We normally think of the flow of blood as directly related to blood pressure. The higher the pressure the better the flow. A better definition of shock is low blood flow rather than low blood pressure. The relationship between BP and blood flow would be linear and directly related if Systemic Vascular Resistance - SVR were a constant value. Unfortunately and annoyingly the systemic vascular resistance is not a constant. The higher the resistance the lower the flow for any given blood pressure. The precise relationship of BP to blood flow is described by the equation of Olm’s law of blood flow which states that Blood Flow is equal to BP divided by the resistance to blood flow, SVR.
In a more accurate way shock is low flow, a more difficult parameter than low blood pressure to observe. A fact that common practice and practicality often chooses to ignore.
The lack of evidence in shock is vascular resistance which is not observable.