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PubMed This is a summary of 13 peer-reviewed journal articles Updated
Pulmonology · Acute Respiratory Distress Syndrome

Biology & Causes: How Did This Happen?

At a Glance

Acute respiratory distress syndrome (ARDS) is triggered by massive inflammation from a direct lung injury (like pneumonia) or an indirect injury (like sepsis). The condition progresses through three phases: fluid flooding (exudative), tissue repair (proliferative), and sometimes scarring (fibrotic).

Understanding how ARDS develops can help make sense of the complex care your loved one is receiving. ARDS is not a single event but a process that unfolds in stages. It begins when an injury—either directly to the lungs or indirectly from elsewhere in the body—triggers a massive wave of inflammation [1].

The Two Ways ARDS Starts

Doctors categorize the “trigger” of ARDS into two main types. Identifying the root cause is critical because it helps the medical team decide which treatments, such as specific antibiotics or fluid management strategies, will be most effective [2][3].

  • Direct Lung Injury: This happens when the insult starts inside the lungs themselves. Common causes include:
    • Pneumonia: A severe lung or bacterial infection [1].
    • Aspiration: Accidentally breathing stomach contents or foreign liquids into the lungs [1].
    • Inhalation Injury: Damage from smoke, chemicals, or vaping.
  • Indirect Lung Injury: This occurs when an injury or illness elsewhere in the body sends inflammatory signals through the bloodstream to the lungs. Common causes include:
    • Sepsis: A body-wide, life-threatening response to an infection [3].
    • Severe Trauma: Major physical injuries from accidents [4].
    • Pancreatitis: Severe inflammation of the pancreas.

The Three Phases of ARDS

Once the injury occurs, the lungs typically move through three biological phases. Knowing these phases helps you understand why recovery often feels like a “slow climb.”

1. The Exudative Phase (The Inflammatory “Flood”)

This is the earliest stage, usually occurring within the first week [5]. Intense inflammation damages the lining of the lungs’ air sacs, making them “leaky” [1][6]. Fluid, proteins, and inflammatory cells flood the air sacs, making it very difficult for oxygen to get into the blood [1]. This is why patients require the most intensive ventilator support during this time.

2. The Proliferative Phase (The “Repair” Stage)

During this phase, the body tries to clean up the fluid and repair the damage [5]. The body grows new cells to replace those that were lost [6]. While this is a sign of healing, the lungs may still feel “stiff” or less flexible than usual, and the patient may still need significant help from the ventilator [7].

3. The Fibrotic Phase (The “Scarring” Stage)

Important: Not every patient enters this phase [8]. In some cases, the inflammation causes the lung tissue to be replaced by fibrosis (scar tissue) [8][9]. If scarring occurs, the lungs can become permanently less efficient at moving oxygen. The medical team works hard during the first two phases to limit inflammation and prevent this stage from occurring [10][11].

Why the Root Cause Matters

While the ventilator support is often similar for both types, the “indirect” causes (like sepsis) often involve other organs, such as the kidneys or heart [3]. In these cases, the team must balance healing the lungs with protecting the rest of the body [11][12]. In “direct” injuries, the focus is often more heavily on clearing the specific lung infection or irritation [13].

Common questions in this guide

What is the difference between direct and indirect lung injury in ARDS?
Direct lung injury starts inside the lungs from causes like pneumonia, smoke inhalation, or aspiration. Indirect injury happens when severe illness elsewhere in the body, such as sepsis or trauma, sends inflammatory signals through the bloodstream to the lungs.
What are the three phases of ARDS?
ARDS typically develops in three biological stages: the exudative phase where inflammatory fluid floods the lungs, the proliferative phase where the body attempts to repair the tissue damage, and a possible fibrotic phase where scar tissue forms.
Does every patient with ARDS develop permanent lung scarring?
No, not every patient enters the fibrotic phase. In some cases, inflammation causes lung tissue to be replaced by permanent scar tissue, but medical teams actively work during the earlier stages to limit inflammation and prevent this permanent damage.
Why is it important for doctors to find the root cause of ARDS?
Identifying the exact trigger helps the critical care team decide which specific treatments will be most effective. For instance, indirect causes like sepsis require balancing lung healing with protecting other major organs, while direct causes may focus more on clearing a specific lung infection.
Why is a ventilator needed during the first phase of ARDS?
During the initial exudative phase, intense inflammation damages the air sacs and causes them to flood with fluid. This makes it incredibly difficult for oxygen to get into the blood naturally, requiring intensive support from a ventilator to keep the patient oxygenated.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Does my loved one have 'direct' or 'indirect' lung injury, and how does that change the treatment plan?
  2. 2.What stage of ARDS is my loved one currently in—exudative, proliferative, or fibrotic?
  3. 3.Are the current ventilator settings focused on 'lung-protective' strategies to prevent further damage during the repair phase?
  4. 4.How are we managing the underlying cause (like sepsis or pneumonia) while the lungs are healing?
  5. 5.Are you seeing any signs of lung scarring (fibrosis), and if so, what are the long-term implications?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

References (13)
  1. 1

    Acute Respiratory Distress Syndrome: Etiology, Pathogenesis, and Summary on Management.

