The right lung is larger than the left in virtually every healthy human. CT scans put the difference at roughly 20 percent more air volume on the right side, and the size gap is not random — it exists because the heart and liver, two of the body’s largest organs, sit off-center and physically reshape the space available to each lung. That two-organ explanation sounds tidy, but the full picture involves lobe counts, airway geometry, diaphragm mechanics, and even molecular signals that set up left-right asymmetry before birth.
How Much Bigger the Right Lung Actually Is
A CT-based study measuring air volume during normal breathing found that the right lung held about 1,035 milliliters of air on average, compared to about 864 milliliters in the left lung. Both measurements came with wide person-to-person variation, but the right side was consistently larger, and the difference was statistically overwhelming. The right lung also packed air more densely into its tissue: its fractional air-volume concentration averaged about 0.68 versus 0.66 on the left.
1PubMed Central. Determination of regional lung air volume distribution at mid-tidal breathing from computed tomography: a retrospective study of normal variability and reproducibilityThose numbers translate into real functional differences. Under normal conditions the right lung handles about 50 to 55 percent of perfusion and roughly 55 percent of ventilation, while the left lung accounts for the rest. That split tracks closely with the size difference and matters in clinical settings like lung-function scanning, where interpreting results requires accounting for the natural right-left imbalance.
2PubMed Central. Scintigraphic Assessment of Lung Perfusion and Ventilation in Patients After PneumonectomyThe Heart Steals Space From the Left Lung
The most familiar explanation for the size difference is the heart. It sits slightly left of the body’s midline, and its bulk presses into the left lung, creating a concave indentation called the cardiac notch. You can see this in any anatomy diagram: the left lung has a scooped-out inner border that the right lung does not. This notch effectively removes a wedge of tissue that would otherwise be filled with functioning lung.
What’s less obvious is how much the heart’s position matters depending on how you’re lying down. When you’re on your back, the heart rests against the spine and compresses mainly the inner, rear portion of the left lung. Roll onto your side and the compression shifts: the heart sinks toward whichever lung is underneath, pressing on its front surface instead. The right lung, sitting farther from the heart, escapes most of this compression in any position.
3PubMed Central. Compression of the lungs by the heart in supine, side-lying, semi-prone positionsThe cardiac notch also explains why the left lung has only two lobes while the right has three. With less room to work with, the left lung is divided into an upper and a lower lobe, separated by a single oblique fissure. The right lung has enough space for an additional horizontal fissure that carves out a distinct middle lobe between the upper and lower ones. More lobes means more independent segments of tissue, which gives the right lung greater total surface area for gas exchange.
The Liver Pushes From Below
The heart gets most of the credit for making the left lung smaller, but the liver plays a subtler and often overlooked role. Sitting in the upper right abdomen, the liver is the heaviest internal organ besides the skin, and it pushes the right side of the diaphragm upward. Research on normal subjects has found that the right dome of the diaphragm sits higher than the left dome, and the right side of the diaphragm has a greater surface area — both effects attributed to the liver’s bulk underneath.
4American Journal of Respiratory and Critical Care Medicine. Diaphragm Dimensions After Single-Lung Transplantation for EmphysemaThis seems like it should shrink the right lung rather than the left, and in one sense it does: the right lung’s base is pushed slightly upward compared to where it would sit without a large liver underneath. But the right chest cavity is already wider than the left because the heart is displaced leftward, so the net effect still favors the right lung in total volume. The liver mostly affects the vertical dimension of the right lung, trimming it at the bottom while the heart carves a much larger chunk from the left lung’s medial surface. The left lung loses more overall.
Airway Architecture Is Asymmetric Too
The size difference is not limited to the lung tissue itself. The airways branching into each lung are also unequal. At the bottom of the trachea, the airway splits into two main bronchi. The right main bronchus is shorter, wider, and drops more steeply downward, while the left main bronchus angles off more gently and travels a longer path to reach its lung.
5PubMed Central. The comparison of the lengths and diameters of main bronchi measured from two-dimensional and three-dimensional images in the same patientsThe steeper, wider right bronchus is a direct consequence of the right lung being larger and needing more airflow. But it also has a well-known clinical side effect: inhaled objects tend to fall into the right bronchus more easily. Aspirated food, small toys in children, and accidentally swallowed dental hardware overwhelmingly end up on the right side. Doctors performing bronchoscopy or placing a double-lumen endotracheal tube need to account for these differences in angle and diameter on each side.
Beyond the main bronchi, the branching pattern continues to diverge. The right lung typically has ten bronchopulmonary segments, while the left has eight or nine, depending on how anatomists count the variations. Each segment has its own dedicated airway and blood supply, functioning almost as a self-contained unit, which is why surgeons can remove individual segments without necessarily sacrificing an entire lobe.
How the Diaphragm Moves Differently on Each Side
Because the diaphragm sits higher on the right and the structures pressing against each lung differ, the two halves of the diaphragm do not move in perfect symmetry during breathing. An ultrasound study of healthy adults measured diaphragmatic excursion — how far each half of the diaphragm travels with each breath. During deep breathing, the right hemidiaphragm moved an average of about 5.5 centimeters while the left moved about 5.3 centimeters, a statistically significant difference. Even during quiet breathing and sniffing, the two sides showed measurable asymmetry.
