The pressure that drives blood flow is a change of pressure, called the perfusion pressure, the difference
between the two from Pin – P out. For most organs, it is the Pressure of the arteries minus the Pressure
of the veins. Most of the time, the perfusion pressure is almost the same as the artery pressure, since
arterial pressure is many times greater than the venous pressure.
Pin = 100 mmHg, P out = 10 mmHg, perfusion pressure = 90mmHg, assume the flow is 10mL per minute.
If we then change the Pin=500, Pout=410, the perfusion pressure is still 90, the flow remains the same at
10mL per minute. The flow is only dependent upon the perfusion pressure.
The flow = Perfusion Pressure/ Resistance to flow (this differs from one vessel to another). The
resistance is measured in PRU (peripheral resistance unit), with standard units of mmHg second per mL.
Laminar or Parabolic Flow: fluids, when flowing, is formed of many many layers. The layers flow at
different speeds. The inner layer flows the fastest, while the outer layer flows the slowest. Resistance is
calculated with the equation R = 8pi*viscosity*length of the vessel/(area squared). Which can be
simplified with 8vL/pi*r . This equation is valid for laminar flow only.
The arterial vessels are the resistance vessels since they control the resistance of the blood flow in the
body. E.g. 20% increase in radius reduce res