A pulmonary embolism causes a red (hemorrhagic) infarct because the lung has two blood supplies. When one is blocked, the other keeps delivering blood into already-dead tissue — and that blood has nowhere organized to go, so it floods the necrotic zone and makes it hemorrhagic. White infarcts occur only where a single artery supplies a region with no backup, leaving the dead zone bloodless.
The common mistake
On a stem describing a post-hip-replacement patient with sudden dyspnea and a wedge-shaped peripheral lung opacity — a textbook pulmonary embolism — Yuki chose "white infarct — inflammatory cells fill the void left by necrosis." Two errors in one answer: wrong color and a completely invented mechanism.
This is a very common trap. Students know PE → infarct, and they know infarcts exist. But the red/white distinction requires knowing why a given organ produces which type, and many students have not built the underlying principle — they have only memorized scattered organ examples.
The "inflammatory cells fill the void" reasoning shows what happens when the mechanism is unknown: the student invents a plausible-sounding explanation. Inflammatory cells do appear at infarct sites, but they do not determine the color.
The actual mechanism
Red (hemorrhagic) infarcts occur when a second blood supply bleeds into the necrotic zone after the primary arterial supply is blocked. The tissue is already dead, but it is not isolated from perfusion — blood from the collateral source leaks into the dead tissue. This produces a hemorrhagic, red-appearing infarct.
Three settings produce red infarcts: 1. Dual blood supply — the lung (pulmonary arteries + bronchial arteries) and the liver (hepatic artery + portal vein) 2. Collateral supply — the small intestine, with its anastomosing mesenteric vessels 3. Venous occlusion — when a vein is blocked, arterial blood keeps coming in but has nowhere to drain; it backs up into the tissue
White (anemic) infarcts occur when a single end-artery supplies the region and nothing else backs it up. When that artery is blocked, the zone is cut off entirely. No blood enters, no hemorrhage occurs — the tissue just dies and turns pale.
Classic white-infarct organs: heart, kidney, spleen. All are supplied by end-arteries with no meaningful collateral circulation.
The wedge shape of a pulmonary infarct is a structural clue: a wedge-shaped peripheral opacity means a branch artery was occluded at the periphery, where it terminates. The bronchial arterial supply then bleeds into the dead zone. Wedge shape + peripheral location = PE infarct = red.
The logic to apply to any organ you're not sure about:
Ask two questions: 1. Does this organ have collateral supply or dual supply? 2. Is this an arterial or venous occlusion?
If either answer points to blood entering the dead zone from a second source — it is red. If the organ is an isolated end-artery system — it is white.
The small intestine is worth noting separately: it has anastomotic mesenteric vessels, so arterial occlusion in the gut typically produces a red infarct. See coagulative vs. liquefactive necrosis for how the underlying cell death pattern (coagulative) is the same in both red and white infarcts — the color refers to the hemorrhagic appearance, not a different type of necrosis.
How to remember it
End-artery, no backup → white. Two supplies or collateral → red.
Think of white infarcts as isolated zones the blood abandoned completely, and red infarcts as zones that drowned in their own backup blood.
Check yourself
A 55-year-old woman with mitral stenosis develops atrial fibrillation and a clot embolizes to the splenic artery. What color infarct does she develop in the spleen, and why?
A) Red — the spleen has dual blood supply from adjacent mesenteric vessels
B) White — the splenic artery is an end-artery with no collateral supply
C) Red — venous occlusion prevents drainage and causes hemorrhage
D) White — inflammatory cells fill the necrotic zone
Correct answer: B. The spleen is an end-artery organ. When the splenic artery is occluded, no backup supply floods the zone, so it goes pale — a classic white infarct. The wedge-shaped pale zone on gross pathology is the signature.
Close the gap
The tutor that caught Yuki's "white infarct" answer and built the dual-supply logic from scratch is available to you right now. Try Gradual Learning free →