Principles of Haemodynamics Flashcards
What is the significance of the CVS being a closed system?
It means that what happens in one part of the CVS has a major impact on the other parts. For example, reduced blood flow to one area increases the pressure upstream and alters flow to other areas.
How does Darcy’s Law come into blood flow?
It takes into account the role of pressure energy in blood flow.
Flow = (Pa - CVP)/ TPR
How does Bernoulli’s Law come into blood flow?
It takes into account the role of pressure, kinetic and potential energies in blood flow.
Flow = Pressure (PV) + Kinetic (ρV²/2) + Potential (ρgh)
What is perfusion?
It is the blood flow per given mass of tissue (ml/min/g).
What is the relationship between blood flow and velocity?
Cross-sectional area brings blood flow and veolcity together.
The greater the cross-sectional area, the slower the flow.
For example, the velocity of the blood flow in the aorta is high. The branching of the arteries slows the blood velocity. The velocity is the slowest in the capillaries.
The velocity increases with the veins coming together.
Describe the three types of blood flow.
- LAMINAR FLOW: IN MOST ARTERIES, ARTERIOLES, VENULES AND VEINS
The blood flow is in concentric shells, with near zero velocity at the walls (due to molecular interactions) and maximum velocity near the centre.
This moves RBCs towards the centre and speeds up blood flow through narrow vessels. - TURBULENT FLOW: VENTRICLES (MIXING), AORTA (PEAK FLOW), ATHEROMA (BRUITS)
Here, the blood does not flow linearly and smoothly in adjacent layers. There are whirlpools, eddies and vortices due to the increased pressure and velocity, or obstructions. - BOLUS FLOW: CAPILLARIES
RBCs have a larger diameter than the diameter of the capillaries, so they move in a single file. There are plasma columns trapped between RBCs.
Here, there is uniform velocity, little internal friction and very low resistance.
What is Reynold’s number?
It describes what determines the change from laminar to turbulent.
Turbulence occurs when Reynold’s number exceeds a critical value (>2000), eg. bruits, ejection number, increased blood velocity.
Re = (ρVD)/μ
(density x velocity x diameter) / viscosity
Arterial blood pressure involves interactions between four key (pressure) relationships. What are they?
- SYSTOLIC PRESSURE: pressure when ejecting
- DIASTOLIC PRESSURE: pressure when relaxing
- PULSE PRESSURE: difference between diastolic and systolic pressure
- MEAN BLOOD PRESSURE: average pressure
Describe the role of the aorta and arteries in blood pressure.
The recoil of the elastic fibers of the aorta and large arteries helps to propel the blood into the circulation.
DURING LV SYSTOLE:
60-80% of the stroke volume is stored in the aorta and arteries as these structures expand. The energy is stored in the stretched elastin.
DURING LV DIASTOLE:
Energy is returned to the blood as the walls of the aorta and arteries contract. This sustains the diastolic blood pressure and blood flow when the heart is relaxed.
Describe pulse pressure.
Pulse pressure is what the finger senses, for example, at the wrist (radial artery). It tells you about the stroke volume and the arterial compliance (stretchiness).
Pulse pressure = stroke volume / compliance
What does age have to do with arterial pulse pressure?
Age increases the stiffness of vessels – particularly the aorta – this means that a large pulse pressure is present throughout arterial tree.
Also, the further a vessel is from the aorta, the higher the pulse pressure.
List some factors that affect the mean blood pressure.
- age
- disease
- distance along the arterial tree
- blood volume (SV, CO)
- exercise (SV, CO)
- emotion (anger, stress, fear, etc.)
- wake/sleep (decreased pressure during sleep)
