How Do We Breathe? — An Upside-Down Tree, and the Leaves Where Air Meets Blood

When You Hold Your Breath

If you hold your breath right now, it feels manageable at first. Then your chest feels tight, and the urge to breathe grows stronger and stronger. Breath-holding is something you do on purpose, but inside your body there is an involuntary force that usually overrides that intention and makes you breathe again (PMID 16272264).

We breathe all day long without usually being aware of it. When we speak, we pace our breath; when we climb stairs, we get short of breath; and even while we sleep, breathing continues. You may also have had a clip-like device attached to your fingertip at a hospital and seen a number appear on a screen.

This article follows what a single breath does. It covers where air goes and where it meets blood, where the signal that makes us breathe comes from, how dust that enters along with it gets back out, and even what the number on the fingertip measures and where it can go wrong. The diseases where breathing breaks down are handled by other pieces in this series.

The Upside-Down Tree — The Pathway of Air

The airway is the system of tubes through which air travels from the nose and mouth deep into the lungs. The lungs are organs, one on each side inside the chest, that exchange oxygen and carbon dioxide between inhaled air and the blood.

Air entering through the nose and mouth first passes through the back of the throat. The pharynx is the space at the back of the throat extending downward from behind the nose and mouth, serving as a shared passageway for both food and air. Below the pharynx, air and food separate paths. The larynx is the entrance to the airway, containing the vocal cords that produce sound. This junction where the food path and airway divide is covered in detail in "Swallowing, Speech and Language — The Crossing Where the Food Path Meets the Airway".

Past the larynx, the pathway descends into the chest. The trachea is the thick airway extending straight down from the larynx into the chest. The trachea splits into two branches inside the chest. The bronchi are the tubes that branch off from the airway coming down the neck into both lungs, becoming progressively narrower. Inside the lungs, the bronchi branch and branch again. This shape closely resembles an upside-down tree. The trachea is the trunk, and the bronchi are the branches.

At the ends of the branches hang leaves. Alveoli are tiny air sacs hanging like clusters of grapes at the ends of the airway, where oxygen and carbon dioxide are exchanged with the blood. Compared to a tree, the alveoli are the leaves.

The trunk, branches, and leaves have different jobs. From the trachea down to the smaller bronchi is generally a passageway for air to travel. These branches are much narrower in total surface area than the alveolar side, and the wall separating air from blood is also thicker. Therefore, the exchange of oxygen and carbon dioxide takes place on the alveolar side (PMID 23733690). Just as more leaves give a tree a wider surface area to catch sunlight, the number of alveoli formed determines the surface area available for gas exchange in the lungs (PMID 8815817).

The airway is the tree that carries air, and the place where oxygen and carbon dioxide are actually exchanged is the leaves at its end, the alveoli.

Inhalation and Exhalation — Muscles That Expand the Chest

Air enters when the chest expands. The diaphragm is a dome-shaped muscle stretching between the chest and the abdomen, and it is the main muscle that draws in a breath. It is also called the phrenic muscle. The contraction of the diaphragm is essential for breathing. If disease strikes the nerve traveling to the diaphragm, the muscle itself, or the junction between the muscle and the chest wall, the diaphragm may fail to do its job, and some people with trouble in only one side of the diaphragm experience no symptoms (PMID 30509855). When the diaphragm contracts, the inside of the chest widens, and outside air flows in along the airway by that much. Muscles between the ribs also pitch in to help widen the chest.

Exhalation during quiet rest takes almost no effort. Expanded lungs have the elasticity to return to their original size, so when the diaphragm relaxes, the lungs shrink and push the air out. As people age, the recoil force of the lungs decreases, and the strength of the breathing muscles also weakens. Even so, the entire breathing body fills the blood with the necessary oxygen throughout a lifetime, both at rest and during activity, so that oxygen in the arteries drops by only a small amount. However, the reserve to endure when an acute illness strikes decreases as age increases. Also, as people age, breathing increases less when oxygen is lacking or carbon dioxide rises, and the narrowing of the airway is sensed less, which can cause diseases to be noticed late (PMID 10836348).

Inhalation is the diaphragm expanding the chest to pull in air, and quiet exhalation happens on its own as the stretched lungs return to their place.

