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Role of mitochondria

Tricarboxylic acid cycle, oxidative phosphorylation.

cellular respiration

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cellular respiration , the process by which organisms combine oxygen with foodstuff molecules , diverting the chemical energy in these substances into life-sustaining activities and discarding, as waste products, carbon dioxide and water. Organisms that do not depend on oxygen degrade foodstuffs in a process called fermentation . (For longer treatments of various aspects of cellular respiration, see tricarboxylic acid cycle and metabolism .)

One objective of the degradation of foodstuffs is to convert the energy contained in chemical bonds into the energy-rich compound adenosine triphosphate (ATP), which captures the chemical energy obtained from the breakdown of food molecules and releases it to fuel other cellular processes. In eukaryotic cells (that is, any cells or organisms that possess a clearly defined nucleus and membrane-bound organelles) the enzymes that catalyze the individual steps involved in respiration and energy conservation are located in highly organized rod-shaped compartments called mitochondria . In microorganisms the enzymes occur as components of the cell membrane . A liver cell has about 1,000 mitochondria; large egg cells of some vertebrates have up to 200,000.

Main metabolic processes

Biologists differ somewhat with respect to the names, descriptions, and the number of stages of cellular respiration . The overall process, however, can be distilled into three main metabolic stages or steps: glycolysis , the tricarboxylic acid cycle (TCA cycle), and oxidative phosphorylation (respiratory-chain phosphorylation).

Glycolysis (which is also known as the glycolytic pathway or the Embden-Meyerhof-Parnas pathway) is a sequence of 10 chemical reactions taking place in most cells that breaks down a glucose molecule into two pyruvate (pyruvic acid) molecules. Energy released during the breakdown of glucose and other organic fuel molecules from carbohydrates , fats , and proteins during glycolysis is captured and stored in ATP. In addition, the compound nicotinamide adenine dinucleotide (NAD + ) is converted to NADH during this step ( see below ). Pyruvate molecules produced during glycolysis then enter the mitochondria, where they are each converted into a compound known as acetyl coenzyme A, which then enters the TCA cycle. (Some sources consider the conversion of pyruvate into acetyl coenzyme A as a distinct step, called pyruvate oxidation or the transition reaction, in the process of cellular respiration.)

The TCA cycle (which is also known as the Krebs, or citric acid , cycle) plays a central role in the breakdown, or catabolism , of organic fuel molecules. The cycle is made up of eight steps catalyzed by eight different enzymes that produce energy at several different stages. Most of the energy obtained from the TCA cycle, however, is captured by the compounds NAD + and flavin adenine dinucleotide (FAD) and converted later to ATP. The products of a single turn of the TCA cycle consist of three NAD + molecules, which are reduced (through the process of adding hydrogen , H + ) to the same number of NADH molecules, and one FAD molecule, which is similarly reduced to a single FADH 2 molecule. These molecules go on to fuel the third stage of cellular respiration, whereas carbon dioxide, which is also produced by the TCA cycle, is released as a waste product.

In the oxidative phosphorylation stage, each pair of hydrogen atoms removed from NADH and FADH 2 provides a pair of electrons that—through the action of a series of iron -containing hemoproteins, the cytochromes —eventually reduces one atom of oxygen to form water . In 1951 it was discovered that the transfer of one pair of electrons to oxygen results in the formation of three molecules of ATP .

Oxidative phosphorylation is the major mechanism by which the large amounts of energy in foodstuffs are conserved and made available to the cell . The series of steps by which electrons flow to oxygen permits a gradual lowering of the energy of the electrons. This part of the oxidative phosphorylation stage is sometimes called the electron transport chain . Some descriptions of cellular respiration that focus on the importance of the electron transport chain have changed the name of the oxidative phosphorylation stage to the electron transport chain.

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Cellular Respiration

Reviewed by: BD Editors

Cellular Respiration Definition

Cellular respiration is the process through which cells convert sugars into energy. To create ATP and other forms of energy to power cellular reactions, cells require fuel and an electron acceptor which drives the chemical process of turning energy into a useable form.

Cellular Respiration Overview

Eukaryotes , including all multicellular organisms and some single-celled organisms, use aerobic respiration to produce energy. Aerobic respiration uses oxygen – the most powerful electron acceptor available in nature.

Aerobic respiration is an extremely efficient process allows eukaryotes to have complicated life functions and active lifestyles. However, it also means that they require a constant supply of oxygen, or they will be unable to obtain energy to stay alive.

Prokaryotic organisms such as bacteria and archaebacteria can use other forms of respiration, which are somewhat less efficient. This allows them to live in environments where eukaryotic organisms could not, because they do not require oxygen.

Examples of different pathways for how sugars are broken down by organisms are illustrated below:

More detailed articles on aerobic respiration and anaerobic respiration can be found on this site. Here we will give an overview of the different types of cellular respiration.

Cellular Respiration Equation

Aerobic respiration equation.

The equation for aerobic respiration shows glucose being combined with oxygen and ADP to produce carbon dioxide, water, and ATP:

C 6 H 12 O 6 (glucose)+ 6O 2 + 36 ADP (depleted ATP) + 36 P i (phosphate groups)→ 6CO 2 + 6H 2 O + 36 ATP

You can see that once it is completely broken down, the carbon molecules of glucose are exhaled as six molecules of carbon dioxide.

Lactic Acid Fermentation Equation

In lactic acid fermentation, one molecule of glucose is broken down into two molecules of lactic acid. The chemical energy that was stored in the broken glucose bonds is moved into bonds between ADP and a phosphate group.

C 6 H 12 O 6 (glucose) + 2 ADP (depleted ATP) + 2 P i (phosphate groups) → 2 CH 3 CHOHCOOH (lactic acid) + 2 ATP

Alcoholic Fermentation Equation

Alcohol fermentation is similar to lactic acid fermentation in that oxygen is not the final electron acceptor. Here, instead of oxygen, the cell uses a converted form of pyruvate to accept the final electrons. This creates ethyl alcohol, which is what is found in alcoholic beverages. Brewers and distillers use yeast cells to create this alcohol, which are very good at this form of fermentation.

C 6 H 12 O 6 (glucose) + 2 ADP (depleted ATP) + 2 P i (phosphate groups)→ 2 C 2 H 5 OH (ethyl alcohol) + 2 CO 2 + 2 ATP

Cellular Respiration Steps

Glycolysis is the only step which is shared by all types of respiration. In glycolysis, a sugar molecule such as glucose is split in half, generating two molecules of ATP.

The equation for glycolysis is:

C 6 H 12 O 6 (glucose) + 2 NAD+ + 2 ADP + 2 P i → 2 CH 3 COCOO− + 2 NADH + 2 ATP + 2 H 2 O + 2H +

The name “glycolysis” comes from the Greek “glyco,” for “sugar” and “lysis” for “to split.” This may help you to remember that glycolysis it the process of splitting a sugar.

In most pathways, glycolysis starts with glucose, which is then split into two molecules of pyruvic acid. These two molecules of pyruvic acid are then processed further to form different end products, such as ethyl alcohol or lactic acid.

Reduction is the next part of the process. In chemical terms, to “reduce” a molecule means to add electrons to it.

In the case of lactic acid fermentation, NADH donates an electron to pyruvic acid, resulting in the end products of lactic acid and NAD+. This is helpful to the cell because NAD+ is necessary for glycolysis. In the case of alcoholic fermentation, pyruvic acid undergoes an additional step in which it loses an atom of carbon in the form of CO 2 . The resulting intermediate molecule, called acetaldehyde, is then reduced to produce NAD+ plus ethyl alcohol.

