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 voltage 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, and this is important, related to resistance. Increasing resistance reduces flow and the most important effect of that is that increasing resistance reduces the delivery of oxygen.
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 an intact body deviations in measured values to CO = BP/SVR occur as physiologic processes work to preserve blood flow and oxygen delivery. At a single point in time this equation CO = BP/SVR will always hold true. As one of these parameters is modified by an intervention such as using a medication to affect blood pressure the resulting change in the other parameters will be affected by autoregulation. Blood flow or Cardiac Output is still directly related to Blood Pressure* and inversely related to resistance.
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. The often seems counter intuitive and has been mistakenly believed for many years. It will raise the blood pressure but results in reduced blood flow and organ perfusion. A drug known as Levophed is colloquially known as “Leave um Dead” because this counter intuitive impression. The short term results of increased blood pressure results in long term negative consequences, deteriorating physical condition and sometimes 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 almost instantly increases what we can see, and measure, that is, blood pressure. The negative consequences are grave but not as obvious because the effect is delayed and because blood flow is so difficult to measure.
A drug such as a vasoconstrictor that increases the resistance to blood flow will reduce blood flow. The blood pressure may increase which is immediately apparent but flow will decrease. This is contrary to the misguided clinical impression that increasing blood pressure no matter how it is achieved benefits blood flow. The measurement of the actual blood flow especially into internal organs such as the liver is in most cases is impossible. Because of this most clinicians equate increased blood pressure to increased blood flow. Skepticism is warented. Sometimes blood flow might be redistributed from one vascular bed to another but it is easy to use this argument to reject a universal law of physics, asking for trouble. The practice of using blood pressure as a substitute for flow is unfortunately encoded into many 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 consider them as such.
* Technically the flow is related to the pressure gradient Arterial Pressure minus Central Venous pressure but generally the Central Venous Pressure is low enough to ignore and we relate flow directly to arterial blood pressure