The lungs do not have formally named “posterior lobes,” but the term makes intuitive anatomical sense: the lower lobes of both lungs sit overwhelmingly in the back of the chest, extending from roughly the mid-thorax down to the diaphragm. When clinicians, radiologists, or respiratory therapists talk about posterior lung regions, they almost always mean these lower lobes and their constituent segments. The lower lobes account for roughly 60% of total lung tissue, and their posterior position has surprisingly far-reaching consequences for blood flow, disease patterns, and treatment strategies.
What “Posterior Lung Lobes” Actually Refers To
Each lung is divided by fissures into lobes: three on the right (upper, middle, and lower) and two on the left (upper and lower). The upper lobes occupy the front and top of the chest, while the lower lobes sweep behind the heart, tucking against the back of the ribcage and sitting on top of the diaphragm. If you placed your hand flat against someone’s upper back, between the shoulder blades and the waistline, you would be covering the lower lobes almost entirely. The middle lobe on the right side is a thin wedge that sits anteriorly and laterally, meaning it has almost no posterior footprint at all.
Within each lower lobe, the tissue is further divided into segments, each supplied by its own bronchus, artery, and vein. On the right, the lower lobe typically has five segments: the superior segment (S6) at the top of the lobe and four basal segments (medial, anterior, lateral, and posterior). The left side mirrors this layout with slight differences, usually having four or five segments. A CT-based study of over 200 lower lobes found that while the basic segmental arrangement is quite consistent from person to person, variation from the dominant pattern occurred in as many as 20% of cases, particularly in the number and position of the arteries feeding each segment.
Why Lower Lobes Sit So Far Back
The posterior dominance of the lower lobes traces partly to how the lungs develop in utero and partly to the constraints imposed by the heart and great vessels. During embryonic development, the lung buds branch off the foregut and expand into the surrounding tissue, with the visceral pleura folding inward to create the fissures that separate each lobe. The pleuroperitoneal membranes grow toward the posterior body wall, anchoring the developing lungs in a position where the lower lobes naturally extend behind the heart.
In an upright human, the heart occupies the anterior-central chest and pushes the lower lobes backward. This is a distinctly human arrangement. In four-legged mammals, what is “ventral” becomes “anterior” in our upright posture, meaning the spatial relationships between the heart and lungs shifted dramatically during the evolutionary transition to bipedalism. The lower lobes, no longer hanging beneath the heart, instead drape behind it. The result is a chest where the posterior thorax is overwhelmingly lower-lobe territory, and the anterior thorax is split between the upper lobes, the middle lobe, and the lingula (the tongue-shaped projection of the left upper lobe that acts as a functional equivalent of the right middle lobe).
The size difference matters clinically. Because the lower lobes constitute about 60% of the lung’s volume, any condition that preferentially affects posterior or lower lung zones involves a disproportionately large share of functioning tissue. A blood clot lodged in a lower-lobe artery, for instance, tends to have a more significant impact on both breathing and blood circulation than one affecting an upper-lobe branch of similar caliber.
Blood Flow Favors the Back
Even setting gravity aside, the posterior lower lobes receive more blood flow per unit of tissue than the rest of the lung. An animal study that mapped blood flow across the entire lung piece by piece found a region of consistently high blood flow in the dorsal area of the lower lobes in six of seven subjects, and this high-flow zone persisted regardless of whether the animal was upright, supine, or prone.
Gravity does add a further tilt, though. When you lie on your back, the posterior lung regions are the dependent (lowest) parts, and blood pools there under gravitational pull. One MRI-based study in healthy humans found that blood flow increased by 16 to 33% to whichever lung was in the dependent position during lateral lying compared to the supine baseline. Venous vessel cross-sectional area in the dependent lung also expanded by 22 to 27%, reflecting the engorgement of blood vessels under gravitational load. In contrast, the non-dependent lung became more distensible, stretching open its vessels even as it received less blood.
This interplay between structural blood-flow preference and gravitational redistribution means that in a typical supine patient, the posterior lower lobes are receiving an outsized share of the cardiac output. That has consequences for gas exchange: well-perfused regions need adequate ventilation to match, and when ventilation falls behind, these areas become hotspots for oxygen-carbon dioxide mismatch.
