Apical bulkheads increase the ability of BC to hold pressure. (a) Mechanical model of a BC. Colored regions have different elastic moduli. (i) Outside view. (ii) Inside view with one bulkhead ablated, as indicated by the yellow arrowheads. (b) Simulated expansion ratios as a function of apical bulkhead modulus and ablation depth (curves inside axes). For each experimentally measured expansion ratio, minimum and maximum ablation depths yield maximum and minimum inferences on apical bulkhead modulus (below axes). There is a range of moduli consistent with all data (gray region below axis). In this range, the lowest modulus is the most likely (below axis, dark gray line). Bulkhead modulus (x-axis) is normalized to the modulus of the cortex. (c) The pressure at which BC with apical bulkheads have circular cross section (D*) is about double the pressure of equivalent BC with no bulkheads (red arrow). The increase in pressure before the critical BC diameter Dbleb reached is about 35% greater in BC with apical bulkheads (ΔP₂) than without (ΔP₁).