Air and Blood Meet in the Alveoli

The air you inhale travels all the way to the alveoli at the ends of the branches. That is where air and blood meet. Gas exchange is the process by which air and blood in the alveoli swap oxygen and carbon dioxide. The outside of each alveolus is wrapped in very thin blood vessels like a net. Capillaries are the narrowest vessels connecting arteries and veins, and they are the sites where oxygen and substances actually move back and forth between blood and tissues. In the lungs, that counterpart is the air inside the alveoli. Between the alveolar air and the capillary blood lies only a single thin wall.

Crossing that wall is handled by diffusion. Diffusion is the process by which a substance naturally spreads from where it is more concentrated to where it is less concentrated. Alveolar air has more oxygen than the blood returning from a trip around the body. Therefore, oxygen crosses from the alveoli into the blood. Carbon dioxide is the reverse; it is abundant in the blood and scarce in alveolar air, so it crosses from the blood into the alveoli and is carried out in the exhalation.

As long as the wall does not block the passage of gases, the oxygen and carbon dioxide in the blood leaving the lungs become similar to the average values of the alveolar air. What determines the value of that alveolar air is how much air flows in and out of the alveoli (PMID 9052034). Breathing, ultimately, is the process of refreshing the air in the alveoli.

The surface area for gas exchange varies depending on how it is measured. One study that calculated it backward based on the degree to which gas actually crosses in a resting, sitting person obtained about 18.7 square meters, which is much smaller than the value obtained by measuring lung tissue under a microscope. The authors attribute this to the fact that the lungs have a large reserve that is left unused (PMID 39954718).

Oxygen that has crossed into the blood is loaded onto red blood cells. Hemoglobin is the protein filling red blood cells that the blood uses to carry oxygen. The oxygen in the blood is divided between the portion dissolved in the blood and the portion attached to hemoglobin. The portion attached to hemoglobin is what we will meet again later as the number on the fingertip (PMID 17449336). When oxygen-carrying blood reaches various parts of the body, the oxygen leaves the red blood cells, passes through the liquid part of the blood and the vessel wall, and crosses into the cells (PMID 2673661).

There is also a separate pathway that sends blood to the capillaries surrounding the alveoli and receives it back. The pulmonary circulation is the short route from the heart to the lungs and back to the heart. It is one of the two paths by which the heart sends blood, and "How the Heart Moves Blood — Four Chambers, Four Doors, Two Loops" follows it alongside the other path that tours the entire body.

The pulmonary circulation has a habit unique to this pathway. When oxygen is low, the blood vessels in the lungs narrow. This response is unique to the pulmonary circulation and is one of the self-regulatory mechanisms that automatically adjusts blood flow in the lung capillaries to the air entering the alveoli, matching air and blood (PMID 10523066). Since sending blood to some alveoli that receive little air would yield little oxygen to load, this device directs blood toward the areas receiving air. The situation is different when the entire lung breathes air low in oxygen. Even in healthy people, breathing air low in oxygen causes the lung vessels to narrow and lung blood pressure to rise, though the degree is usually not large (PMID 1483782).

In the alveoli, oxygen crosses into the blood and carbon dioxide crosses into the alveoli, moving from where they are more concentrated to where they are less concentrated. The blood loads that oxygen onto hemoglobin and carries it through the heart to the whole body.

The Signal to Breathe — The Brain and the Sensors

If you hold your breath and then let go, your breathing becomes deep and fast for a while. That signal comes from the brain. The brainstem is the stem-like part at the bottom of the brain that connects toward the neck. A net of nerve cells that manages breathing sits in the brainstem, and this net adjusts the activity of the nerves that move the breathing muscles. This net receives reports from sensors and also gets signals from the front part of the brain—which bundles together various tasks like speaking or moving (PMID 18784597). It is thanks to these signals that you can steady your breath to speak.

Chemoreceptors are sensors that read the levels of carbon dioxide and oxygen in the blood and inform the brain that manages breathing. These sensors are located in small lumps attached near the arteries of the neck and inside the brainstem (PMID 10501632).

Carbon dioxide plays a particularly large role in fine-tuning the usual strength of breathing. When the gas composition of the blood, especially carbon dioxide, moves outside the normal range, chemoreceptors spot this and change the amount of air entering the alveoli to reverse that change (PMID 9052034). Even a slight increase in blood carbon dioxide greatly increases breathing (PMID 10501632). Conversely, when carbon dioxide drops below normal, breathing decreases while asleep or under anesthesia, and can even stop (PMID 10501632).

