Fig. 3.

Coordinate and orientation definitions. a Definition of cylindrical coordinate system using radial ($\mathit{\rho }$), angular ($\mathit{\theta }$), and axial ($z_{\mathrm{c}}$) coordinates. b, c Orientations of SMCs can be visualised as points on a unit hemisphere. b The orientation of such a hemisphere with respect to the vessel. c The two spherical coordinates that are used to quantify SMC orientation. ${\mathit{\theta }}_{\mathrm{h}}$ and ${\mathit{\theta }}_{\mathrm{t}}$ are an SMC’s helix and transverse angles, respectively. d Exact definitions of ${\mathit{\theta }}_{\mathrm{h}}$ and ${\mathit{\theta }}_{\mathrm{t}}$. ${\mathit{\theta }}_{\mathrm{h}}$ is the angle that the orientation’s projection on the axial-circumferential plane makes with the circumferential direction. ${\mathit{\theta }}_{\mathrm{t}}$ is the angle that the orientation makes with the axial–circumferential plane. SMC smooth muscle cell