A Course-long Approach to Teaching Clearance Using the Core Concept of Mass Balance

The shift towards incorporating core concepts into physiology curricula provides an exciting opportunity to rethink how traditionally challenging topics are taught. One such topic is the principle of clearance, defined as the volume of plasma cleared of a substance per unit time. In many physiology courses and textbooks, clearance is introduced in the discussion of renal physiology, which often leads students to associate clearance exclusively with kidney function. This narrow focus fosters misconceptions, such as conflating clearance with renal solute handling and concluding that clearance always results in excretion. In reality, clearance is not just a renal process: it is a much broader concept that applies to multiple body systems. Any tissue that removes solute from the blood is involved in clearance: the lungs excrete volatile substances, sweat glands help remove xenobiotics, and the liver and kidneys clear endogenous substances as well as xenobiotics. Solute can also be cleared from the plasma through means other than excretion, such as metabolic conversion to other substances. One way to expand teaching clearance is to frame it as an illustration of the core concept of mass balance. Mass balance, one of the foundational physiology core concepts, describes the quantity of mass in a biological system as the initial quantity plus input minus output. Clearance, the removal of a substance from plasma by one of several body systems, directly represents the output variable in mass balance of blood components. This poster presents an expanded approach to teaching clearance using the core concept of mass balance. We begin by introducing the principle of clearance early during the semester as a demonstration of mass balance and a basic element of pharmacokinetics. We then highlight mass balance in the discussion of renal clearance by having students work on simple interactive exercises. We provide additional examples where educators can use clearance to demonstrate the core concept of mass balance. Introducing clearance early and revisiting it at different times during the standard “march through the systems” offers educators the opportunity to solidify student understanding of both mass balance and clearance throughout an introductory physiology course.

This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.