Cardiovascular Systems 4 - Cardiovascular Mechanics 3 (blood flow) Flashcards
Briefly summarise the characteristics of the systemic circulation.
- Small arteries and arterioles have smooth muscle to regulate diameter and show resistance to blood flow
- Veins and venules are highly compliant, so act as a reservoir for blood volume
How is blood pressure calculated?
Blood pressure (MAP) = cardiac output (CO) x resistance (PVR)
What assumptions does the blood pressure (MAP) equation assume?
- Steady flow (this does not occur due to pumping of the heart)
- Rigid vessels
- Right atrial pressure is neglegable
How is regulation of flow achieved physiologically?
- Variation in resistance in the vessels
- Blood pressure remains constant
What three variables does resistance to flow depend on?
- Fluid viscosity
- The length of the tube
- The radius of the tube
What are the two types of blood flow?
- Laminar flow
- Turbulent flow
What is laminar flow?
- The velocity of the fluid is constant, and the fluid flows in lines
- Blood flows fastest at the centre of the lumen
What is turbulent flow?
- Blood flows erratically, forming eddys.
- Prone to pooling
- Associated with pathophysiological changes in the endothelial lining of blood vessels.
Why does the fluid in the middle of the lumen flow fastest in laminar flow?
As there are adhesive forces between fluid and the surface of the blood vessel.
What is the shear rate?
The gradient of the velocity profile at any point.
What is the shear stress?
The shear rate multiplied by the viscosity of the blood.
What are the characteristics of laminar shear stress?
- High sheer stress
- Promotes endothelial cell survival and quiescence
- Cells are aligned in the direction of flow
- Secretions promote vasodilation and anticoagulation
What are the characteristics of turbulent shear stress?
- Low shear stress
- Promotes endothelial proliferation, apoptosis and shape change
- Secretions promote vasoconstriction, coagulation and platelet aggregation
How does blood pressure calculation work?
- Measured at the upper arm as it is heart level
- Slow deflation of the cuff causes turbulent flow, which can be heard with a stethoscope
Why do the ventricular and aortic pressures differ?
- Once the aortic valve closes, ventricular pressure falls rapidly while aortic pressure falls slowly
- Can be explained by elasticity of the aorta and large elastic arteries which act to buffer changes in pulse pressure
What is the windkessel effect?
- If the wall of the vessel is rigid, then there will be a burst of liquid then nothing then another burst
- If there is elasticity, there will be a more continuous flow.
Describe the relationship between the law of laplace and blood flow.
- Transmural pressure causes a tension force on the wall
- High stress causes vessel distension (dilation)
Describe the formation of ruptured aneurysms using the law of Laplace.
- Vessel walls weaken causing a distention
- This increases the radius, so the pressure increases
- The inward force cant be maintained, so the aneurysm will continue to expand until it ruptures.
Define compliance in relation to vessels
- The relationship between the transmural pressure and the vessel volume (depends on elasticity)
Describe how gravity affects blood flow when standing still.
- Veins are highly compliant, so the blood pools in the veins due to gravity increasing hydrostatic pressure in the legs
- This reduces cardiac output and blood pressure, so bloodflow to the organs falls and you may faint.
- Wiggling toes pushes blood back up to the heart.
What are the two pumps for blood (not including the heart)?
- Skeletal muscle pump (positive pressure)
- Respiratory pump (negative pressure)
What causes varicose veins?
- Incompetent valves
- Superficial veins in the leg dilate