Aerobic respiration takes these processes to another level. Instead of directly reducing intermediates of the Krebs cycle, aerobic respiration uses oxygen as the final electron receptor. But first, the electrons and protons bound to electron carriers (such as NADH), are processed through the electron transport chain . This chain of proteins within the mitochondrial membrane uses the energy from these electrons to pump protons to one side of the membrane. This creates an electromotive force, which is utilized by the protein complex ATP synthase phosphorylate a large number of ATD molecules, creating ATP.

Products of Cellular Respiration

The main product of any cellular respiration is the molecule adenosine triphosphate (ATP) . This molecule stores the energy released during respiration and allows the cell to transfer this energy to various parts of the cell. ATP is used by a number of cellular components as a source of energy. For example, an enzyme may need energy from ATP to combine two molecules. ATP is also commonly used on transporters , which are proteins that function to move molecules across the cell membrane.

Carbon Dioxide

Carbon dioxide is a universal product created by cellular respiration. Typically, carbon dioxide is considered a waste product and must be removed. In an aqueous solution, carbon dioxide creates acidic ions. This can drastically lower the pH of the cell, and eventually will cause normal cellular functions to cease. To avoid this, cells must actively expel carbon dioxide.

Other Products

While ATP and carbon dioxide are regularly produced by all forms of cellular respiration, different types of respiration rely on different molecules to be the final acceptors of the electrons used in the process.

Purpose of Cellular Respiration

All cells need to be able to obtain and transport energy to power their life functions. For cells to continue living, they must be able to operate essential machinery, such as pumps in their cell membranes which maintain the cell’s internal environment in a way that’s suitable for life.

The most common “energy currency” of cells is ATP – a molecule which stores a lot of energy in its phosphate bonds. These bonds can be broken to release that energy and bring about changes to other molecules, such as those needed to power cell membrane pumps.

Because ATP is not stable over long periods of time, it is not used for long-term energy storage. Instead, sugars and fats are used as a long-term form of storage, and cells must constantly process those molecules to produce new ATP. This is the process of respiration.

The process of aerobic respiration produces a huge amount of ATP from each molecule of sugar. In fact, each molecule of sugar digested by a plant or animal cell yields 36 molecules of ATP! By comparison, fermentation usually only produces 2-4 molecules of ATP.

Anaerobic respiration processes used by bacteria and archaebacteria yield smaller amounts of ATP, but they can take place without oxygen. Below, we’ll discuss how different types of cellular respiration produce ATP.

Types of Cellular Respiration

Aerobic respiration.

Eukaryotic organisms perform cellular respiration in their mitochondria – organelles that are designed to break down sugars and produce ATP very efficiently. Mitochondria are often called “the powerhouse of the cell” because they are able to produce so much ATP!

Aerobic respiration is so efficient because oxygen is the most powerful electron acceptor found in nature. Oxygen “loves” electrons – and its love of electrons “pulls” them through the electron transport chain of the mitochondria.

The specialized anatomy of the mitochondria – which bring together all the necessary reactants for cellular respiration in a small, membrane-bound space within the cell – also contributes to the high efficiency of aerobic respiration.

In the absence of oxygen, most eukaryotic cells can also perform different types of anaerobic respiration, such as lactic acid fermentation. However, these processes do not produce enough ATP to maintain the cell’s life functions, and without oxygen, cells will eventually die or cease to function.

Fermentation

Fermentation is the name given to many different types of anaerobic respiration, which are performed by different species of bacteria and archaebacteria, and by some eukaryotic cells in the absence of oxygen.

These processes can use a variety of electron acceptors and produce a variety of byproducts. A few types of fermentation are:

• Alcoholic fermentation – This type of fermentation, performed by yeast cells and some other cells, metabolizes sugar and produces alcohol and carbon dioxide as byproducts. This is why beers are fizzy: during fermentation, their yeasts release both carbon dioxide gas, which forms bubbles and ethyl alcohol.
• Lactic acid fermentation – This type of fermentation is performed by human muscle cells in the absence of oxygen, and by some bacteria. Lactic acid fermentation is actually used by humans to make yogurt. To make yogurt, harmless bacteria are grown in milk. The lactic acid produced by these bacteria gives yogurt its distinctive sharp-sour taste and also reacts with milk proteins to create a thick, creamy texture.
• Proprionic acid fermentation – This type of fermentation is performed by some bacteria, and is used to make swiss cheese. Proprionic acid is responsible for the distinctive sharp, nutty flavor of Swiss cheese. The gas bubbles created by these bacteria are responsible for the holes found in the cheese.
• Acetogenesis – Acetogenesis is a type of fermentation performed by bacteria, which produces acetic acid as its byproduct. Acetic acid is the distinctive ingredient in vinegar which gives it its sharp, sour taste and smell. Interestingly, the bacteria that produce acetic acid use ethyl alcohol as their fuel. This means that to produce vinegar, a sugar-containing solution must be first fermented with yeast to produce alcohol, then fermented again with bacteria that turn the alcohol into acetic acid!

Methanogenesis

Methanogenesis is a unique type of anaerobic respiration that can only be performed by archaebacteria. In methanogenesis, a fuel source carbohydrate is broken down to produce carbon dioxide and methane.

Methanogenesis is performed by some symbiotic bacteria in the digestive tracts of humans, cows, and some other animals. Some of these bacteria are able to digest cellulose, a sugar found in plants that cannot be broken down through cellular respiration. Symbiotic bacteria allow cows and other animals to obtain some energy from these otherwise undigestible sugars!

1. Which of the following is NOT necessary for cellular respiration?

2. Which of the following types of cells CANNOT survive by using fermentation alone?

3. Which of the following is NOT a reason why multicellular organisms need oxygen to survive?

4. How is the process of  respiration  (breathing) related to  cellular respiration ?

5. Which of the following forms of cellular respiration is responsible for creating beer, wine, and spirits?

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8.3 Cellular Respiration

Learning objectives.

By the end of this section, you will be able to:

• Compare and contrast the electron transport system location and function in a prokaryotic cell and a eukaryotic cell
• Compare and contrast the differences between substrate-level and oxidative phosphorylation
• Explain the relationship between chemiosmosis and proton motive force
• Describe the function and location of ATP synthase in a prokaryotic versus eukaryotic cell
• Compare and contrast aerobic and anaerobic respiration

We have just discussed two pathways in glucose catabolism—glycolysis and the Krebs cycle—that generate ATP by substrate-level phosphorylation. Most ATP, however, is generated during a separate process called oxidative phosphorylation , which occurs during cellular respiration. Cellular respiration begins when electrons are transferred from NADH and FADH 2 —made in glycolysis, the transition reaction, and the Krebs cycle—through a series of chemical reactions to a final inorganic electron acceptor (either oxygen in aerobic respiration or non-oxygen inorganic molecules in anaerobic respiration). These electron transfers take place on the inner part of the cell membrane of prokaryotic cells or in specialized protein complexes in the inner membrane of the mitochondria of eukaryotic cells. The energy of the electrons is harvested to generate an electrochemical gradient across the membrane, which is used to make ATP by oxidative phosphorylation.