The oxygen sensors do not rest either. The sensors in the neck read oxygen and steadily keep waking up the nerve cells that manage breathing even during normal times, and they strongly boost breathing when oxygen drops (PMID 10501632). The sensors inside the brainstem are especially important for protecting breathing during sleep, while other parts of the regulatory apparatus can keep breathing going when awake. This role during sleep has been determined from observations of human illness and laboratory studies (PMID 10501632).

Let us return to holding your breath. Even while holding your breath, the rhythm of breathing made by the brain appears to keep running. A person merely holds back that rhythm from showing and keeps the chest fixed at the desired size. After taking a deep breath or breathing in a lot of oxygen, when blood carbon dioxide is low, one can hold out longer; the opposite, or when the body is doing a lot of work, makes it break out early. Yet the exact moment when breathing breaks out is not explained by any single factor among the degree to which the lungs have shrunk, the gases in the blood, or the sensors in the neck. A 2006 review notes that there is still no simple explanation that encompasses all of these (PMID 16272264).

There are also unresolved aspects to why breathlessness occurs when climbing stairs. The increase in breathing during exercise is thought to be produced jointly by signals coming from nerves and signals carried in the blood, but a 1992 review noted that the exact nature of those signals remains a matter of debate (PMID 1314799).

The rhythm of breathing is made by the brainstem, and its strength is adjusted mainly by reading carbon dioxide in the blood. There are also separate sensors that boost breathing when oxygen drops.

How Dust That Enters Is Removed

Every time we breathe, dust and microorganisms come in along with the air. Breathing tissue faces billions of particles every day. If these accumulate, the tissue can be damaged or infected, which can disrupt airflow and gas exchange (PMID 30431350).

The airway catches and pushes these out. Cilia are very fine hairs densely packed on the inner cellular surface of the airway, moving like waves. Mucus-cilia clearance is the process in which mucus covering the inside of the airway traps incoming particles and pathogens, and cilia push that mucus out of the airway. This work is the primary innate defense of the lungs. The mucus layer covering the top, the watery fluid layer beneath it, and the cilia that sequentially wave and push the mucus work together (PMID 27864314). In the upper airway, cilia move mucus toward the stomach so that trapped things are swallowed (PMID 20525503).

For mucus to flow well, the moisture must be right. How quickly mucus moves depends heavily on how thick the mucus is. When healthy, airway cells regulate the movement of salt and water to maintain the moisture of the mucus (PMID 35001665). When diseased and mucus becomes excessively thick, it sticks to the airway walls, forms clumps, and blocks airflow (PMID 35001665). In common lung diseases such as COPD or asthma, the failure to clear mucus properly plays a part in worsening the disease (PMID 30431340), and these diseases are covered in other parts of this series.

Coughing also helps. Mucus is cleared by cilia and airflow, and that flow includes the flow created by coughing (PMID 35001665). Ciliary movement and coughing together push out particles and pathogens (PMID 30431340). Coughing and swallowing are interlocked reflexes that share the same purpose of protecting the airway (PMID 24297325). What happens when this interlocking goes wrong is covered in "Is Aspiration Pneumonia Really Caused by Aspiration?".

The inside of the airway is covered by a moving belt where mucus catches and cilia push upward, and coughing repeats that work.

The Number at Your Fingertips

A small device clipped to a fingertip at the hospital displays a number in just a few seconds. That number is the story of hemoglobin we saw earlier. Oxygen saturation is the percentage of hemoglobin in the blood that is carrying oxygen. A pulse oximeter is a device that estimates oxygen saturation by shining light onto a fingertip or earlobe and reading the light that has passed through the blood vessels beneath the skin. The device emits light of specific wavelengths and receives and reads that light after it has passed through the blood vessels under the skin (PMID 37900526). The lights used are red light and infrared light (PMID 40849175). What this device estimates is the oxygen saturation of arterial blood, and it also measures the pulse rate at the same time (PMID 42351472).

