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Blood is a fluid containing biological cells that transports substances through a circulatory system. Its viscosity depends on its cellular content and deformation rate. Treating it as a Newtonian fluid in the Hagen-Poiseuille equation is an approximation that depends on the vessel size and flow conditions.

A blood vessel is a tubular structure conducting blood within the circulatory system. Its endothelium and supporting wall differ among arteries, capillaries and veins. Conduction, exchange and volume storage are therefore emphasized differently in different vessel classes.

The circulatory system transports blood through the heart and vessels, connecting exchange surfaces with tissues. In humans, pulmonary and systemic pathways operate in series. Pressure gradients drive flow, while the distribution of resistance helps determine how total output is divided among organs.