Hemodynamics (Cardiovascular System Organization)

Pressure

The force applied perpendicularly to a surface, per unit of surface area.

• arterial blood pressure

  • systolic blood pressure

  • diastolic blood pressure

  • mean arteriole blood pressure

• Central venous pressure (pressure in distal inferior and superior vena cava.)

  • substitute for right atrial pressure

• pulmonary artery pressure

• pulmonary capillary wedge pressure

  • sub for left atrial pressure

hydrostatic pressure

the pressure present in a fluid at rest due to gravity

ΔP = p g Δh

• the shape or orientation of the vessel is irrelevant in determining the pressure.

• think of how a mercury barometer works. a closed vacuum capillary tube with the open end in mercury will cause the mercury to rise in the capillary tube. how much it rises is equal to the pressure of the atmosphere acting on the surface of the mercury pool.

• internal jugular vein is used as a blood-filled barometer to measure/estimate the right atria pressure. we can do this by measuring how high the jugular vein’s distension extends.

  • this also works because the jugular vein is very compliant. it dilates when filled with blood or collapses when empty

  • only the vertical distance to which the jugular vein;s distension extends matters, which is way this pressure measurement stays the same when the patient is laying at an angle.

Flow

• The volume of fluid that moves past a certain point per unit of time

• Q = V/t

• in hemodynamics, this would be cardiac output

• Q = area*velocity

• area₁*velocity₁=area₂*velocity₂

  • So, if cross sectional area decreases, velocity must increase in a singular vessel.

  • as total cross sectional area increases (for example, it is very high in the capillaries), the mean velocity of blood decreases.

Resistance

• Poiseuille’s Law

• R = (8*viscosity*Length)/(pi*r^4)

• limitations to strictly applying it to hemodynamics:

  • requires flow to be constant

  • conduit for flow should be rigid

  • fluid must have a constant velocity

  • requires the absence of turbulence

• pulsatile nature of blood flow gets attenuated as the vessels become smaller (blood flow is smooth by the time it reaches the capillaries, venules, etc. But there is also a steep drop in pressure as the blood passes through the arterioles (the site of the greatest resistance to blood flow).

• conductance is the inverse of resistance.

Ohms Law in hemodynamics

Pressure = Flow * Resistance (Pressure is generated by the heart pumping, the flow is the blood, and the parallel resistors are the vascular beds to individual organs).

Mean Arterial Pressure - Central Venous Pressure = Cardiac Output * Systemic Vascular Resistance (applied qualitatively, not greatly in quantitative analysis)