Ventilation Gradients and the Density Problem
The posterior lung regions are not just better perfused when you lie down; they are also denser. CT measurements of two human subjects found that the gradient in lung tissue density from front to back in the supine position was roughly twice as steep as in the prone position. One subject showed a density gradient of about −4.3% per centimeter when supine, compared to only about −2.7% per centimeter when prone. The corresponding pleural pressure gradients followed the same pattern, approximately 0.55 cmH₂O per centimeter supine versus about 0.28 cmH₂O per centimeter prone.
What does this mean in practical terms? When you lie on your back, the weight of the lung tissue above compresses the posterior regions, squeezing alveoli partially closed. The heart, sitting anteriorly, also presses down through the lung. This compression reduces the volume of air reaching the posterior lower lobes at exactly the moment blood flow there is highest, creating a mismatch that worsens gas exchange. In healthy awake people, this effect is manageable. In anesthetized or critically ill patients, it becomes a serious clinical problem. Studies of mechanically ventilated patients in the supine position have shown atelectatic (collapsed) areas developing in dependent lung regions, along with increased blood flowing through poorly ventilated tissue and a corresponding drop in blood oxygen levels.
Prone Positioning and Why It Works
Flipping a patient onto their stomach is one of the more dramatic interventions in critical care, and its effectiveness is directly tied to the anatomy of the posterior lung. Prone positioning was first described as a therapeutic strategy about 40 years ago and is now a well-established treatment for patients with acute respiratory distress syndrome (ARDS).
The logic follows from the physics described above. When a patient with ARDS is supine, the posterior lower lobes tend to be the most damaged, flooded, and collapsed regions, yet they still receive the majority of blood flow. Turning the patient prone makes the previously compressed posterior regions non-dependent. They open up, recruit collapsed alveoli, and begin participating in gas exchange again. Meanwhile, the anterior lung, which was relatively well-aerated in the supine position, is now dependent and takes on the higher blood flow. Because the density gradient is roughly half as steep in the prone position as in the supine, the overall distribution of air and blood becomes more even.
ARDS research frames this through two complementary models. The “sponge lung” model treats the edematous lung as a heavy, wet sponge: lower regions get squeezed by the weight above, and flipping the sponge redistributes the compression. The “shape matching” model focuses on how well the lung’s shape fits within the chest wall; the prone position allows the lung to inflate more uniformly because the heart no longer rests on the posterior lung tissue. Both models predict that prone positioning generates a more even distribution of gas-to-tissue ratios and more homogeneous lung stress, which is exactly what imaging studies confirm.
Diseases That Concentrate in the Posterior Lower Zones
Several lung diseases show a striking preference for the posterior lower lobes, and recognizing this pattern is one of the first things radiologists and pulmonologists learn.
Idiopathic pulmonary fibrosis (IPF) is a progressive scarring disease of the lung that typically begins in the posterior and lower zones. On physical exam, clinicians listen for early inspiratory crackles, which in IPF are predominantly located in the lower posterior lung fields. On high-resolution CT, the hallmark findings are peripheral, subpleural reticular opacities in the lower lobes, often accompanied by honeycomb-pattern destruction of the tissue. The posterior-basal predilection is so characteristic that its absence raises doubts about the diagnosis.
Aspiration pneumonia follows a related but mechanistically simpler pattern. When a person inhales food, liquid, or stomach contents, gravity directs the material into the most dependent airway. In a supine patient, this means the posterior segments of the upper lobes and the superior segments of the lower lobes (S6). In an upright or semi-reclined person, the aspirated material tends to settle into the basal segments of the lower lobes. Either way, the posterior lower lung bears the brunt.
Pulmonary embolism also demonstrates a lower-lobe bias, though the mechanism is vascular rather than gravitational. Because the lower lobes receive the lion’s share of pulmonary blood flow, clots traveling through the venous system and into the pulmonary arteries are statistically more likely to lodge in lower-lobe branches. When they do, the clinical consequences tend to be more severe than upper-lobe emboli because a larger fraction of total lung perfusion is compromised.
Tumor Location and Prognosis
Where a lung cancer arises within the lungs has implications beyond just the surgical approach. Research on primary tumor location in non-small-cell lung cancer has shown that the specific lobe of origin is associated with distinct patterns of lymph node involvement and distant metastasis. Lobe-specific location has been identified as an independent factor in prognosis, meaning a tumor in the right lower lobe may behave differently from an identical-size tumor in the left upper lobe, partly because of differences in lymphatic drainage and partly because of the vascular anatomy described earlier.
The practical importance of this for treatment planning is still being refined. Surgeons already tailor their approach based on which lobe is affected, but emerging data suggest that the specific segment within the lower lobe may also matter for predicting outcomes. This is one area where the growing precision of CT-based segmental mapping is starting to change clinical decision-making.