Electron Transport System

The electron transport system (ETS) is the last component involved in the process of cellular respiration; it comprises a series of membrane-associated protein complexes and associated mobile accessory electron carriers ( Figure 8.15 ). Electron transport is a series of chemical reactions that resembles a bucket brigade in that electrons from NADH and FADH 2 are passed rapidly from one ETS electron carrier to the next. These carriers can pass electrons along in the ETS because of their redox potential . For a protein or chemical to accept electrons, it must have a more positive redox potential than the electron donor. Therefore, electrons move from electron carriers with more negative redox potential to those with more positive redox potential. The four major classes of electron carriers involved in both eukaryotic and prokaryotic electron transport systems are the cytochrome s, flavoprotein s, iron-sulfur protein s, and the quinone s.

In aerobic respiration , the final electron acceptor (i.e., the one having the most positive redox potential) at the end of the ETS is an oxygen molecule (O 2 ) that becomes reduced to water (H 2 O) by the final ETS carrier. This electron carrier, cytochrome oxidase , differs between bacterial types and can be used to differentiate closely related bacteria for diagnoses. For example, the gram-negative opportunist Pseudomonas aeruginosa and the gram-negative cholera-causing Vibrio cholerae use cytochrome c oxidase, which can be detected by the oxidase test, whereas other gram-negative Enterobacteriaceae, like E. coli , are negative for this test because they produce different cytochrome oxidase types.

There are many circumstances under which aerobic respiration is not possible, including any one or more of the following:

• The cell lacks genes encoding an appropriate cytochrome oxidase for transferring electrons to oxygen at the end of the electron transport system.
• The cell lacks genes encoding enzymes to minimize the severely damaging effects of dangerous oxygen radicals produced during aerobic respiration, such as hydrogen peroxide (H 2 O 2 ) or superoxide ( O 2 – ) . ( O 2 – ) .
• The cell lacks a sufficient amount of oxygen to carry out aerobic respiration.

One possible alternative to aerobic respiration is anaerobic respiration , using an inorganic molecule other than oxygen as a final electron acceptor. There are many types of anaerobic respiration found in bacteria and archaea. Denitrifiers are important soil bacteria that use nitrate ( NO 3 – ) ( NO 3 – ) and nitrite ( NO 2 – ) ( NO 2 – ) as final electron acceptors, producing nitrogen gas (N 2 ). Many aerobically respiring bacteria, including E. coli , switch to using nitrate as a final electron acceptor and producing nitrite when oxygen levels have been depleted.

Microbes using anaerobic respiration commonly have an intact Krebs cycle, so these organisms can access the energy of the NADH and FADH 2 molecules formed. However, anaerobic respirers use altered ETS carriers encoded by their genomes, including distinct complexes for electron transfer to their final electron acceptors. Smaller electrochemical gradients are generated from these electron transfer systems, so less ATP is formed through anaerobic respiration.

Check Your Understanding

• Do both aerobic respiration and anaerobic respiration use an electron transport chain?

Chemiosmosis, Proton Motive Force, and Oxidative Phosphorylation

In each transfer of an electron through the ETS, the electron loses energy, but with some transfers, the energy is stored as potential energy by using it to pump hydrogen ions (H + ) across a membrane. In prokaryotic cells, H + is pumped to the outside of the cytoplasmic membrane (called the periplasmic space in gram-negative and gram-positive bacteria), and in eukaryotic cells, they are pumped from the mitochondrial matrix across the inner mitochondrial membrane into the intermembrane space. There is an uneven distribution of H + across the membrane that establishes an electrochemical gradient because H + ions are positively charged (electrical) and there is a higher concentration (chemical) on one side of the membrane. This electrochemical gradient formed by the accumulation of H + (also known as a proton) on one side of the membrane compared with the other is referred to as the proton motive force (PMF). Because the ions involved are H + , a pH gradient is also established, with the side of the membrane having the higher concentration of H + being more acidic. Beyond the use of the PMF to make ATP, as discussed in this chapter, the PMF can also be used to drive other energetically unfavorable processes, including nutrient transport and flagella rotation for motility.

The potential energy of this electrochemical gradient generated by the ETS causes the H + to diffuse across a membrane (the plasma membrane in prokaryotic cells and the inner membrane in mitochondria in eukaryotic cells). This flow of hydrogen ions across the membrane, called chemiosmosis , must occur through a channel in the membrane via a membrane-bound enzyme complex called ATP synthase ( Figure 8.15 ). The tendency for movement in this way is much like water accumulated on one side of a dam, moving through the dam when opened. ATP synthase (like a combination of the intake and generator of a hydroelectric dam) is a complex protein that acts as a tiny generator, turning by the force of the H + diffusing through the enzyme, down their electrochemical gradient from where there are many mutually repelling H + to where there are fewer H + . In prokaryotic cells, H + flows from the outside of the cytoplasmic membrane into the cytoplasm, whereas in eukaryotic mitochondria, H + flows from the intermembrane space to the mitochondrial matrix. The turning of the parts of this molecular machine regenerates ATP from ADP and inorganic phosphate (P i ) by oxidative phosphorylation , a second mechanism for making ATP that harvests the potential energy stored within an electrochemical gradient.

The number of ATP molecules generated from the catabolism of glucose varies. For example, the number of hydrogen ions that the electron transport system complexes can pump through the membrane varies between different species of organisms. In aerobic respiration in mitochondria, the passage of electrons from one molecule of NADH generates enough proton motive force to make three ATP molecules by oxidative phosphorylation, whereas the passage of electrons from one molecule of FADH 2 generates enough proton motive force to make only two ATP molecules. Thus, the 10 NADH molecules made per glucose during glycolysis, the transition reaction, and the Krebs cycle carry enough energy to make 30 ATP molecules, whereas the two FADH 2 molecules made per glucose during these processes provide enough energy to make four ATP molecules. Overall, the theoretical maximum yield of ATP made during the complete aerobic respiration of glucose is 38 molecules, with four being made by substrate-level phosphorylation and 34 being made by oxidative phosphorylation ( Figure 8.16 ). In reality, the total ATP yield is usually less, ranging from one to 34 ATP molecules, depending on whether the cell is using aerobic respiration or anaerobic respiration; in eukaryotic cells, some energy is expended to transport intermediates from the cytoplasm into the mitochondria, affecting ATP yield.

Figure 8.16 summarizes the theoretical maximum yields of ATP from various processes during the complete aerobic respiration of one glucose molecule.

• What are the functions of the proton motive force?

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Photosynthesis and Cellular Respiration Essay

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Photosynthesis is one of the primary sources of energy for living organisms. The fossilized photosynthetic fuels account for almost 90% of the energy in the world (Johnson, 2016). Cellular respiration is a process that takes place in the living organism and converts nutrients into energy. This essay will examine photosynthesis and cellular respiration separately and identify similarities, differences, and interconnectedness between two processes. Two processes are similar in that they both deals with energy, but they are different because one process involves catabolic reactions and another anabolic one.

The purpose of photosynthesis is to convert atmospheric carbon dioxide into carbohydrates using light energy. The light splits one of the reactants, water in the mesophyll of the leaf into oxygen, electrons, and protons during the light-dependent phase (Johnson, 2016). Then carbon dioxide enters the mesophyll of the leaf through openings, stomata, during the light-independent phase. These two reactions differ in light utilization and molecules production. The first reaction products are oxygen, adenosine triphosphate (ATP), and nicotinamide adenine dinucleotide phosphate (NADPH) that are used as energy storages, while by the end of the second reaction, the carbohydrate is obtained, and molecules mentioned above are used (Flügge et al., 2016). Photosynthesis occurs in the chloroplast with the light-dependent reaction taking place in the thylakoid membrane, and light-independent reaction in the stroma. The energy produced in the light reaction is used to fix carbon dioxide and produce carbohydrates while oxygen is released outside. According to the following equation of the photosynthesis, C → O2 + 2H20 + photons (CH2O)n + electrons + O2 carbon monoxide and water are transferred into carbohydrates under the light with the release of atmospheric oxygen.

