Pressure, Flow, and Resistance

Pressure, Flow, and Resistance

Pressure

• Pressure arises from the continuous random motion of fluid molecules colliding with the walls of their container.
• Pressure is equal to the force exerted per unit area.
• Pressure is measured in kg/m² or dyne/cm² in science and mmHg (millimeters of mercury) in cardiology.

Pressure Effects

• Pressure inside a vessel can cause distension of the vessel walls if they are distensible, leading to an increased diameter and flow.
• Increased arterial pressure during systole distends the aorta and large blood vessels.
• Fluid will leak out if there are small holes and the pressure inside is higher than outside.
• Plasma tends to leak out of capillaries due to fluid pressure, however, this is counteracted by plasma oncotic pressure.
• Fluid flows from an area of higher pressure to lower pressure.
• The pressure at the entrance of a typical arteriole is around 85 mmHg, while at the exit it is around 30 mmHg, causing blood to flow through.

Flow

• Flow is defined as the volume of fluid per unit of time (min, sec or hr).
• Cerebral blood flow is 750 ml/min.
• Cardiac output is 5 L/min in an adult man at rest.
• Velocity refers to the distance an object moves with respect to time, often in cm/sec for blood in a vessel, not volume.

Resistance

• The higher the resistance, the lower the flow.
• Flow is proportional to the pressure difference (ΔP) and inversely proportional to the resistance (R): F = ΔP / R.
• According to Poiseuille's law, resistance (R) is determined by: R = (8/π) (ηL/r^4)
  • L is length
  • r is radius
  • η is viscosity of the fluid.
• Viscosity, a function of friction between molecules in a flowing fluid, is 4-5 times greater in whole blood compared to water, and depends on hematocrit (RBCs).
• Radius is the most important determinant of blood vessel resistance.
• Vasoconstriction decreases the radius by smooth muscle contraction, which then increases resistance.
• Vasodilation increases the radius by smooth muscle relaxation, decreasing resistance.
• When the radius is doubled, r^4 increases by a factor of 16, resulting in a resistance decrease by a factor of 16.

Resistance Problems

• If the radius of a tube is doubled, the resistance decreases by a factor of 16, given R is inversely proportional to r^4.
• If radius doubles, the flow increases 16 times
• For a tube with an entrance pressure of 40 mmHg and an exit pressure of 20 mmHg, a resistance of 5 PRU leads to a flow of 4 ml/min.

Resistance in Vascular Beds

• A vascular bed consists of arteries, arterioles, capillaries, venules, and veins.
• Blood flow (F) is proportional to the pressure difference (ΔP) across the vascular bed and inversely proportional to the resistance (R) of the vascular bed.
• The entrance of each vascular bed connects to the aorta via an artery, while the exit connects to the vena cava via a vein.
• The perfusion pressure (ΔP) is the same for all vascular beds, equivalent to MAP-VP
  • MAP: Mean Arterial Pressure
  • VP: Venous Pressure
• Blood flows along the path of least resistance; organs with the lowest resistance receive the highest flow.
• Coronary circulation (heart muscle supply) typically flows at 250 ml/min at rest.
• Coronary vascular beds decrease their resistance through vasodilation of smooth muscle cells in blood vessel walls to respond to increased blood flow demand.
• Most of the resistance decrease occurs in arterioles, which contain the most smooth muscle, which then allows the largest resistance adjustment.

Organ Blood Flow Problems

• Given a Mean Arterial Pressure (MAP) of 92 mmHg, Venous Pressure (VP) of 2 mmHg, and cerebral vascular resistance (R) of 0.12 PRU, the cerebral blood flow is 750 ml/min.
• If the MAP falls from 92 to 74 mmHg with unchanged VP and resistance, cerebral blood flow decreases to 600 ml/min.
• Cerebral circulation has a built-in mechanism for local control, which quickly restores cerebral blood flow to normal.

Total Peripheral Resistance (TPR)

• TPR is the resistance of every arteriole for every organ and tissue.
• Systemic circulation consists of many vascular beds in parallel, it also has resistance known as TPR - Flow: Cardiac Output (CO) - Pressure at the entrance: Mean Arterial Pressure (MAP) - Pressure at the exit: pressure in the Right Atrium (RAP)
• Therefore, CO = (MAP – RAP) / TPR
• If RAP is close to zero for healthy persons, CO is about MAP/TPR
• The total peripheral resistance is much smaller than the resistance of any single cerebral vascular bed because those beds are in parallel.
• Given MAP = 92 mmHg, VP = 2 mmHg, and CO = 6 L/min, TPR is 0.015 PRU. Given CO = 5 L/min and TPR = 0.015 PRU, MAP = 75 mmHg.
• CO would have to increase to 6 L/min, given MAP=CO x TPR, and TPR is constant, for the MAP to increase to 90 mmHg.
• TPR would need increase to 0.018 PRU, given MAP=CO x TPR, and CO is constant, for the MAP to increase to 90 mmHg.

MAP Considerations

• Control systems increase CO and TPR when the MAP drops below its set point for vascular beds to get blood supply from the aorta (where the MAP is maintained).
• TPR consists of systemic vascular beds branching from the aorta: increasing their resistance increases TPR).
• In response to falling MAP, most vascular beds increase resistance, while some beds (cerebral and coronary) decrease resistance through flow autoregulation.

Pressure Profile

• Pressure at the entrance = MAP (90 mmHg)
• Pressure at the exit of each vascular bed = VP (close to zero).
• Most pressure reduction is due to arteries converting to veins, due to friction between fluid particles
• Arterioles experience the greatest pressure drop, thus, produce the highest pressures, however, the same flow passes through each section of the vascular bed.
• Arteries: 90-85 mmHg (low resistance)
• Arterioles: 85-32 mmHg (high resistance)
• Capillaries: 32-15 mmHg (intermediate resistance)
• The remaining pressure dissipates when blood goes through venules and veins