    Kaku S, Nguyen CD, Htet NN, et al.

    Journal of intensive care medicine 2020; (35(8)):723-737 doi:10.1177/0885066619855021.

    PMID: 31208266
  2. 2

    Fifty Years of Research in ARDS. Genomic Contributions and Opportunities.

    Reilly JP, Christie JD, Meyer NJ

    American journal of respiratory and critical care medicine 2017; (196(9)):1113-1121 doi:10.1164/rccm.201702-0405CP.

    PMID: 28481621
  3. 3

    Sepsis-related pediatric acute respiratory distress syndrome: A multicenter prospective cohort study.

    Prasertsan P, Anantasit N, Walanchapruk S, et al.

    Turkish journal of emergency medicine 2023; (23(2)):96-103 doi:10.4103/tjem.tjem_237_22.

    PMID: 37169028
  4. 4

    Morbidity and Mortality Among Critically Injured Children With Acute Respiratory Distress Syndrome.

    Killien EY, Mills B, Watson RS, et al.

    Critical care medicine 2019; (47(2)):e112-e119 doi:10.1097/CCM.0000000000003525.

    PMID: 30379667
  5. 5

    Integrated histopathology, spatial and single cell transcriptomics resolve cellular drivers of early and late alveolar damage in COVID-19.

    Lee JTH, Barnett SN, Roberts K, et al.

    Nature communications 2025; (16(1)):1979 doi:10.1038/s41467-025-56473-x.

    PMID: 40064844
  6. 6

    COVID-19 post-mortem findings: how the departed can teach us.

    Nimir M, Tank A, Snead D

    Diagnostic histopathology (Oxford, England) 2021; (27(10)):422-424 doi:10.1016/j.mpdhp.2021.07.003.

    PMID: 34341671
  7. 7

    Histopathological autopsy findings in lungs of pregnant and postpartum women who died of COVID-19 infections.

    Bisharyan MS, Arsenyan KA, Khachatryan PS, et al.

    Rechtsmedizin (Berlin, Germany) 2023; 1-7 doi:10.1007/s00194-023-00618-z.

    PMID: 37363653
  8. 8

    Umbilical cord mesenchymal stem cells overexpressing CXCR7 facilitate treatment of ARDS-associated pulmonary fibrosis via inhibition of Notch/Jag1 mediated by the Wnt/β-catenin pathway.

    Xiao K, Liu C, Wang H, et al.

    Biomedicine & pharmacotherapy = Biomedecine & pharmacotherapie 2023; (165()):115124 doi:10.1016/j.biopha.2023.115124.

    PMID: 37454589
  9. 9

    Acute respiratory distress syndrome in adults: diagnosis, outcomes, long-term sequelae, and management.

    Gorman EA, O'Kane CM, McAuley DF

    Lancet (London, England) 2022; (400(10358)):1157-1170 doi:10.1016/S0140-6736(22)01439-8.

    PMID: 36070788
  10. 10

    Lung-Protective Mechanical Ventilation Strategies in Pediatric Acute Respiratory Distress Syndrome.

    Wong JJM, Lee SW, Tan HL, et al.

    Pediatric critical care medicine : a journal of the Society of Critical Care Medicine and the World Federation of Pediatric Intensive and Critical Care Societies 2020; (21(8)):720-728 doi:10.1097/PCC.0000000000002324.

    PMID: 32205663
  11. 11

    Fluid Therapy for Critically Ill Adults With Sepsis: A Review.

    Zampieri FG, Bagshaw SM, Semler MW

    JAMA 2023; (329(22)):1967-1980 doi:10.1001/jama.2023.7560.

    PMID: 37314271
  12. 12

    Positive Cumulative Fluid Balance Is Associated With Mortality in Pediatric Acute Respiratory Distress Syndrome in the Setting of Acute Kidney Injury.

    Zinter MS, Spicer AC, Liu KD, et al.

    Pediatric critical care medicine : a journal of the Society of Critical Care Medicine and the World Federation of Pediatric Intensive and Critical Care Societies 2019; (20(4)):323-331 doi:10.1097/PCC.0000000000001845.

    PMID: 30672838
  13. 13

    Respiratory Mechanics, Lung Recruitability, and Gas Exchange in Pulmonary and Extrapulmonary Acute Respiratory Distress Syndrome.

    Coppola S, Froio S, Marino A, et al.

    Critical care medicine 2019; (47(6)):792-799 doi:10.1097/CCM.0000000000003715.

    PMID: 30908313

This page explains the biology and causes of ARDS for educational purposes. Always consult your critical care team for specific information about your loved one's condition, stage of ARDS, and treatment plan.

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