It's a branch of physiology that focuses on:

* Composition and properties of bodily fluids: This includes blood, lymph, cerebrospinal fluid, and interstitial fluid.

* Regulation of fluid balance: This involves mechanisms for maintaining proper fluid levels and electrolyte concentrations.

* Transport of substances: How fluids move nutrients, waste products, and other substances throughout the body.

* Role in various bodily functions: The contribution of fluids to processes like thermoregulation, blood pressure regulation, and cellular function.

Note: While "body fluid physiology" is the most accurate term, you might also encounter related terms like "hemodynamics" (specifically focusing on blood flow) or "renal physiology" (focusing on kidney function and fluid balance).