The purpose of cellular respiration is to convert nutrients into energy. The reactants of the respiration are glucose circulating in the blood and oxygen obtained from breathing, while the product is ATP. Cellular respiration starts from glycolysis in the mitochondria’s stroma, where the glucose is broken down into pyruvate (Bentley & Connaughton, 2017). Then it continues with the citric acid cycle that generates ATP, NADH, and FADH2. In the final stage, the electron transport chain uses these molecules to generate more ATP. The energy produced is then used for metabolic processes in the organism, while carbon dioxide is released with breathing (BBC Bitesize, n.d.). According to the following equation of the cellular respiration, C → 6H12O6 + 6O2 6CO2 + 6H2O the glucose is broken down into carbon dioxide and water with the presence of oxygen.

There are two main differences between photosynthesis and cellular respiration. The first one is the anabolic process, during which complex compounds are synthesized, while the second one is catabolic, which involves breaking down the compounds (Panawala, 2017). The second crucial difference is that photosynthesis is found only in chloroplasts, while cellular respiration is found in any living cell, making it a universal process. There are also two main similarities between photosynthesis and respiration. The first similarity is that both processes involve the production of ATP (Stauffer et al., 2018). The second similarity is that both processes utilize ATP but for different purposes.

Photosynthesis and cellular respiration are connected in such a way that they allow to perform metabolic functions normally. Moreover, these processes help to regulate the concentration of oxygen and carbon dioxide in the atmosphere. If photosynthesis stopped occurring, the level of oxygen would drop dramatically This would lead to deaths of all living organisms whose lives depend on this molecule. Whereas if cellular respiration stopped happening, living creatures would not be able to generate energy and sustain life.

To conclude, photosynthesis plays a crucial role in maintaining life on Earth. Photosynthesis uses light energy to produce oxygen, while cellular respiration uses oxygen to break down complex molecules and provide energy. These processes are different in their metabolic nature, but similar in terms of energy storage. If photosynthesis did not exist, the life for oxygen-dependent creatures would become extinct. Similarly, in the case of cellular respiration disappearing, living organisms would not be able to produce energy.

BBC Bitesize . (n.d.). Respiration. 2020. Web.

Bentley, M., & Connaughton, V, P. (2017). A simple way for students to visualize cellular respiration: Adapting the board game MousetrapTM to model complexity . CourseSource. 4, 1-6. Web.

Flügge, W., Westhoff, P., & Leister, D. (2016). Recent advances in understanding photosynthesis. F1000 Research, 5, 1-10.

Johnson, M. P. (2016). Photosynthesis. Essays Biochemistry , 60 (3), 255-273.

Panawala, L. (2017). Difference between photosynthesis and respiration. IE PEDIAA. Web.

Stauffer S., Gardner A., Ungu D.A.K., López-Córdoba A., & Heim M. (2018). Cellular respiration. In Labster virtual lab experiments: Basic biology (pp. 43-55). Springer.

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The Physiology and Maintenance of Respiration: A Narrative Review

Lynn r. webster.

1 PRA Health Sciences, Salt Lake City, UT USA

Suzanne Karan

2 University of Rochester Medical Center, Rochester, NY USA

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Chronic pain is one of the most common reasons adults seek medical care and is often managed with opioid analgesics; however, opioids may cause respiratory depression by suppressing various components of respiration. Respiration is the physiological process that facilitates gas exchange and is mediated through the proper function of and communication among central neural control (respiratory drive), sensory input systems, the lungs, and the muscles involved in respiration. Normal respiratory function can be dampened with the use of central nervous system (CNS) depressants and/or underlying health conditions. Patients with chronic pain are often exposed to CNS depressants other than opioids, including benzodiazepines, barbiturates, nonbenzodiazepine sedative-hypnotics, and ethanol, which can function synergistically with opioids to increase the risk of respiratory depression. Some patients may also have underlying health issues, such as obstructive sleep apnea, that can be exacerbated with the use of opioids and other CNS depressants and further contribute to respiratory depression. Clinicians should have a thorough understanding of respiration, recognize how various CNS depressants suppress it, and take necessary steps to mitigate the risk of opioid-induced respiratory depression by collaborating with a multidisciplinary team (i.e., sleep and pain specialists), choosing appropriate medications, and educating patients on the proper use and storage of opioids.

Key Summary Points

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Introduction

In the USA, chronic pain is one of the most frequent reasons adults seek medical attention [ 1 , 2 ]. Results from the 2012 National Health Interview Survey showed that approximately 25.3 million adults in the USA experienced chronic pain [ 3 ]. Chronic pain is often managed with opioid analgesic medication [ 2 , 4 , 5 ]. Opioid analgesics are frequently diverted and used improperly, which has contributed to a national crisis of opioid overdose deaths [ 5 , 6 ]. Opioids act primarily on mu-opioid receptors in the central nervous system (CNS), including receptors located on neuronal centers that play a key role in regulating respiratory drive , the ability of neuronal respiratory centers to control ventilation [ 7 ]. Failure in one or more steps of respiratory drive leads to respiratory depression , the medical outcome that occurs when inadequate ventilation of the lungs decreases the rate of gas exchange [ 8 ]. Opioid-induced respiratory depression is the leading cause of death in patients who overdose with or abuse opioids [ 4 , 5 , 9 , 10 ].

Patients with chronic pain may not be treated only with opioid analgesics but may also be exposed to other CNS depressants, or a combination of them, including benzodiazepines, barbiturates, nonbenzodiazepine sedative-hypnotics (also known as Z drugs), and ethanol. Along with opioids, each of these agents can cause respiratory depression. Therefore, it is important that clinicians understand the mechanisms behind respiration and the impact of suppressant agents on respiratory drive and patency.

The purpose of this review is to provide an overview of the physiology behind respiration, maintenance of ventilation, and the various CNS depressant agents that affect normal physiological responses. The information presented here can be used as an educational resource for clinicians to improve decision-making and medication selection regarding potential impacts on respiratory function.

This article is based on previously conducted studies and does not contain any studies with human participants or animals performed by any of the authors.

Respiration

Respiration encompasses the processes that facilitate gas exchange on a cellular level, which involves the intake of O 2 and the removal of CO 2 [ 11 ]. Respiration involves the synchronization of various components, including central neural control (respiratory drive), sensory input systems, respiratory muscles, and lungs (Fig.  1 ) [ 11 – 13 ]. Central neural control and sensory input systems coordinate the timing and rate of ventilation and air volume intake, which is signaled to the respiratory muscles and lungs for the mechanical exchange of inspired gases (i.e., air) [ 11 , 14 ].

Three primary components that facilitate respiration. Respiration is mediated by central neural control through the cortex and the respiratory center in the brain (the medulla and pons); sensory input systems, including mechanoreceptors, metaboreceptors, and peripheral and central chemoreceptors located in the CNS, carotid arteries, and muscles; and respiratory muscles, which include the upper airway, diaphragm, external and internal intercostals, and abdominal muscles and the lungs. Each of these systems coordinates with one another to control the diffusion of essential chemicals and gases between the circulatory system and the environment. CNS central nervous system

Central Neural Control

The respiratory center in the brain is controlled by the pons and medulla (Fig.  2 a) [ 15 ]. These neural control centers work collectively to regulate inspiration and exhalation [ 15 ]. The cerebral cortex influences the respiratory centers of the brain to control conscious (i.e., breath holding) or unconscious (i.e., speech, singing, coughing) respiration [ 16 – 18 ].

