Lungs don't just help us breathe, they also make blood

Introduction: an organ with a hidden double life

The certainty that just fell apart

For generations, physiology textbooks have taught a simple rule: blood is made in the bone marrow, full stop. That certainty has just been seriously shaken. Researchers at the University of California, San Francisco (UCSF) have shown that human lungs are themselves a major site of blood cell production, particularly of platelets, the small cell fragments essential for clotting. A discovery that forces us to rewrite part of the biology textbooks taught for decades around the world.

Published on February 27, 2025, in the scientific journal Blood, this work shows that the lung harbors hematopoietic stem cells, meaning the mother cells capable of giving rise to every type of blood cell. A finding since confirmed by follow-up studies, reinforcing the idea that our body has a second, largely overlooked, blood-making site.

A colossal daily demand

To grasp the scale of what's at stake, it helps to recall a staggering figure: our body must produce roughly 200 billion new red blood cells every day to keep oxygen flowing to every organ. This continuous production, called hematopoiesis, has always been attributed almost exclusively to the bone marrow inside our bones. The fact that an organ as busy as the lung actively contributes to this daily effort changes the picture of how blood-making work is really distributed across the human body.

How researchers made this discovery

Comparing human lung, bone marrow, and blood

The team led by Dr. Mark Looney, a professor of medicine at UCSF, and his colleague Catharina Conrad, a postdoctoral researcher and the study's first author, compared donated samples of lung tissue, bone marrow, and blood from the same human donors. Examining a volume of lung tissue barely bigger than a golf ball, the researchers detected hematopoietic stem cells at rates comparable to those seen in bone marrow itself.

They then grew these cells in petri dishes to see what they produced once stimulated. The striking result: colonies derived from the lung generated more red blood cells and megakaryocytes — the giant cells that give rise to blood platelets — while bone marrow colonies produced relatively more immune cells. A functional specialization suggesting that the two organs aren't simply duplicating each other's work, but playing complementary roles.

A decisive test in mice

To verify that these human lung stem cells were truly functional, researchers injected them into mice lacking blood stem cells. The result: these cells were able to restore the bone marrow of the rodents, proof that they retain all the regenerative abilities expected of genuine hematopoietic stem cells. This gold-standard test in the field confirms that the presence of these cells in the lung is not a passing artifact, but a real, functional biological phenomenon.

Researchers also pinpointed exactly where these cells sit in lung tissue: they nestle between blood vessels, in an arrangement strongly reminiscent of what's seen in bone marrow. This observation reinforces the idea that the lung hosts a genuine hematopoietic niche, rather than simple cells passing through on their way somewhere else via the bloodstream.

A precedent in mice dating back to 2017

Half of platelets already observed in the mouse lung

This discovery in humans didn't come out of nowhere. As early as 2017, the same UCSF team had shown, in mice, that cells located in the lung were responsible for producing roughly 50% of the animal's blood platelets. At the time, that result already surprised the scientific community, but it remained to be seen whether a similar mechanism existed in humans, whose lung physiology differs from that of laboratory rodents in many ways.

The researchers had also identified, still in mice, lung stem cells capable of producing every component of blood: red blood cells, megakaryocytes, and several types of immune cells as well. This diversity of output already suggested that the lung might act as a versatile hematopoietic reservoir, activated according to the body's needs.

From mice to humans: an awaited confirmation

The leap from observation in mice to demonstration in humans marks a decisive step. Model organisms like mice let scientists formulate hypotheses, but human physiology can hold surprises of its own. By confirming the presence and functionality of these stem cells directly in donated human lung tissue, the UCSF team turned a laboratory curiosity into an established fact of human biology.

This continuity between the 2017 and 2025 studies also illustrates the pace at which scientific research unfolds: it took nearly eight years of follow-up work to move from an observation in animals to solid proof in humans, funded in part by the National Heart, Lung, and Blood Institute, part of the American National Institutes of Health.

The implications for transplant medicine

A second reservoir of valuable stem cells

Bone marrow transplants have for decades been a pillar of treatment for cancers like leukemia. These transplants depend entirely on the availability of compatible hematopoietic stem cells, which are often hard to find. The discovery that the lung harbors a significant stock of these same cells opens up an entirely new avenue: it could serve as a second reservoir for certain future therapeutic applications.

