pneumonia, liver disease, cancer, sepsis, and pulmonary embolism. As the fluid builds up, it gradually restricts lung movement and begins to create enough pressure that cardiac output can be effected.34 This combination of restricted lung movement and reduced blood volume can dramatically hamper the gas exchange process.

Anaphylaxis: Anaphylaxis is a severe systemic allergic reaction that causes pharyngeal/laryngeal edema, bronchospasms, and hypotension.35 Any one of these conditions by themselves can significantly hamper the gas exchange process, however if all of these exist the gas exchange process can stop entirely. Rapid treatment is the key in restoration of proper gas exchange.

Environmental: Many environmental and industrial factors can affect the gas exchange process. Essentially, any environmental condition that affects the lungs, blood, tissues, or the heart can affect the gas exchange process. These factors would include exposure to toxic gases, such as smoke, carbon monoxide, or other airborne toxins that may damage lung tissue or occupy valuable hemoglobin needed for carrying oxygen. Essentially, any adverse environmental variation that affects the content and quality of air, including air pressures and partial pressures can have a dramatic effect on diffusion rates and the gas exchange process.

Conclusion

The human body depends completely on the effective absorption and elimination of vital and potentially dangerous respiratory gasses. Many medical and environmental factors can affect the lungs’ ability to facilitate the movement of oxygen from a gaseous state into a liquid state and carbon dioxide from a liquid state to a gaseous state. These conditions typically result in cellular oxygen demand exceeding oxygen supply. When this happens, the body’s natural oxygen reserves are quickly used up. This can decrease, or worse, completely stop, cellular function.

The high metabolic demands of the body require a constant supply of oxygen and the efficient removal of carbon dioxide. Even though the gas-exchange process is a passive diffusion process in which gases diffuse down a concentration gradient, adequate partial pressures must still be present within the lungs for this process to happen. Additionally, the mechanical movement of air in and out of the lungs and the circulation of blood through the pulmonary and systemic circuits must provide sufficient quantities of oxygen while still facilitating effective removal of carbon dioxide.

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Published: January 6, 2017
Revised: February 27, 2017