What Happens When a Lower Lobe Is Removed
When lung cancer or another condition necessitates removing an entire lobe (lobectomy), the posterior lower lobes present a particular challenge because they are larger and contribute more to overall lung function than the upper lobes. Preoperative imaging studies have shown that lower lobectomy removes roughly 27% of functional lung volume on average, compared to about 17% for upper lobectomy.
The body compensates for this loss. After any lobectomy, the remaining lung tissue expands to fill the space, and this expansion comes with a proportionate gain in function. The compensatory response is actually more vigorous after lower lobectomy than after upper lobectomy, as if the remaining tissue works harder to make up the bigger deficit. Despite the larger initial volume loss, the net functional loss at follow-up ends up similar for both: about 12% for upper lobectomy and about 14% for lower lobectomy, a difference that is not statistically meaningful.
Recovery from lower lobectomy does take longer, however. A multicenter longitudinal study found that patients who had a lower lobe removed reported greater interference with daily activities, mood, and work capacity during the first year after surgery compared to those who had an upper lobe removed. Median recovery time for daily activities was 15 days after lower lobectomy versus 4 days after upper lobectomy, and walking recovery took about 7 days versus 4 days. These differences likely reflect both the larger initial volume loss and the posterior location, which affects the mechanics of breathing differently than losing an anterior upper lobe.
The Subsuperior Segment, an Often Overlooked Variant
One anatomical feature of the lower lobes that surprises even some clinicians is the subsuperior segment, sometimes called S*, which sits just below the standard superior segment (S6) on the posterior surface of the lower lobe. Not everyone has one. A large imaging study found the subsuperior segment in about 32% of individuals, with a slight right-sided predominance (roughly 19% right versus 16% left). When present, it averages about 53 cubic centimeters in volume and is supplied by a single bronchus with an average diameter of about 2.5 millimeters. Most subsuperior segments have a single dedicated artery, though about 11% have two.
The subsuperior segment matters most during segmentectomy, a lung-sparing surgery where the surgeon removes individual segments rather than an entire lobe. If the subsuperior segment exists and the surgeon does not account for it, the resection margin may be inadequate, or the blood supply to adjacent tissue may be inadvertently compromised. Preoperative CT mapping to identify or rule out a subsuperior segment has become standard practice at many thoracic surgery centers.
Emphysema and the Diaphragm Connection
The posterior lower lobes sit directly on top of the diaphragm, and this intimate contact means that disease in the lower lobes can impair diaphragmatic function in ways that upper-lobe disease does not. In patients with chronic obstructive pulmonary disease (COPD), the distribution of emphysema across the lungs is not uniform, and where the destruction is concentrated matters for breathing mechanics. Imaging research has found that the extent of emphysema in the lower lung zones correlates with measurable reductions in diaphragmatic dome movement and posterior diaphragm excursion. Lower-zone emphysema was also linked to worse airflow limitation and greater impairment of physical activity.
Upper-lobe-predominant emphysema, by contrast, tends to spare diaphragmatic mechanics to a greater degree, which is one reason lung volume reduction surgery (which removes the most destroyed tissue to allow healthier tissue to expand) has historically targeted the upper lobes. When emphysema is lower-lobe-predominant, the surgical calculus changes significantly, because removing lower-lobe tissue sacrifices the lung’s most physiologically active region while also risking further compromise of the diaphragm.
Fetal Development and the Instability of Lower Lobe Segments
The segmental arrangement of the lower lobes is not fixed early in fetal life and then left alone. Three-dimensional studies of human fetal lungs have revealed that the segments of the lower lobe undergo substantial remodeling throughout gestation. In early and mid-term fetuses, the lateral basal segment (S9) dominates the lateral half of the lower lobe, with adjacent segments like S6b, S8a, and S10b gradually expanding outward to encroach on S9’s territory as the lung grows in volume. Some subsegmental bronchi present in earlier stages are absent or underdeveloped at later stages, and others that initially fail to reach the lung surface eventually do so as surrounding tissue shifts.
This prolonged period of remodeling may explain why the lower lobes show more anatomical variation than the upper lobes. Delayed growth of certain lateral subsegments can create mechanical stress within the developing lobe, potentially producing aberrant notches or fissures. In some cases, these developmental irregularities persist into adulthood as incomplete fissures or accessory lobes, which can complicate both imaging interpretation and surgical planning.