Collapsed veins illustrate which transmural pressure state?

• A. High transmural pressure
• B. Low transmural pressure
• C. Negative transmural pressure
• D. Zero transmural pressure

Answer: (B)

Transmural pressure is the difference between the pressure inside a vessel and the pressure outside it. When the external pressure on a vein becomes greater than the internal venous pressure, the transmural pressure falls to a low value (and can become negative). That reduced pull outward allows the vein walls to collapse, which is exactly what you see as a collapsed vein on ultrasound. So, collapsed veins reflect a low transmural pressure state because the external forces exceed the internal pressure, preventing the lumen from staying open. If internal pressure were higher than outside, the vein would stay distended; if they were equal, you’d have zero transmural pressure and the vein would be at the edge of collapse.