Control of respiration. a The respiratory center in the brain controls various components of respiratory drive, including inhalation, airway defense, exhalation, and breathing patterns. b The sensory input systems are composed of mechanoreceptors, metaboreceptors, and peripheral and central chemoreceptors that sense chemical changes and influence various components of respiration, such as breathing, lung space, and irritation triggers. c Neuronal processes and sensory input systems are communicated to the respiratory muscles and lungs to control the mechanical aspects of respiration

Medullary Groups

The dorsal medulla is responsible for inhalation and airway defense, while the ventral medulla is responsible for exhalation [ 11 , 19 , 20 ]. The strength of the signal from the dorsal medulla can influence breathing, whereby increased impulse frequency results in stronger muscle contractions and deeper breathing, and decreased frequency results in passive expiration [ 21 ]. The dorsal medulla communicates with the ventral medulla by integrating input from central and peripheral receptors prior to relaying information to respiratory muscles to generate respiratory rhythm [ 21 ]. The pre-Bötzinger complex is a group of neurons located between the ventral respiratory group and the Bötzinger complex in the brainstem that also functions to control inspiration [ 7 ]. The pre-Bötzinger complex interacts with respiratory centers to ensure a smooth transition between different breathing phases, while also preventing the activation of opposing muscle groups [ 7 ]. The principal neurotransmitters involved in modulating the generation and transmission of respiratory rhythm are glutamate, gamma-aminobutyric acid (GABA), and glycine [ 22 ].

Pontine Grouping

The pontine grouping allows for modulation of the intensity and frequency of medullary signals to control breathing patterns while promoting a smooth transition between inspiration and expiration [ 21 , 23 ]. More specifically, the pneumotaxic center in the upper portion of the pons coordinates the speed of breathing, sends inhibitory impulses to the respiratory center, and is involved in the fine-tuning of respiratory rate [ 24 ]. The apneustic center in the lower portion of the pons also coordinates the speed of breathing but can be overridden by the pneumotaxic center to end inhalation; this region is mostly responsible for sending stimulatory impulses to the inspiratory area (prolonging inhalation) [ 24 ]. Suprapontine areas also contribute to respiration by responding to changes in internal or external environmental conditions such as exercise, hypoxia, hypercapnia, and thermal changes, as well as other processes like swallowing and coughing [ 18 , 25 ]. These neuronal processes are modulated by incoming sensory input systems [ 26 ].

Sensory Input Systems

Sensory input systems consist of mechanoreceptors, metaboreceptors, and peripheral and central chemoreceptors that coordinate with other components of the respiratory response system to control breathing (Fig.  2 b) [ 11 , 27 , 28 ].

Mechanoreceptors and Metaboreceptors

Mechanoreceptors are located throughout the respiratory tract in the airways, trachea, lungs, and pulmonary vessels. They provide sensory information to the respiratory center of the brain regarding the mechanical status of the lungs and chest, including the rate of breathing, lung space, and irritation triggers [ 11 , 27 ]. Pulmonary stretch receptors can be classified as slowly or rapidly adapting, where slowly adapting receptors are activated during inflation of the lungs and play a critical role in the Hering–Breuer reflex (termination of inspiration and prolongation of expiration), and rapidly adapting receptors initiate defensive respiratory reflexes in response to irritants [ 27 , 29 ]. Bronchopulmonary C-fiber receptors also play a role in initiating defensive respiratory reflexes in response to inhaled irritants or rapid changes in lung volume [ 27 ]. Other receptors called metaboreceptors are found in the skeletal muscle and are activated by metabolic byproducts to stimulate breathing during exercise [ 30 ].

Peripheral Chemoreceptors

Peripheral chemoreceptors consist of fast-acting carotid and aortic bodies that monitor the partial pressure of arterial O 2 in the blood and respond to hypercapnia or acidosis [ 28 , 31 ]. Carotid body chemoreceptors are located at the bifurcation of the common carotid arteries and are responsible for the majority of the peripheral control of ventilation [ 28 , 31 , 32 ]. Carotid bodies are also able to sense arterial gas concentrations and pH, which initiates a rapid response (within 1–3 s) by stimulating ventilation via communication with medullary response neurons [ 24 , 28 ]. Aortic bodies are located near the arch of the aorta and play a large role in regulating circulation while also responding to changes in gas concentrations [ 24 , 28 ].

Central Chemoreceptors

Central chemoreceptors are located within the ventral surface of the medulla and retrotrapezoid nucleus and are responsible for sensing pH, O 2 , or CO 2 concentration changes in the brain and cerebrospinal fluid [ 7 , 11 , 26 , 32 , 33 ]. An acidic environment (increased hydrogen ions) in the brain triggers the respiratory center to initiate contraction of the diaphragm and intercostal muscles [ 26 ]. As a result, the rate and depth of respiration increase, allowing for more CO 2 to be expelled and thereby reducing CO 2 levels and hydrogen ions in the blood [ 26 ]. Conversely, low levels of CO 2 in the blood cause low levels of hydrogen ions in the brain, ultimately leading to a decrease in the rate and depth of ventilation and the slowing of breathing [ 26 ]. Taken together, these processes function to normalize pH [ 31 ]. Of all the sensory input systems, central chemoreceptors are thought to have primary control over respiration, which is ultimately carried out by respiratory muscles and the lungs [ 11 , 33 ].

Respiratory Muscles and Lungs

Information from neuronal and sensory input systems signals to the diaphragm and other respiratory muscles to control the mechanical aspects of respiration (Figs.  2 c, ​ c,3) 3 ) [ 21 , 34 – 37 ]. The upper airway is composed of many soft tissues, muscles, and bony structures that modulate patency for respiratory functions [ 38 ]. The upper airway can be influenced by cortical states, sensory input, drugs, and passive changes in lung volume, which ultimately functions to modulate reflex activity and provide defense/protection and maintenance of the airway [ 32 , 38 , 39 ]. The depth of inspiration during breathing is based on the level of activity of the respiratory center in the brain and subsequent stimulation of motor neurons [ 40 ]. With more stimulation, an increased number of motor units are excited, leading to respiratory muscles contracting with greater force [ 21 ].

Respiration is mediated by communication among central neural control, sensory input systems, respiratory muscles, and the lungs. Neuronal and sensory input systems coordinate with respiratory muscles and the lungs to control the mechanical aspects of respiration

Contraction and relaxation of the diaphragm and external intercostals are responsible for most of the pressure changes that result in inspiration [ 26 ]. Accessory muscles of inspiration include the scaleni, sternocleidomastoid, and anterior serrati; however, they do not play a role in passive breathing [ 41 ]. The most important muscles for expiration are the abdominal muscles and internal intercostals, which contract and compress the abdominal organs pushing them up into the diaphragm, raising pleural pressure and alveolar pressure, and driving air out of the lungs [ 41 ]. Respiratory exchange surfaces in the lungs transfer O 2 and CO 2 between the air and the blood [ 28 ]. O 2 crosses the alveoli in the lungs to the blood and is transported to tissues, while CO 2 is removed from the blood and transferred to the alveoli prior to exchange back to the environment [ 42 ]. The combination of these processes results in an average resting respiratory rate of approximately 12 breaths per minute in a healthy adult [ 11 , 41 ]. Minute ventilation is the product of respiratory rate and tidal volume, with a normal tidal volume being approximately 7 mL/kg of ideal body weight (approximately 500 mL in an average healthy adult male and 400 mL in a healthy adult female) [ 41 , 43 ]. Assessment of this and other components of respiration can provide insight into a patient’s overall respiratory health.

