# Thrombosis and Embolism

> Thrombosis and embolism for MBBS Pathology — Virchow's triad, arterial vs venous thrombi, embolism types and exam high-yield points.

- Canonical URL: https://prepelephant.com/topics/mbbs/pathology/thrombosis-and-embolism
- Exam / course: MBBS · Subject: Pathology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Thrombosis and Embolism", PrepElephant, https://prepelephant.com/topics/mbbs/pathology/thrombosis-and-embolism

## Direct answer

Thrombosis is the formation of a solid mass (thrombus) from blood constituents within the circulation during life, governed by Virchow's triad — endothelial injury, abnormal blood flow (stasis or turbulence) and hypercoagulability. An embolus is any detached intravascular mass, solid, liquid or gaseous, carried by the blood to a point distant from its origin; thromboembolism accounts for the vast majority of embolic events.

## What you must remember

- Arterial thrombi are pale, platelet-rich masses forming on injured or atherosclerotic endothelium under high flow; their laminated lines of Zahn (alternating platelet-fibrin and red-cell layers) prove formation during life.
- Venous thrombi are red, coagulation-rich and form in stagnation, classically in the deep veins of the calf and popliteal region; leg venous thrombosis above the knee is the classic source of pulmonary embolism.
- Hypercoagulability includes inherited states such as factor V Leiden, prothrombin and protein C, S and antithrombin deficiencies, and acquired states — surgery, immobility, malignancy (Trousseau syndrome), pregnancy, oral contraceptives and nephrotic syndrome.
- Lines of Zahn distinguish an antemortem thrombus from a post-mortem clot, which is gelatinous, unattached and separates into "chicken fat" and "currant jelly" layers.
- Thromboembolism: pulmonary embolism presents with sudden dyspnoea, pleuritic chest pain, haemoptysis and, with large saddle emboli, sudden death; a paradoxical embolus crosses a patent foramen ovale to reach the systemic circulation.
- Non-thrombotic emboli: fat embolism syndrome one to three days after long-bone fracture (respiratory distress, cerebral signs, thrombocytopenia and petechiae); decompression sickness from nitrogen bubbles (the "bends", treated with hyperbaric oxygen); amniotic fluid embolism during labour causing abrupt hypoxia, hypotension and disseminated intravascular coagulation; tumour and atherosclerotic cholesterol emboli.
- Fates of a thrombus: propagation, embolisation, dissolution by fibrinolysis, and organisation with recanalisation; prophylaxis in hospitalised patients uses low-molecular-weight heparin and early ambulation.

## Common confusion

The commonest error is equating embolism with infarction. An embolus lodges where vessel size no longer permits passage, but infarction follows only when collateral supply is inadequate — dual blood flow protects lung and liver. Also keep thrombus and embolus straight: the thrombus is the attached clot, the embolus is the detached traveller.

## Exam-focused takeaway

Theory answers should state Virchow's triad with examples, contrast arterial and venous thrombi, list embolus varieties with their settings, and describe the fates of a thrombus. Viva examiners ask the significance of lines of Zahn and why venous thrombosis targets the legs. MCQs pair fat embolism with long-bone fractures and petechiae, amniotic fluid embolism with disseminated intravascular coagulation, Trousseau with malignancy and factor V Leiden with inherited thrombophilia.

## Frequently asked questions

### What is Virchow's triad?

Endothelial injury, abnormal blood flow (stasis or turbulence) and hypercoagulability of blood — the three forces that favour thrombus formation.

### How does an antemortem thrombus differ from a post-mortem clot?

The thrombus is firm, attached and shows lines of Zahn; a post-mortem clot is gelatinous, unattached and lacks lamination.

### What is paradoxical embolism?

A venous thrombus crossing a patent foramen ovale or other right-to-left shunt to occlude a systemic artery, causing stroke or limb ischaemia.

### When does fat embolism syndrome appear and how?

Typically 24 to 72 hours after long-bone fracture or orthopaedic surgery, with respiratory distress, confusion, thrombocytopenia and a petechial rash.

### What are the possible fates of a thrombus?

Propagation, embolisation, dissolution by fibrinolysis, and organisation with recanalisation to restore partial flow.