Section 4

• Flow = pressure gradient / resistance
• Pressure is the force exerted by blood onto the vessel wall (mmHg)
• Resistance is the friction that impedes that flow
• Mean arterial pressure (MAP) is the average pressure during cardiac cycle in the aorta
• Pressure gradient = MAP
• Mean arterial pressure (MAP) is what drives blood flow
• Resistance to blood flow is determined by three factors:

  1. Length of blood vessels
  2. Viscosity of blood
  3. Radius (main driver of resistance in body)
• The combined resistance of all the blood vessels is called the Total Peripheral Resistance (TPR)
• Flow is inversely proportional to the resistance
• Blood flow is proportional to the 4th power of vessel radius
• Smooth muscle controls the radius and thus resistance in blood vessels
• Vasoconstriction: Decreases the radius of vessel, and so you can increase resistance and decrease blood flow
• Vasodilation: Increases the radius of the vessel and so decreases the resistance and increases the blood flow
• Resistance = Total Peripheral Resistance (TPR) (mmHg xmm/L)
• Flow = Cardiac output (L/min)
• Cardiac output = MAP/TPR
• MAP = heart rate x stroke volume x TPR
• Diastolic pressure is the pressure in the aorta as it falls to its minimum
• Arteries are pressure reservoirs as arterial walls are able to expand and recoil due to the pressure of elastic fibers
• Systolic pressure is the maximum pressure occurring during systole
• Pulse pressure = Systolic pressure - Diastolic pressure (mmHg)
• MAP = diastolic pressure + 1/3 (Pulse pressure)
• MAP is closer to diastolic pressure because diastole lasts twice as long as systole
• Vasoconstriction increases MAP
• Vasodilation decreases MAP