depends on pressure difference/gradient and vascular resistance (antagonism of flow). Pressure gradient of systemic circ = 100 mmHg , pulmonary circ = 14mmHg
Ohm’s Law, F = (Pin – Pout)/R –> F = ∆P/R
Higher pressure gradient –> higher flow
BUT Higher resistance –> lower flow
Negative ∆P –> reverse flow
Resistance decrease pressure gradient trough Shear stress: stress of friction of blood elements against the vascular –> energy dissipation –> decrease pressure
Laminar flow: parallel layers with no contact, internal velocity higher than close to walls. Small vessels.
Turbolent flow: crosswise, large vessels.
PRESSURE
Force exerted by blood against vessel wall
P = F/A
Main pressure drop occurs in arterioles with a progressive switch from pulsatile to steady state. P = 0 in right heart
RESISTANCE
Impediment of blood flow (friction) in a vessel. Higher resistance –> lower flow
Depends on geometrical parameters (length, radius) and blood viscosity. Resistance increases with length/viscosity and decreases with radius
Increasing the radius, decreases the resistance –> flow increases
Other regulatory mechanisms
Vascular distensibility: ability of vessels to store more blood. Veins more distensible than arteries
Compliance: ratio between increase in volume and increase in pressure C = ∆V/∆P