One striking detail reinforces this prospect: according to the researchers, nearly one-fifth of the stem cells isolated during standard bone marrow transplants may already carry the molecular signature specific to lung-derived stem cells. This suggests these cells likely circulate more widely through the body than previously thought, and may already be quietly contributing, without anyone realizing it, to transplants performed daily in hospitals.

An emergency reservoir activated on demand

Researchers propose that lung stem cells might act as a backup reservoir, mobilized when the body needs a surge in platelet, red blood cell, or immune cell production — for instance during major blood loss or a severe infection. This backup function would complement the more continuous role played by bone marrow on a day-to-day basis.

Two questions remain open for researchers, though: why must the lung, an organ already busy with gas exchange, also produce blood? And do the various reservoirs of hematopoietic stem cells scattered throughout the body play distinct therapeutic roles? These are avenues that could shape medical research in the years ahead.

What this changes about our understanding of the body

A map of the human body that needs revising

This discovery shows just how incomplete our understanding of the human body remains, even for an organ as thoroughly studied as the lung. The lung has long been seen as a simple gas-exchange organ, dedicated to bringing in oxygen and expelling carbon dioxide. Seeing it now recognized as an active site of blood production highlights the need to keep questioning certainties long taken for granted in school textbooks.

This revision joins other recent discoveries in physiology showing that certain organs perform multiple, interconnected functions, well beyond the primary role traditionally taught. The human body increasingly looks like a system where the boundaries between organs are more porous than we imagined.

Research still to be pursued

UCSF scientists plan to continue their work to precisely determine the actual proportion of blood cells produced by the lung in humans, under normal conditions and under physiological stress — for example after a hemorrhage or during a chronic disease affecting bone marrow. This work could eventually influence how doctors assess and make use of a patient's stem cell reserves.

What this could mean for tomorrow's patients

Rethinking how blood disorders are monitored

If the lung genuinely serves as an active site of blood production in humans, that could eventually influence how doctors interpret certain blood disorders in patients whose bone marrow has been weakened by chronic illness or a harsh treatment like chemotherapy. A healthy lung could, to some extent, partially compensate for insufficient blood cell production elsewhere in the body, a hypothesis researchers now want to test more formally.

This lead remains speculative for now, but it illustrates how a fundamental discovery can open up concrete clinical avenues. Chronic lung diseases, for example, could themselves have unsuspected consequences for a patient's blood production, an angle almost entirely absent from research conducted so far on these respiratory conditions.

A better grasp of functional anatomy

This discovery also fits into a broader movement in medical research seeking to map, with ever greater precision, the overlapping functions between organs. The liver, the spleen, and now the lung appear as additional players in hematopoiesis, a function long considered the exclusive domain of bone marrow. This distributed view of blood production could, in time, transform how we assess a person's overall hematological health.

Researchers hope this work will encourage other teams around the world to systematically explore other organs in search of still-unsuspected hematopoietic functions, a basic-research push that could substantially redraw our physiological map of the human body.

Conclusion: the lung, an unrecognized player in our blood

A lesson in scientific humility

This research is a reminder that major discoveries still remain to be made about organs we thought were perfectly well mapped. The lung, long reduced to its respiratory function, now appears as a full player in the making of our blood, alongside bone marrow. A discovery that, beyond its scientific reach, illustrates the unsuspected richness of human biology.

It also calls for a degree of caution around well-established certainties: what has been taught for decades can, at any moment, be challenged by a rigorous study carried out with modern tools. Science advances precisely through this kind of methodical, well-documented reassessment.

Toward more complete medicine

Eventually, this discovery could enrich therapeutic strategies related to stem cell transplants and the management of blood diseases, taking into account this long-overlooked second, pulmonary reservoir. A concrete example of how a fundamental advance in biology can, over time, translate into tangible clinical benefits for patients.

By Maxime Marquette, columnist

Sources

Primary sources

ScienceDaily — How our lungs back up the bone marrow to make our blood — February 2025

PubMed Central — Reference study on pulmonary hematopoietic stem cells — 2025

UCSF News — News from the University of California, San Francisco on biomedical research — 2025

Secondary sources

Futura-Sciences — Health section, science coverage on lung physiology — 2025

Sciences et Avenir — Health section, French biomedical news — 2025

Sciencepost — Science outreach outlet, news on biological discoveries — 2025