This number is an estimate. The exact value is obtained by drawing blood from an artery and measuring it directly, and the pulse oximeter's value is an estimate of that value using light (PMID 38368234). There are also several conditions that shake the number. One literature review gathered these conditions and added that the evidence varies widely across studies. If the skin is hot, if there is a genetic abnormality in hemoglobin, or if ambient light enters, it can be read higher than the actual value. When blood circulation is poor or fingers are cold, or in the presence of nail polish, tattoos, or anemia, it can be read lower than the actual value. Hand movement and a fast pulse exceeding 150 beats per minute blur the signal (PMID 38599519). In a systematic review gathering adults with poor circulation, 75% of the devices were judged accurate, and new devices and those attached to the earlobe performed better, but only one study examined skin color (PMID 35119597).

When oxygen saturation drops into a very low range, the accuracy of the device itself falls (PMID 21718156). This is because the device is calibrated using healthy volunteers, and since there is a limit to how safely oxygen can be lowered, the lower range is estimated and extrapolated (PMID 37900526). Several papers note that in this range the device reads higher than the actual value (PMID 37900526, PMID 42105110, PMID 38806220), and a literature review also notes that it reads lower (PMID 38599519). Whether high or low, the number is read together with the patient's overall condition (PMID 40849175).

Skin color also shakes the number. Evidence has accumulated in several systematic reviews that in people with darker skin, the oximeter may read higher than the actual arterial value, and the difference appears to grow larger as oxygen levels drop (PMID 38368234, PMID 41045137). One literature review notes that it is not yet clear how clinically meaningful that difference is (PMID 38368234), while another literature review notes that this can delay the detection and treatment of low oxygen (PMID 41045137).

Hypoxemia is a state where the blood in the arteries lacks sufficient oxygen. There are cases where the oximeter shows a fine value but hypoxemia is actually present, and such occurrences may be more frequent in people with darker skin, though they are not uncommon in people with lighter skin either. Viewed as a whole group, even if the average difference between the oximeter and the actual value is almost zero, the values for individual people can deviate quite widely from reality, and in many people the reading may come out higher than actual. The accuracy stated by manufacturers is measured in healthy people and may differ in hospital patients (PMID 38806220).

The threshold used to make decisions like whether to give more oxygen after looking at that number is determined by clinical guidelines and medical staff. Guidelines for newborn care have also been revised multiple times based on the results of large clinical trials (PMID 30365906). The place where measurement discrepancies changing judgments cause the most trouble is often right around those commonly used thresholds (PMID 42105110). A review discussing use at high altitudes notes that normal values at any given altitude should be viewed as a range rather than a single number, and should not be judged by small differences over time or small differences between people (PMID 21718156). A review covering oxygen therapy in newborns notes that oxygen saturation alone cannot tell whether oxygen is reaching tissues in abundance (PMID 30365906).

Therefore, several reviews note that the oximeter's values should be read together with the patient's overall condition and other clinical information (PMID 40849175, PMID 36166259). This device is cited as the monitoring device that has had the greatest impact on caring for breathing patients (PMID 38806220), and a review covering the perioperative period notes that it greatly improved patient safety by allowing early detection of hypoxemia (PMID 42351472). Which signs to treat as danger signals when experiencing shortness of breath are covered following guidelines in other pieces of this series on colds, pneumonia, and asthma.

The number on a pulse oximeter is an estimate guessed through light, and the threshold for reading that number is up to guidelines and the medical staff.

Following a Single Breath

When the diaphragm contracts and the chest widens, outside air enters through the nose. The air passes through the pharynx and larynx down into the trachea, and travels along the branching bronchi to the very tips of the tree. Some of the dust that enters along the way gets caught in mucus and is pushed back out of the airway by the wave of cilia (PMID 27864314).

The oxygen in the air that reaches the leaves crosses the thin wall to spread into the blood of the capillaries and is loaded onto hemoglobin. The oxygen and carbon dioxide in the blood leaving the lungs have become similar to the alveolar air (PMID 9052034). That blood returns to the heart along the pulmonary circulation and goes out to the whole body, and when it reaches the muscles and brain, the oxygen leaves the red blood cells and crosses over to the cells (PMID 2673661). The carbon dioxide produced by the cells while working is loaded onto the blood and comes back to the lungs.

In the alveoli, carbon dioxide crosses from the blood into the air. When the diaphragm relaxes, the stretched lungs return to their place and send that air out. When carbon dioxide in the blood increases, chemoreceptors read this to deepen breathing and exchange more alveolar air to reverse that change (PMID 9052034, PMID 10501632). When oxygen drops, sensors in the neck stimulate breathing (PMID 10501632). Upon the rhythm made by the brainstem, the values read by the sensors adjust the intensity of the breath (PMID 18784597).