6PubMed Central. Diaphragmatic excursion by ultrasound: reference values for the normal population; a cross-sectional study in EgyptFor most people, this difference is too small to notice. But it becomes clinically relevant when doctors are evaluating diaphragm function, especially in patients on ventilators or those recovering from nerve injuries that affect breathing. Knowing what “normal asymmetry” looks like helps distinguish a healthy diaphragm from one that is weakened or paralyzed on one side. Without these reference ranges, a slight right-left mismatch might be misread as pathology when it is just anatomy.
Body Position Changes the Balance
The 55/45 split in ventilation between the right and left lungs is an average measured while people are upright or lying on their backs. Change your position and the distribution shifts. When you lie on your side, gravity pulls blood and air preferentially toward the lung that is lower, compressing it slightly but also increasing the blood flow through it. The upper lung expands more freely but receives less blood flow. A pilot study of healthy adults found that ventilation distribution changed significantly between right and left lateral positions, even when the same level of breathing support was applied.
7Trends in Anaesthesia and Critical Care. Distribution of pulmonary ventilation in the lateral decubitus position of healthy adults under Non-Invasive Mechanical Ventilation: a pilot studyThis is why critical care teams sometimes roll patients onto one side or the other — or even face-down in the prone position — to improve oxygenation. If one lung is badly diseased and the other is relatively healthy, putting the patient with the good lung down can improve the match between where air goes and where blood flows. The inherent size difference between right and left lungs adds another variable to these decisions: because the right lung already handles more of the work, losing right-lung function to disease or fluid buildup tends to cause more dramatic drops in blood oxygen than equivalent damage on the left.
Why Losing the Right Lung Is More Dangerous Than Losing the Left
The practical stakes of the size difference become starkest in surgery. A pneumonectomy, the complete removal of one lung, is sometimes the only option for certain cancers or severe infections. Removing the right lung is substantially more dangerous than removing the left. A meta-analysis that combined surgical data from multiple centers found that mortality after right pneumonectomy was about 3.4 times higher than after left pneumonectomy.
8The Annals of Thoracic Surgery. Risk of a Right Pneumonectomy: Role of Bronchopleural FistulaPart of the explanation is straightforward: removing the bigger lung leaves the patient with less remaining capacity. But complications also play a role. The right bronchial stump, where the airway is sutured shut after surgery, has a less robust blood supply than the left, which makes it more vulnerable to a dangerous complication called bronchopleural fistula — essentially, the sutured stump breaks down and creates a leak between the airway and the chest cavity. The combination of greater volume loss and higher complication risk makes surgeons cautious about right pneumonectomy, sometimes opting for lobe removal instead when the cancer’s location allows it.
Even outside of pneumonectomy, the size difference influences transplant planning. Donor-recipient lung matching takes volume into account, and a right lung from a smaller donor may not adequately fill the chest cavity of a larger recipient, or vice versa. Getting the fit wrong can impair the transplanted lung’s ability to expand fully and contribute to breathing.
The Asymmetry Is Set Before Birth
The left-right difference is not something that develops gradually as the body grows around the heart. It is baked in early during embryonic development. In the first weeks after conception, a cluster of molecular signals establishes left-right asymmetry throughout the body. Signaling molecules guide the heart to loop toward the right and eventually settle left of center, which in turn shapes the space that each developing lung bud can grow into.
9Current Biology. The origin of left-right asymmetry: left and right go to headDynamic MRI studies of children and adults have confirmed that the ventilatory asymmetry between the two lungs is present from an early age and changes as people grow. Right and left ventilatory volumes show considerable asymmetry in both their structure and the dynamics of how they expand and contract, and this pattern evolves over the lifespan.
10PubMed Central. Thoracic Quantitative Dynamic MRI to Understand Developmental Changes in Normal Ventilatory DynamicsWhen the molecular signaling goes wrong, the results can be dramatic. In a rare condition called situs inversus, the entire arrangement of internal organs is mirrored. The heart points right, the liver sits on the left, and the lung asymmetry flips: the left lung becomes the three-lobed, larger lung, and the right becomes the smaller one. People with situs inversus usually function normally because the mirror image is internally consistent — everything is reversed but still fits together. More problematic is situs ambiguus, where the reversal is incomplete and organs end up in a jumbled arrangement, which can cause serious heart and lung defects.
What Happens in Other Species
Lung asymmetry is not unique to humans, and some animals take it to an extreme that makes the human 55/45 split look modest. Snakes are the most striking example. Because their bodies are so elongated and narrow, there is simply no room for two full-sized lungs side by side. The right lung in snakes is always fully developed, while the left lung ranges from completely absent to vestigial to present but much smaller than the right, depending on the species.
11PLoS ONE. Heterochrony and Early Left-Right Asymmetry in the Development of the Cardiorespiratory System of SnakesThe developmental cues driving this asymmetry appear to share deep evolutionary roots with the same left-right signaling pathways found in mammals. In snake embryos, the timing of how these signals activate differs between the right and left lung buds, leading to vastly different growth trajectories for each side. It’s an extreme version of the same biological logic at work in your own chest: the body’s left-right patterning machinery does not build a symmetric pair and then trim one side down. Instead, asymmetry is the plan from the start, and each side develops according to its own schedule, shaped by the organs and body plan that surround it.
Among mammals, the details vary but the overall pattern holds. Most have a larger right lung with more lobes than the left. Dogs, cats, and cattle follow this rule. Some marine mammals that have adapted to deep diving show modifications in lobe number and airway reinforcement, but the right-side-dominant pattern persists. The consistency across such different body plans reinforces that lung asymmetry is an ancient, deeply conserved feature of vertebrate anatomy rather than a quirk specific to how the human chest happens to be shaped.