In healthy adults, the rise in ventilation as the partial pressure of CO 2 (PaCO 2 ) increases can be observed in a linear fashion (Fig.  4 ) [ 28 , 44 – 46 ]. In the setting of hypoxia, the body is more sensitive to changes in PaCO 2 , as both central and peripheral chemoreceptors cause an additive increase in respiratory drive [ 46 ]. During sedation, metabolism and minute ventilation are both decreased [ 28 ]. CNS depressants that cause respiratory depression, such as opioids, can further dampen the CO 2 response curve, causing a right-shift (decreased threshold) [ 46 ]. The further the shift to the right, the more likely respiratory depression is to occur. Opioids and other CNS depressants can also relax airway muscles, decrease upper airway patency, disengage protective arousal mechanisms, and cause obstructed breathing, which further reduces effective ventilation in the setting of decreased respiratory drive [ 47 – 51 ].

CO 2 response curve during normal respiration, sedation, and CNS depressant exposure. This figure was modified from Benner et al. [ 46 ] under the terms of the Creative Commons Attribution 4.0 International License ( https://creativecommons.org/licenses/by/4.0/ ). The CO 2 response curve functions as a graphical depiction of the linear relationship between increases in PaCO 2 and alveolar ventilation. These response curves vary for each person within a population, with roughly a 1 to 4 L/min increase in minute ventilation for each 1 mm Hg increase in PaCO 2 . As the CO 2 response is dampened, a right-shift occurs. CNS central nervous system, PaCO 2 partial pressure CO 2

Clinical Manifestations of Respiratory Depression

The clinical manifestations of respiratory depression and failure exist on a continuum and may vary depending on the reason(s) for onset (i.e., disease and/or drug-induced) and the severity of hypoventilation, onset of hypercapnia, and degree of respiratory acidosis [ 52 ]. Typically, during the early stages of respiratory depression, patients are asymptomatic or may experience anxiousness and dyspnea upon exertion [ 52 ]. As the degree of respiratory depression progresses, dyspnea at rest, disturbed sleep, and daytime hypersomnolence can occur. With continued progression, cyanosis, delirium, somnolence, asterixis, seizures, and papilledema may occur and be followed by respiratory failure that requires ventilatory support or death [ 52 ]. Progression to respiratory failure may not follow this continuum for all patients and can be exacerbated with the use of CNS depressant agents or from the presence of underlying conditions that impact one or more steps in respiration.

Respiratory Depressants

Pharmaceutical and recreational agents classified as CNS depressants can suppress one or more steps in respiration and patency. Common agents that are known as inducers of respiratory depression are benzodiazepines, barbiturates, Z drugs, opioids, and ethanol [ 53 ]. These different classes suppress respiration by affecting various components of the respiratory system (Table  1 ; Fig.  5 ) [ 18 , 47 – 51 , 54 – 62 ].

Table 1

Effects of CNS depressants on respiration

CNS central nervous system, GABA A gamma-aminobutyric acid A , ORL-1 opioid receptor-like 1

Effect of CNS depressants on respiration. Pharmaceutical and recreational agents such as barbiturates, benzodiazepines, Z drugs, opioids, and ethanol can suppress multiple steps in respiration to cause respiratory depression

Barbiturates

Barbiturates are a class of sedative-hypnotic drugs used for a variety of purposes [ 54 ]. Phenobarbital and primidone are often used in the treatment of seizures, amobarbital is used as an investigative agent for the neurological assessment of cerebral hemispheres, and secobarbital is used for insomnia [ 54 , 63 ]. Barbiturates generally act on GABA A receptors by increasing the amount of time the chloride ion channel is opened, which increases GABA receptor affinity (Table  1 ) [ 54 ]. These drugs can also act to increase chloride influx in the absence of GABA, which further depresses the CNS [ 54 ]. Ultimately, these processes can lead to suppression of upper airway patency and central neuronal control of ventilation [ 55 ].

Benzodiazepines

Benzodiazepines are widely prescribed in general practice for their anxiolytic, sedative, anticonvulsant, and muscle relaxant effects [ 64 ]. These agents bind directly to various GABA A receptor subtypes and increase the inhibitory effects of endogenous GABA (Table  1 ) [ 18 , 56 ]. Benzodiazepines can depress central respiratory drive, chemoreceptor responsiveness to hypercapnia, peripheral chemoreceptors, and inspiratory and expiratory respiratory muscle strength in a dose-dependent manner, thus reducing respiration [ 47 , 57 , 58 ]. Benzodiazepines can also cause upper airway obstruction via relaxation of the tongue and neck [ 47 ]. Dose-dependent effects on reducing resting ventilation and the ventilatory response to hypoxia and hypercapnia can occur [ 18 ]. As such, benzodiazepine use has been associated with an increased risk of respiratory depression, especially when used in combination with opioids and alcohol [ 58 , 65 ].

Z Drugs

Nonbenzodiazepine hypnotics, such as zolpidem and zaleplon, are also referred to as Z drugs and are extremely useful in the treatment of insomnia owing to their quick onset and short duration [ 56 ]. Similarly to benzodiazepines, they too act at the GABA A receptor, but in a more specific manner (Table  1 ) [ 56 ]. A consequence of their greater specificity is less anxiolytic and anticonvulsant activity [ 56 ]. Similarly to benzodiazepines, these drugs may cause respiratory depression by suppressing central respiratory drive, decreasing respiratory muscle strength, and increasing upper airway resistance [ 59 , 60 ]. However, a recent clinical study found differential effects on the respiratory arousal threshold, with no reduction in upper airway muscle activity or alteration of airway collapsibility during sleep—instead, muscle activity increased during airway narrowing [ 66 ].

Opioids are often prescribed for acute and chronic pain relief, as they provide analgesia by acting on mu-opioid receptors located throughout the CNS (Table  1 ; Fig.  6 ) [ 47 , 50 , 51 ]. Opioid-induced respiratory depression can occur through the suppression of respiratory drive by interacting with neurons in the pons and medulla, including the pre-Bötzinger complex, or suppressing peripheral or central chemoreceptors, which impacts respiratory rhythm and blunts the normal response to hypoxemia and hypercapnia (Fig.  7 ) [ 47 – 51 ]. Opioids also suppress neural signals to the upper airway dilator muscles, thereby impacting upper airway patency [ 61 ].

