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This is the pathophysiology of sepsis - a classic diagram showing how microbial infection triggers a destructive systemic inflammatory cascade. Here is a full walkthrough:
Sepsis Pathophysiology Flowchart - Complete Explanation
STEP 1: The Trigger
Gram-negative and gram-positive bacteria (and other organisms) release microbial products into the bloodstream. This is the initiating event.
From microbial products, three parallel pathways are activated simultaneously:
PATHWAY A: Coagulation Cascade (Far Left)
- Endotoxin (from gram-negative bacterial cell walls, e.g., lipopolysaccharide/LPS) activates Factor XII (Hageman factor)
- Factor XII triggers activation of coagulation
- Simultaneously, endothelial injury (see below) causes:
- Increased Tissue Factor (TF) - a potent coagulation initiator
- Decreased anticoagulants (protein C, antithrombin)
- Together these cause microvascular thrombi and DIC (Disseminated Intravascular Coagulation)
- DIC leads to tissue ischemia (blocked microcirculation starves organs of oxygen)
- Tissue ischemia causes:
- Multiorgan failure (via ischemic damage)
- Adrenal insufficiency (adrenal glands are particularly sensitive to ischemia)
PATHWAY B: Complement and Immune Activation (Right Side)
- Microbial products activate the complement cascade: C3 is cleaved into C3a (and C3b)
- C3a and complement products activate neutrophils and macrophages
- Activated macrophages and neutrophils release cytokines: TNF, IL-1, HMGB1
- These cytokines feed back onto endothelial cells, worsening injury
- Cytokines also produce:
- Secondary anti-inflammatory mediators → Immune suppression (this is why late sepsis causes immunoparalysis and susceptibility to secondary infections)
- Metabolic abnormalities (hyperglycemia, lactic acidosis, etc.)
PATHWAY C: Endothelial Cell Activation/Injury (Center)
The endothelium is the "command center" of sepsis pathology. It receives inputs from ALL pathways above and generates three dangerous downstream effects:
C1 - Inflammatory Mediators
- Endothelial cells release IL-6, IL-8, NO (nitric oxide), PAF (platelet activating factor), reactive oxygen species (ROS)
- These cause:
- Vasodilation (NO causes blood vessels to dilate)
- Increased vascular permeability (fluid leaks out of vessels - "capillary leak")
- Decreased perfusion (despite vasodilation, effective tissue perfusion drops)
- Result: Multiorgan failure from poor tissue delivery of oxygen
C2 - Cardiac Depression
- Mediators directly suppress the heart:
- Decreased myocardial contractility
- Decreased cardiac output
- This leads to organ dysfunction → multiorgan failure
C3 - Coagulation Imbalance
- Endothelial injury upregulates TF and downregulates anticoagulants, feeding the DIC pathway (see Pathway A)
FINAL COMMON PATHWAY: Multiorgan Failure
All pathways converge on multiorgan failure, which is the life-threatening endpoint of sepsis. The organs most commonly affected include:
- Kidneys (acute kidney injury)
- Lungs (ARDS)
- Liver
- Brain (septic encephalopathy)
- Adrenal glands (adrenal insufficiency - shown separately)
Key Concepts to Remember
| Mechanism | Clinical Consequence |
|---|---|
| DIC / microvascular thrombi | Bleeding + clotting simultaneously |
| NO-mediated vasodilation | Warm, hypotensive septic shock |
| Capillary leak | Edema, hypoalbuminemia |
| Myocardial depression | Cardiogenic component of septic shock |
| Immune suppression | Susceptibility to secondary infections |
| Adrenal insufficiency | Refractory hypotension (needs steroids) |
| Metabolic abnormalities | Lactic acidosis, hyperglycemia |
This diagram is from Robbins Pathology (or a similar pathology textbook) and represents the core mechanism behind why sepsis is so deadly - it is not just the infection itself, but the host's own dysregulated response that causes multiorgan damage.
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