A single breath is a loop that draws air down to the alveoli, loads oxygen onto the blood, sends out carbon dioxide, and has sensors read the results to adjust the intensity of the breathing.

Where to Look Next

How blood carrying oxygen from the lungs passes through the heart to the entire body is traced in "How the Heart Moves Blood — Four Chambers, Four Doors, Two Loops". The narrowing or closing of the airway in the back of the throat during sleep is covered in "Insomnia and Sleep Apnea — The Alarm That Won't Switch Off, the Airway That Closes", and the place where the food path and the airway diverge is found in "Swallowing, Speech and Language — The Crossing Where the Food Path Meets the Airway". A more general guide on how to read whether measured values are correct or incorrect is dealt with in "What a Test Actually Tells You — An Introduction to Sensitivity, Specificity, and Pretest Probability".

How to read the report of a pulmonary function test, which measures the volume and speed of the breath, is traced in the piece right after this one, "Reading a Spirometry Report — One Point Among Your Peers, Two Graphs Drawn by a Breath".

Diseases where breathing breaks down are each handled by other parts of this series: "How Do a Cold, the Flu, and Pneumonia Differ? — An Infection That Stays in the Entryway, and One That Reaches the Inner Room", "What Is Asthma? — An Airway That Tightens Too Easily, and the Inflammation That Keeps It Swollen", "What Is COPD? — A Balloon That Won't Empty, and the Breath That Leaves in the First Second", "What Is Tuberculosis? — The Sleeping Ember, and the Fire That Takes Months to Put Out", "Lung Cancer and Low-Dose CT Screening — Reading a Small Spot in the Picture Over Time", "What Is Pulmonary Fibrosis? — Thickening Paper Windows, Scars on the Wall Oxygen Crosses", and "Bronchiolitis, RSV and Croup — In a Thin Straw, the Same Swelling Blocks More of the Way".

The moment when breath-holding finally bursts and the signal of increased breathlessness during exercise are problems noted by the review articles this piece relies on (from 2006 and 1992) as not yet resolved into a single explanation.

