Critical threshold phenomena in a one dimensional quasi-linear
hyperbolic model of blood flow with viscous damping are
investigated. We prove global in time regularity and finite time
singularity formation of solutions simultaneously by showing the
critical threshold phenomena associated with the blood flow model.
New results are obtained showing that the class of data that leads
to global smooth solutions includes the data with negative initial
Riemann invariant slopes and that the magnitude of the negative
slope is not necessarily small, but it is determined by the
magnitude of the viscous damping. For the data that leads to shock
formation, we show that shock formation is delayed due to viscous
damping.