Opioid effects on central neural control. Opioids interact with mu-opioid receptors located throughout the respiratory centers in the brainstem, which under certain circumstances may suppress various components of respiratory drive. Pre-BötC pre-Bötzinger complex

Feedback response to hypercapnia. Hypercapnia results from a stimulus such as an inhaled gas mixture, a disease, or CNS depressants. Respiratory drive depends on the activity of central and peripheral chemoreceptors that respond to changes in CO 2 and pH levels in the blood and CSF. A feedback loop among central and peripheral chemoreceptors, central neural control, and respiratory muscles and lungs allows for coordination of respiration to normalize CO 2 and pH levels. Measuring the ventilatory response to hypercapnia is a common research assessment that reflects the ability of central chemoreceptors to carry out a normal respiratory response. BBB blood–brain barrier, CNS central nervous system,  CSF cerebrospinal fluid, MV minute ventilation, PaCO 2 partial pressure of carbon dioxide

Based on their agonistic activity at the mu-opioid receptor, the impact on respiratory drive and extent of respiratory depression varies among different opioids [ 67 , 68 ]. The risk of respiratory depression is greater with classic full mu-opioid receptor agonists, such as morphine, hydrocodone, and oxycodone, than it is with atypical opioids, such as tramadol, tapentadol, and buprenorphine, because these atypical opioids are mixed-mechanism drugs [ 1 , 48 , 67 , 69 – 71 ]. For example, in addition to being full mu-opioid receptor agonists, tramadol and tapentadol have been shown to activate descending pain inhibitory pathways through norepinephrine reuptake inhibition, with tramadol also acting as a serotonin reuptake inhibitor [ 72 , 73 ]. Buprenorphine is a partial mu-opioid receptor agonist, an antagonist at the kappa- and delta-opioid receptors, and a full agonist at opioid receptor-like 1 [ 74 ]. Of the classic and atypical opioids, buprenorphine has unique partial agonism and signaling profiles at the mu-opioid receptor, which may result in a ceiling effect on respiratory depression but not on analgesia (Fig.  8 ) [ 67 , 70 , 75 – 79 ]. Mu-opioid receptors are coupled to inhibitory proteins that, upon activation, separate from one another to engage in a variety of intracellular signaling cascades that depress neural functions [ 79 ]. Full mu-opioid receptor agonists recruit (an adaptor protein) that is associated with signaling events that lead to poor outcomes such as respiratory depression, whereas the biased signaling mechanism elicited by buprenorphine limits β-arrestin recruitment, which may contribute to an enhanced safety profile [ 74 ].

Biased signaling: full vs partial mu-opioid receptor agonists. Buprenorphine has less intrinsic activity compared with full mu-opioid receptor agonists, which translates to downstream signaling events that lead to effective analgesia but a decreased risk of respiratory depression

Alcohol is a widely used and abused recreational substance that contributes to approximately 15% of opioid overdose deaths [ 62 , 80 ]. Alcohol is primarily metabolized in the liver by alcohol dehydrogenase to acetaldehyde, which increases CNS inhibition and decreases excitation through interactions with GABA receptors (Table  1 ) [ 62 ]. This can suppress central neuronal control and upper airway patency, thereby resulting in respiratory depression [ 62 ].

Diphenhydramine may also impact respiratory drive (Table  1 ) [ 81 ]. In addition to its use in the relief of allergy symptoms, diphenhydramine is frequently used to treat pruritus and nausea in patients who have received neuraxial opioids and was found to augment the interaction between hypoxic and hypercapnic ventilatory drive in healthy patients [ 81 ]. However, additional research is needed to determine any additional effects of diphenhydramine on respiratory drive.

Herbal Supplements

Patients with chronic pain are likely to seek out herbal supplements for pain relief, which may be used in conjunction with prescription medications [ 82 ]. Some common supplements utilized for their analgesic properties, but that may also suppress the CNS, include kratom, synthetic cannabinoids, valerian root, and kava [ 82 – 86 ]. Although additional research is needed to determine the specific effects of these agents on respiratory drive, clinicians must provide proper education to patients with chronic pain regarding the risks associated with concomitant use of supplements with their current prescriptions.

Concomitant Use

When analgesic drugs from different classes are combined, the resulting effects are typically synergistic rather than merely additive [ 87 ]. Synergy between CNS depressants is more common when drugs acting primarily on GABA receptors (e.g., barbiturates, benzodiazepines, Z drugs, ethanol) are combined with drugs acting on other receptor types (e.g., opioids) [ 87 ]. It is especially important to note that respiratory depression is a potentially fatal complication of opioid use and may be exacerbated by simultaneous ethanol intake [ 88 ]. One clinical trial showed that ethanol ingestion with concomitant oxycodone use resulted in clinically relevant ventilatory depression to a greater extent than when either agent was used alone [ 88 ]. The combination of an opioid, benzodiazepine, and the skeletal muscle relaxant carisoprodol is commonly referred to as the “Holy Trinity,” and these medications behave synergistically to induce respiratory depression, which could collectively result in death [ 89 , 90 ]. Compared to treatment alone, the combination of opioids with benzodiazepines or alcohol has been shown to increase the risk of serious adverse respiratory events by 31% [ 91 ]. Thus, clinicians must take caution when prescribing agents that can collectively enhance the potential for serious adverse events caused by CNS depressants [ 92 ].

In addition to sedative herbal supplements and antihistamines, the concomitant use of prescription analgesics with other drug classes such as gabapentinoids can also lead to increased CNS depression [ 82 , 93 ]. The prescription of the gabapentinoid drugs gabapentin and pregabalin has recently increased because physicians and patients are seeking opioid alternatives for pain relief during the opioid crisis; however, gabapentinoids are often being co-prescribed with CNS depressants, such as opioids, thereby increasing the risk of life-threatening and fatal respiratory depression [ 93 , 94 ]. The increased risk of adverse events with concomitant use of CNS depressants and other medications and supplements must be considered prior to the prescription or recommendation of multiple agents.

Assessment and Risks

To promote patient safety, it is important that clinicians have a deep understanding of respiration as a whole, and how various pharmaceutical and recreational agents can impact each step of the process to cause respiratory depression. If a patient presents with chronic pain and opioid treatment is under consideration, a risk–benefit analysis should be performed, including the assessment of factors that influence respiration. Medical history should be carefully reviewed to identify conditions known to increase the risk of respiratory depression.

Stroke, encephalitis, and brainstem diseases decrease central respiratory drive and minute ventilation, thereby limiting respiratory function [ 45 ]. Primary spinal cord/lower motor neuron and muscle disorders, thoracic cage disorders, and metabolic disorders can decrease respiratory neuromuscular and thoracic cage function and lead to respiratory depression [ 45 ]. In addition, gas exchange abnormalities, such as pulmonary embolism and vascular disease, chronic obstructive pulmonary disease, fever, metabolic acidosis, and upper airway disorders, can decrease ventilation [ 45 ]. Congestive heart failure or chronic liver and kidney diseases may make some patients (i.e., the elderly) more sensitive to certain medications because of inadequate clearance or metabolism by the liver or kidney. This can cause fluid retention, which ultimately impairs diaphragm function and lung volume/reserve [ 95 ]. These and other underlying factors must be considered prior to prescribing CNS depressants, as they can cause synergistic depression of a patient’s respiratory function.

Assessment of sleep disturbances is a key metric for evaluating patient risk as well as for monitoring opioid therapy [ 96 ]. During an exam, high blood pressure, body mass index (> 28), age (≥ 50), neck circumference (> 17″ for men; > 16″ for women), and sex (male) should be taken into account, as these factors may indicate greater risk of moderate to severe obstructive sleep apnea (OSA) [ 96 – 98 ]. A history of snoring, tiredness, and obstruction symptoms are also indicative of OSA [ 96 – 98 ]. Referral to a sleep specialist may be appropriate for some patients so that OSA can be diagnosed and treated or the response to opioid therapy can be gauged [ 99 , 100 ]. If a patient is receiving automatic positive airway pressure, the pressure can be interrogated to determine appropriate settings, but the optimization of sleep-related breathing therapies should be done in conjunction with a sleep specialist [ 100 ].

If sleep-related breathing is not an issue, incentive spirometry can be performed at home as a preventative measure to increase functional residual capacity. O 2 saturation can be measured independently using arterial blood tests or as a noninvasive measurement of the percentage of saturated hemoglobin in the capillary bed using co-oximetry with a pulse oximeter [ 101 ]. Preference may also be to utilize the 6-min walk test, an exercise test that can be predictive of underlying lung issues [ 102 ].

