Your lungs have five lobes in total: three in the right lung and two in the left. This asymmetry exists because the heart sits slightly left of center in your chest, taking up space that would otherwise be available to the left lung.
Right Lung: Three Lobes
The right lung is the larger of the two and contains three lobes: the upper lobe, the middle lobe, and the lower lobe. Two deep grooves called fissures separate them. The oblique fissure runs diagonally and divides the lower lobe from the upper and middle lobes. The horizontal fissure branches off from the oblique fissure and separates the upper lobe from the middle lobe.
Because it has three lobes and more overall volume, the right lung handles slightly more air exchange than the left. It also contains ten distinct segments, smaller subdivisions within the lobes that each have their own airway and blood supply. The upper lobe has three segments, the middle lobe has two, and the lower lobe has five.
Left Lung: Two Lobes
The left lung is smaller and narrower, divided into just an upper lobe and a lower lobe by a single oblique fissure. The front edge of the left lung has a curved indentation called the cardiac notch, where the heart and its protective sac (the pericardium) sit without being covered by lung tissue. Below this notch, a small tongue-shaped projection called the lingula extends from the upper lobe. The lingula is sometimes considered the left lung’s equivalent of the right middle lobe, and it contains two segments of its own.
In total, the left lung has eight or nine segments depending on classification, compared to ten on the right. The left lower lobe typically lacks a separate segment on its inner base because the heart occupies that space.
Why the Two Sides Aren’t Symmetrical
The size difference comes down to the heart’s position. Your heart is centered slightly to the left, pushing the left lung back and reducing its volume. This is why the left lung developed with two lobes instead of three, and why the cardiac notch exists. If the notch were absent, the left lung’s front edge would cover the heart entirely, which is an uncommon anatomical variation but one that does occur.
Anatomical Variations in Lobe Count
Not everyone has exactly five lobes. The most well-known variation is the azygos lobe, a small extra section in the upper right lung created when a vein called the azygos vein takes an unusual path through the lung tissue during fetal development. It shows up in roughly 0.4% to 1% of people and is almost always found on the right side (about 86% of cases in one study of over 4,500 patients). The azygos lobe is harmless and usually discovered incidentally on a chest scan. Incomplete fissures are another common variation, where the grooves between lobes don’t fully separate the tissue, effectively blurring the boundary between neighboring lobes.
What Lobes Mean for Surgery
The lobe structure of the lungs is clinically important because each lobe functions as a somewhat independent unit with its own airway and blood supply. This makes it possible to surgically remove a single lobe, a procedure called a lobectomy, without destroying the entire lung. Lobectomy is the most common surgery for early-stage non-small cell lung cancer. It’s also used for carcinoid tumors, small cell lung cancer, lung damage from severe infections, and tumors that have spread to the lungs from other parts of the body.
After a lobectomy, the remaining lobes gradually expand to fill some of the empty space, and most people adapt well over time. Having five lobes rather than one solid organ on each side gives surgeons the option to remove diseased tissue while preserving as much healthy lung as possible.