Professor Saltzman discusses the biophysics of the circulatory system. He begins by describing the anatomy of different types of blood vessels, and states the relationship between pressure difference (ΔP) as the driving force for fluid flow (Q) in a tube (i.e., blood vessel) with some resistance R (ΔP = RQ). R can be calculated using if dimensions of the tube (L, r) and fluid viscosity (μ) are known: R = 8μL/πr4. Next, Professor Saltzman traces the blood flow through the circulatory system and explains how the body can regulate blood flow to specific regions of the body. Finally, he describes the heart and its function as the pressure generator in the system.