If a patient is taking multiple medications, referral to a pain specialist may also be considered to optimize the use of multimodal agents without increasing the risk of respiratory depression [ 100 , 103 ]. Prescribing CNS depressant medications should be a collaborative decision made by the multidisciplinary team responsible for a patient with chronic pain, including having full awareness of potential side effects, access to antidotes, and ensuring proper patient education regarding the administration and storage of medications [ 100 ]. In regard to the prescription of opioids, medication selection of atypical opioids (buprenorphine, tramadol, tapentadol) versus full mu-opioid receptor agonists (e.g., morphine, fentanyl, oxycodone) may limit adverse events, especially in terms of respiratory depression [ 1 ]; however, buprenorphine is unique in that it has a ceiling effect on respiratory depression but not on analgesia [ 1 , 67 , 70 ]. In humans, intravenous (IV) administration of fentanyl has been shown to suppress minute ventilation in a dose-dependent manner, whereas suppression of minute ventilation with IV administration of buprenorphine plateaued as the dose increased [ 67 ]. For this reason, many clinicians feel buprenorphine may be better tolerated than conventional full mu-opioid receptor agonists for the treatment of chronic pain [ 1 ].

Patients with chronic pain are often exposed to one or more pharmaceutical or recreational agents that impact respiration. Respiratory drive is controlled by the activity of a network of neurons within the brainstem. Respiration is influenced by peripheral and central chemoreceptor input (supplies information regarding arterial pH and O 2 and CO 2 levels) and mechanoreceptor input (provides information about the mechanical status of the lungs and chest). These processes work in synchrony to ensure appropriate ventilation by the lungs via signaling to the primary muscles of respiration to initiate the exchange of CO 2 for O 2 . When one or more steps in respiration are suppressed, life-threatening respiratory depression can occur. Certain CNS depressant agents used alone or concomitantly (e.g., benzodiazepines, barbiturates, Z drugs, opioids, and ethanol) are well known to cause respiratory depression. Of note, opioid-induced respiratory depression has triggered a serious national health crisis, which may be mitigated through the prescription of medications less likely to impact respiratory drive, such as buprenorphine. Proper risk–benefit assessments, collaboration with a multidisciplinary team, medication selection, and patient education can aid in limiting the incidence of respiratory depression induced by pharmacologic agents or recreational substances. Thus, prior to prescribing certain medications, the risk of respiratory depression should be considered and precautions should be taken to reduce adverse outcomes, including death.

Acknowledgements

This article and the Rapid Service Fee were funded by BioDelivery Sciences International, Inc.

Medical Writing and Editorial Assistance

The authors thank Jessica Brunquell, PhD, of MedLogix Communications, LLC, Itasca, IL, for medical writing and editorial support, which was funded by BioDelivery Sciences International, Inc.

All named authors meet the International Committee of Medical Journal Editors (ICMJE) criteria for authorship for this article, take responsibility for the integrity of the work as a whole, and have given their approval for this version to be published.

Disclosures

Lynn R. Webster has received consultation, advisory board, and travel fees from Charleston Laboratories, Depomed, Egalet, Insys Therapeutics, Mallinckrodt Pharmaceuticals, Pfizer, Teva, and Trevena; consultation and travel fees from Alcobra, Bonti, Daiichi Sankyo, Elysium Health, Indivior, KemPharm, Pain Therapeutics, Pernix Therapeutics, and Shionogi; advisory board and travel fees from BioDelivery Sciences International, Inc., Ensysce Biosciences, and Inspirion Pharmaceuticals; travel fees from Cara Therapeutics; and consultation fees from Jefferies, Merck, Trevi Therapeutics, Vallon Pharmaceuticals, and Vector Pharma. Suzanne Karan is a co-investigator for SAFeR which is sponsored by the ResMed Foundation.

Compliance with Ethics Guidelines

Data availability.

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Cellular respiration, the metabolic engine that powers life's myriad processes, is encapsulated within a deceptively simple equation, yet its implications reverberate throughout the vast tapestry of biological complexity: C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP This equation, more than a mere chemical formula, represents the alchemy of energy transformation within cells, where the conversion of glucose and oxygen fuels the synthesis of adenosine triphosphate (ATP), the molecular currency of vitality. But within this seemingly straightforward equation lies a […]

Unlocking Energy: the Essential Formula for Cellular Respiration

In the rich mosaic of existence, energy acts as a dynamic thread interlacing every creature, breathing vigor into life itself. From towering sequoias in age-old woods to minuscule beings in hidden worlds, energy pulses across life’s spectrum. Central to this dynamic is cellular respiration—a remarkable process that transforms the potential within organic molecules into usable energy that powers life's manifold forms. Picture a lively marketplace where simple goods are skillfully turned into prized items through detailed exchanges. Similarly, in the […]

Cellular Respiration: the Enigmatic Ballet of Energy Within Cells

Embarking on a mesmerizing journey into the microscopic realm, cellular respiration unfurls as a breathtaking performance, an intricate ballet of molecules that converges to transform nutrients into the lifeblood of energy within cells. This choreography, far from a mundane metabolic routine, is a captivating spectacle, where molecules pirouette and electrons waltz, weaving the intricate tapestry of life. Picture the cellular stage set with the spotlight on glycolysis, the rhythmic initiation of this metabolic symphony. In the cytoplasm, glucose takes center […]

The Powerhouse Formula: Cracking the Code of Cellular Respiration

Let's talk about something that's happening right now in every cell of your body, something so fundamental that life as we know it hinges on it. It's cellular respiration, the process that keeps the lights on and the engines running in the complex vehicle that is life. This isn't just biology textbook material; it's the very essence of energy transformation in living beings, from humans to the humblest of bacteria. Here's the lowdown: C6H12O6 + 6O2 -> 6CO2 + 6H2O […]

From Air to Energy: the Astonishing Saga of Cellular Respiration Explored

In the depths of every living cell lies a grand narrative, a saga that unfolds quietly yet relentlessly, shaping the very essence of life itself. This saga begins with a humble molecule, oxygen, drawn from the surrounding air. But what follows is a tale of transformation, a journey through the intricate pathways of cellular respiration, where molecules dance and react in a choreography of astonishing complexity. At the heart of this saga lies the mitochondrion, a tiny powerhouse within the […]

Breathing Life into Cells: a Deep Dive into the Mechanics of Cellular Respiration

In the realm of biology, the dance of life unfolds within the microscopic confines of our cells, where the intricate machinery of cellular respiration orchestrates the transformation of molecules into energy. As a biologist, I find myself captivated by the intricate choreography that unfolds within the cellular landscape, where every breath taken and molecule moved contributes to the symphony of life. At the heart of cellular respiration lies a delicate balance between chaos and order, where molecules collide and react […]

Breath of Life: the Awesome Outputs of Cellular Respiration

Let's talk about cellular respiration – it's like the unsung hero of our bodies, quietly working behind the scenes to keep us going. It's not just about breathing in oxygen and out carbon dioxide; it's an intricate dance of chemistry that powers every cell in our body. This process is as essential as your morning coffee, and trust me, it's way more interesting than it sounds. Picture this: cellular respiration is a three-act play, happening inside our cells. First up, […]

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Cellular Respiration - Free Essay Examples and Topic Ideas

Cellular respiration is the process by which cells convert nutrients into a usable form of energy called ATP. It is made up of three main stages: glycolysis, the citric acid cycle, and the electron transport chain. During these stages, glucose is broken down into smaller molecules, which undergo various chemical reactions to produce ATP molecules. Cellular respiration occurs in both plants and animals, and is essential for the survival of all living organisms.

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