Flow down gradients: the problem of pressure in this physiology core concept.

The core concepts of physiology, as first published in this journal in 2011, not only provide a noteworthy teaching approach but also encourage reflection on the fundamentals of physiology. Unfortunately, a fundamental flaw has crept into the core concept of flow down gradients. Fluids do not generally flow from high to low pressure, as claimed, but only because of a specific pressure difference, that is, the perfusion pressure. This is related to a problem that is widespread in physiology, from which even the core concepts are not free, namely, the description of mean arterial pressure (MAP) solely by means of Ohm's law of circulation, although this law actually describes perfusion pressure. Both pressures can be numerically approximately equal in the physiological case, but conceptually they remain different in principle. We solved this problem using the extended Bernoulli equation (a combination of Ohm's law and the simple Bernoulli equation). Thereafter, MAP depends on the following pressure components, all of which are essential for a basic understanding of circulation: perfusion, central venous, gravitational, and dynamic pressures. These pressures also have great pathophysiological and clinical importance, which we exemplify here. Toward the end of this article, we provide recommendations that should be considered in teaching, whether it is a beginner or advanced course. We address physiology teachers who are open to critical constructive improvements in their teaching, especially in hemodynamics. In particular, we encourage the authors of the flow down gradients core concept to improve and refine its "unpacking."