Great Q — but arteriolar pressure is usually above CCP, so collapse doesn’t occur and MAP − CCP isn’t flow-limiting in normal physiology. Why don’t more distal vessels with lower pressures collapse? Because CCP applies at the level of arterioles, which are uniquely prone to collapse under their own tone. They’re the only part of the vascular tree where: – Smooth muscle tone is high enough to create inward pressure – That tone can exceed internal pressure if MAP drops (or tone rises) – Transmural pressure becomes negative → collapse occurs → This forms the basis of critical closing pressure and the vascular waterfall 🔁 That’s why: Arterioles are where CCP is generated, and where collapse typically happens — e.g. in shock, vasopressors, or oedema. In contrast: – Capillaries: no tone, structurally supported, low wall tension (Laplace). Collapse from compression – not active tone. – Veins: can collapse, but usually from posture, low volume, or external compression.