Source Papers

• "Breath-holding and its breakpoint." (2006) — PMID 16272264 — Breath-holding is voluntary, but healthy individuals cannot hold their breath until losing consciousness; involuntary mechanisms override it, and central respiratory rhythm continues even while holding. It is prolonged by large lung expansion, hyperoxia, and hypocapnia, and shortened by the opposite conditions and increased metabolic rate. The breakpoint is not explained by lung contraction, blood gases, or carotid chemoreceptors alone, and no simple explanation exists.
• "Airway gas exchange and exhaled biomarkers." (2011) — PMID 23733690 — The conducting airways serve as a passage for respiratory gases, with a much smaller surface area and thicker barrier than the alveolar region; other gases under special conditions are exchanged in the airways as well, but this does not apply to respiratory gases.
• "Formation of pulmonary alveoli and gas-exchange surface area: quantitation and regulation." (1996) — PMID 8815817 — Alveolar formation determines the magnitude of the gas-exchange surface area, and alveolar number is calculated using new stereological methods.
• "Diaphragmatic dysfunction." (2019) — PMID 30509855 — The diaphragm is the primary respiratory muscle, and its contraction is essential for ventilation. Disease anywhere along the neural, muscular, or mechanical connection with the chest wall can cause dysfunction, and asymptomatic patients with unilateral dysfunction are observed.
• "Physiological changes in respiratory function associated with ageing." (1999) — PMID 10836348 — Aging reduces the static elastic recoil of the lungs and respiratory muscle strength, yet adequate gas exchange is maintained during lifelong rest and exercise (with a slight decrease in arterial oxygen partial pressure and no change in carbon dioxide). Reserves decrease during acute illness, and a blunted hypoxic and hypercapnic response of the respiratory center, along with reduced perception of bronchoconstriction, can delay diagnosis.
• "The respiratory system and homeostasis." (1996) — PMID 9052034 — Unless blocked by the exchange barrier, the oxygen and carbon dioxide partial pressures of blood leaving the lungs are similar to alveolar mean values, and alveolar ventilation determines these values. When blood gases (especially carbon dioxide) move outside the homeostatic range, chemoreceptors detect this and alter alveolar ventilation to restore them.
• "Measurement of gas exchange surface area from DLNO and DLCO." (2025) — PMID 39954718 — The alveolar-capillary membrane area of a seated, resting person, calculated by inserting diffusion coefficients into Fick's law, is 18.7 m², which is considerably smaller than stereological estimates, presumably due to a large functional reserve.
• "The role of hemoglobin oxygen affinity in oxygen transport at high altitude." (2007) — PMID 17449336 — Oxygen in the blood is divided into a dissolved fraction ($PO_2$) and a hemoglobin-bound fraction (saturation).
• "Theory of oxygen transport to tissue." (1989) — PMID 2673661 — A mathematical model review; oxygen passes through red blood cells, plasma, endothelial cells, the vascular wall, and extracellular and intracellular spaces. The diffusion-enhancing role of hemoglobin and other factors remain subjects of critical debate and unresolved issues.
• "Hypoxic pulmonary vasoconstriction." (1999) — PMID 10523066 — Hypoxic pulmonary vasoconstriction is unique to the pulmonary circulation; it is an autoregulatory mechanism that automatically matches pulmonary capillary blood flow to alveolar ventilation.
• "Pulmonary circulation in hypoxia." (1992) — PMID 1483782 — Hypoxia constricts pulmonary blood vessels; even in healthy individuals who breathed hypoxic air at sea level, pulmonary vascular resistance increased, and hypoxic pulmonary hypertension is generally moderate.
• "Abnormalities of respiratory control and the respiratory motor unit." (2008) — PMID 18784597 — Ventilation control involves a brainstem neural network regulating the activity of respiratory muscle motor neurons, receiving input from peripheral and central chemoreceptors (carbon dioxide and oxygen) and forebrain structures responsible for integrated behaviors such as speech and movement.
• "CO2, brainstem chemoreceptors and breathing." (1999) — PMID 10501632 — Breathing is regulated by carbon dioxide and oxygen chemical feedback. The carotid body senses oxygen, maintaining tonic excitation normally and exhibiting dramatic excitation during oxygen drops. Carbon dioxide feedback involves the carotid body and brainstem central chemoreceptors; a small increase in carbon dioxide causes a large increase in ventilation, whereas a subnormal decrease can lead to apnea during sleep and anesthesia. Central chemoreceptors are crucial for breathing during sleep, whereas other parts maintain it when awake (based on experiments of nature and laboratory evidence).
• "Potassium and ventilation in exercise." (1992) — PMID 1314799 — The drive to breathe during exercise is thought to be a combination of neural and humoral factors, but the exact regulatory signals remain debated.
• "Mucociliary Defense: Emerging Cellular, Molecular, and Animal Models." (2018) — PMID 30431350 — Respiratory tissues are exposed to billions of particles daily, which, if allowed to accumulate, can impair airflow and gas exchange through damage, inflammation, and infection. Mucus and cilia trap and clear inhaled materials.
• "Cilia and Mucociliary Clearance." (2017) — PMID 27864314 — Mucociliary clearance is the primary innate defense of the lungs, composed of the protective mucus layer, airway surface liquid layer, and cilia. Cilia beat in continuous waves to sweep pathogens and particles trapped in mucus out of the airways.
