Why do sickled erythrocytes block narrow capillaries?

1. Sickle cell disease produces an abnormal hemoglobin, hemoglobin S. 2. Hemoglobin S makes erythrocytes take a sickle or crescent shape, especially at low oxygen concentrations. 3. A normal biconcave disc survives a narrow capillary by folding in on itself and springing back afterwards. 4. The sickled cell cannot perform that fold, so it lodges instead of passing. 5. The obstruction starves tissue downstream, giving painful joints, delayed growth, blindness and stroke. 6. Cell volume is not the mechanism, so the swelling option is wrong even though it sounds mechanical. 7. Platelet clumping describes a thrombus rather than a sickle crisis, which is the trap for a candidate who reaches for the clotting chapter. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 18 "The Cardiovascular System: Blood", section 18.3 Erythrocytes_