According to thermodynamic principles energy flows from high energy states to low energy states. This can be seen as heat moving from hot to cold, electric current from high to low potential and fluids moving from high to low pressure. This always occurs through some type of resistance. Blood flows from an area of high energy that is high pressure to an area of low energy, low pressure. In the case of blood the resistance to flow is determined by the physical properties of the blood vessels. The general equation describing blood flow is Flow = Pressure / Resistance which for the entire body is Cardiac Output, CO = Blood Pressure / Systemic Vascular Resistance written CO = BP/SVR. Flow such as cardiac output is what carries oxygen. This flow is related to pressure, increasing with increased pressure. and also related to resistance. Increasing resistance reduces flow.
Thermodynamic principals are the same in humans, animals, vertebrates and invertebrates, on earth and also in the rest of the universe. We don’t look for experiments to verify the relationship of CO = BP/SVR. The relationship exists as a matter of universal law.
Biologically or physiologically blood flow is modulated by various mechanisms such as feedback loops and auto regulatory processes. In practice, or in vivo, the data from measurements may not follow CO = BP/SVR precisely. In fact, there may be quite significant deviations to measured values as physiologic processes work to preserve blood flow and oxygen delivery. A drug such as a vasoconstrictor that is expected to increase resistance should be expected to reduce blood flow not increase it. It is sometimes said that using a vasoconstrictor that increases resistance will increase flow or perfusion. It certainly will not do that. Only by redistributing flow might flow be shunted from one vascular tree to another. Overall, vasoconstriction and increasing resistance will not increase flow.
Generally is assumed that an intervention that raises blood pressure will increase blood flow and an intervention that reduces blood pressure will decrease blood flow. This is not true where vasoconstriction is used to increase blood pressure. By increasing resistance the blood pressure can be increased at the expense of blood flow. Levophed is known as leave um dead by this principal. It will raise the blood pressure but increasing resistance is not advantageous. Short term results of increased blood pressure results in long term negative consequences, i.e. death. Using vasoconstriction to raise blood pressure is seductive because blood pressure is easily measured and blood flow is essentially impossible to measure under most circumstances. We assume the infallibility of an intervention that increases what we can see, and measure, that is, blood pressure and does it instantly. The practice of using blood pressure as a substitute for flow is unfortunately frequently encoded into inflexible protocols and procedures.
1 https://en.wikipedia.org/wiki/Heresy “In other contexts the term does not necessarily have pejorative overtones and may even be complimentary when used, in areas where innovation is welcome, of ideas that are in fundamental disagreement with the status quo in any practice and branch of knowledge.”
Shock should be considered to be low blood flow. However, as soon as the admonition is made that “shock is not low blood pressure”, blood pressure is, for convenience used to define shock. Practice and science usually diverge in this situation. Blood pressure is simplistically used to assess blood flow or perfusion. Blood pressure and blood flow are not the same but is common practice is to treat them as such.