• "Cilia dysfunction." (2010) — PMID 20525503 — Mucociliary clearance transports mucus from the upper airways to the gastrointestinal tract, serving as the primary means by which the upper airways clear pathogens, allergens, debris, and toxins.
• "Physiology and pathophysiology of human airway mucus." (2022) — PMID 35001665 — Mucus clearance is the dominant mechanical defense of the lungs, achieved by cilia and airflow (including cough-dependent mechanisms). Clearance speed largely depends on mucus concentration; when healthy, ion transport maintains mucus hydration, whereas in disease, mucus becomes hyperconcentrated, forming plates and plugs that obstruct airflow.
• "Airway Epithelial Differentiation and Mucociliary Clearance." (2018) — PMID 30431340 — The mucociliary escalator removes particles and pathogens via the mechanical action of cilia and coughing; impaired clearance contributes to the pathophysiology of common chronic lung diseases such as COPD and asthma.
• "Airway protective mechanisms." (2014) — PMID 24297325 — Cough and swallowing are highly coordinated reflexes sharing the common purpose of protecting the airway; the pharynx is a common conduit for both air and food.
• "Accuracy of Pulse-Oximetry in Non-white Patients." (2025) — PMID 40849175 — In darkly pigmented skin, melanin absorption of red and infrared light sources leads to an overestimation of $SpO_2$ and the risk of occult hypoxemia; overall patient assessment should be considered alongside it.
• "Impacts of Skin Color and Hypoxemia on Noninvasive Assessment of Peripheral Blood Oxygen Saturation: A Scoping Review." (2023) — PMID 37900526 — Pulse oximeters emit specific wavelengths of light into the skin vascular bed of a finger or earlobe and detect them to estimate $SpO_2$. They are calibrated using healthy volunteers, and low ranges are extrapolated. When $SaO_2$ is below 90%, it is overestimated across all races (to a greater extent in minority races), potentially delaying treatment.
• "Effect of skin tone on the accuracy of the estimation of arterial oxygen saturation by pulse oximetry: a systematic review." (2024) — PMID 38368234 — $SpO_2$ is an estimate of actual $SaO_2$ and may be overestimated in darkly pigmented skin. Its clinical significance is unclear but likely increases as $SaO_2$ decreases; some studies show no difference, and meta-analysis was not feasible.
• "Determining factors of pulse oximetry accuracy: a literature review." (2024) — PMID 38599519 — Dark skin, hemoglobinopathies, and high skin temperature lead to overestimation, whereas low perfusion, cold skin, nail polish, tattoos, hypoxia, anemia, and high-altitude training lead to underestimation. Motion, ambient light, pulse rates exceeding 150 beats per minute, electromagnetic interference, and sensor placement distort the signal, and the evidence shows high heterogeneity.
• "Accuracy of pulse oximeters in measuring oxygen saturation in patients with poor peripheral perfusion: a systematic review." (2022) — PMID 35119597 — In adults with low perfusion, 75% of devices are accurate, with newer devices and earlobe placement being more accurate. Only one study controlled for skin color.
• "Do Differences in Skin Pigmentation Affect Detection of Hypoxemia by Pulse Oximetry: A Systematic Review of the Literature." (2025) — PMID 41045137 — Across 42 articles, consistent overestimation of $SpO_2$ was observed in darkly pigmented skin, especially at low saturations, with the potential for unrecognized hypoxemia and delayed critical treatment, systematic bias, and inconsistent methods of racial classification and skin color assessment.
• "Pulse Oximetry: 2023 Year in Review." (2024) — PMID 38806220 — Pulse oximetry is the monitoring device with the greatest impact on respiratory care. Occult hypoxemia may be greater in Black individuals, but cannot be ignored in white individuals either. While group mean bias may be close to zero, wide limits of agreement mean overestimation occurs in many individuals, and manufacturer accuracies derived from normal subjects may differ from clinical settings.
• "Pulse Oximetry, Skin Pigmentation, and Occult Hypoxemia in Pediatric Cardiac Critical Care." (2026) — PMID 42105110 — In darkly pigmented skin and hypoxia, $SpO_2$ frequently overestimates $SaO_2$, and misclassification is clinically most important near commonly used treatment escalation thresholds.
• "Pulse oximetry at high altitude." (2011) — PMID 21718156 — Device accuracy decreases at saturations below 80%. At high altitudes, normal saturations should be reported as a range, clinical decisions should not be made based on small differences over time or between individuals, and errors caused by cold extremities, ambient light, or poorly fitting probes should be minimized.
• "Hemoglobin oxygen saturation targets in the neonatal intensive care unit: Is there a light at the end of the tunnel? 1." (2019) — PMID 30365906 — Large clinical trials have led to multiple revisions of neonatal care guidelines, and oxygen saturation monitoring does not comprehensively assess tissue oxygenation.
• "Racial Disparity in Oxygen Saturation Measurements by Pulse Oximetry: Evidence and Implications." (2022) — PMID 36166259 — Overestimation and occult hypoxemia in darkly pigmented skin are potentially caused by red light absorption by melanin and a lack of darkly pigmented participants during calibration, and clinicians are advised to consider additional clinical information.
• "Pulse Oximetry-A Perioperative Perspective." (2026) — PMID 42351472 — Arterial oxygen saturation and pulse rate are assessed continuously and noninvasively; since its introduction, it has greatly improved patient safety by enabling early detection of hypoxia and physiological deterioration. Inaccurate readings can be caused by motion, low perfusion, abnormal hemoglobins, ambient light, and skin color.