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.
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. The physics of blood flow are not altered by physiologic processes. The physical 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 evidence to verify the relationship of CO = BP/SVR. If we measure these paramters in any actual system we may not find the this relationship. It does not matter. The relationship exists as a matter of universal law. A drug such as a vasoconstrictor that is expected to increase resistance should be expected to reduce blood flow not increase it. It is a leap of faith that using a vasoconstrictor will increase flow or perfusion. through important vascular beds such as the brain or myocardium. It might be true is some cases but we should expect the opposite and always be circumspect of this conclusion. This assumption may be considered heresy1 and unfortunately often opposed by arrogance. The evidence in “evidence based” decision making can be either good or bad. In this case, bad evidence is often convenient as it supports our desired preference. The choice is often marketing over science. Bad evidence is chosen over good while admonishing the value of evidence over reason.
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. There is generally no practical alternative for making this assumption because blood pressure is so easy to measure and the actual flow of blood is essentially impossible to measure is most all circumstances. The harried and chaotic nature of the real world demands this approach. The only argument against it is that it is stupid. It may be true but not a nice thing to say. Remember that.
We know what we are doing to blood pressure because we can measure it and almost never have an idea of what flow is because we cannot measure it. Still we assume the infallibility of an intervention that increases what we can see, perceive, and measure, that is, blood pressure. This practice is encoded into dogmatic and inflexible protocols and procedures. This dilemma and resultant misinformation is unfortunately, usually influenced more by the arrogance and bravado of so called experts than is science and reason. A lot of money is at stake.
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 the management of shock. Blood pressure is universally used to assess blood flow or perfusion. Blood pressure and blood flow are not the same but common practice is to treat them as such.