The Wright Variation of the Darrow‐Yannet Diagram: A New Tool to Teach Water Homeostasis

Abstract

PurposeTo introduce an expanded version of the Darrow‐Yannet (D‐Y) diagram that can both improve basic understanding of water balance homeostasis and be more specifically applied to specific organ system topics (e.g., gastrointestinal secretion) than can the basic diagram.Introduction and hypothesisSince 1935, the Darrow‐Yannet diagram has been used to teach both water balance and homeostasis. Six models of ECF volume expansion or contraction of hypertonic, isotonic, or hypotonic fluid with subsequent water transfer to or from ICF show how ECF and ICF remain in balance. Yet when osmosis is used later in both gastrointestinal and renal physiology to describe movement of water from spaces that are technically outside the body into or out of part of the extracellular fluid compartment, students sometimes struggle with the concept of osmosis (water diffusion between compartments) not from ECF to ICF but from ECF to OB and back. They must also consider that through both structural characteristics of the epithelium and the autonomic and hormonal control of epithelium function, this water transfer can be considerably modified or even prevented. Because the classic D‐Y has no OB in its illustrations, it cannot be used to show either of these aspects of water balance. The question therefore arose, is there some way to expand the D‐Y diagram to show both, and emphasize other aspects of homeostasis too? The author hypothesized that it could be done.ResultIn the Wright variation of the D‐Y diagram, two changes are made to expand the D‐Y diagram and increase its usefulness. First, an OB compartment of open volume is introduced on the one side of the ECF while keeping ICF on the other side as usual. Second, the epithelial layer between OB and ECF is artificially widened so that it is both more easily visualized and potentially drawn with two cell populations (e.g., secretory vs. absorptive cells lining the duodenal lumen). Though volume is undefined in the OB compartment, osmolarity is defined, ranging from zero (pure water) to immeasurably high (dry), so osmotic OB‐to‐ECF gradients can be seen. Using control of the epithelium to determine whether fluid flows through or not regardless of gradient, nervous and hormonal control of water homeostasis can be shown. Finally, not just water movement but hormonal‐ or autonomic‐controlled ion movement across the epithelium can be graphically shown, showing off an important aspect of homeostasis. Loss of control of these ion movements (e.g., in GI secretory cells in cholera) can also be demonstrated, along with its pathophysiological effect.ConclusionAn expanded D‐Y diagram with an artificially emphasized epithelial layer between an outside‐the‐body (OB) compartment and the extracellular fluid (ECF) has the potential to better illustrate the epithelium’s possible control in expanding or contracting ECF volume prior to ICF change than can a D‐Y diagram alone, both in health and disease.