Poiseuille´s law: The volume rate (Vdot) is equal to the driving pressure (DP) divided by the resistance: V° = DP/Resistance. For the left ventricle, the blood flow is actually cardiac output (Q° ), so the equation reads:
Q° = DP/TPVR (l min-1).
The driving pressure (DP) is the mean arterial pressure (MAP) minus the atrial pressure, and TPVR is the total peripheral vascular resistance.
TPVR is directly related to the blood viscosity (h) and to the length (L) of the vascular system, and inversely related to its radius in the 4th power:
TPVR = 8 h L/r4.
Doubling the length of the system only doubles the resistance, but halving the radius increases the resistance sixteen-fold. - Poiseuille´s law is an approximation!