Hemodynamics

See Flow for more details on Hagen-Poiseuille's law and laminar vs turbulent flow.

Laminar flow

Flow = pressure / resistance

Resistance = 8nl/r⁴pi

Resistance is proportional to viscosity, not density.

Turbulent flow

Resistance is proportional to flow.

Resistance is proportional to density, not viscosity

Haematocrit

Apparent viscosity varies with haematocrit ratio.

Haematocrit decreases progressively as vessel diameters reduces below 300 micrometer.
--> RBCs tend to accumulate in the faster axial stream of blood
--> RBCs moves at greater speed in axial stream
--> Relative haematocrit ratio decreased
--> Apparent viscosity decreased
* [BL8:p129]

Also, when flow decreases, cells tend to aggregate and increase viscosity.

When flow increases, viscosity decreases
--> "shear thinning"

Note:

• Hagen-Poiseuille's law does not apply to vessels under 300 micrometers because viscosity is affected by diameter

A few points to note

• Greatest resistance to flow are in the arterioles

Functional cardiac murmur in anaema is due to:

• Reduced viscosity of blood
• High flow and cardiac output