CVS S5 - Blood flow Flashcards
What is the relationship between flow and velocity?
At fixed flow, the velocity is inversely proportional to cross sectional area
Give an example of a vessel through which blood would travel at high velocity at a given flow rate
Artery
How does velocity alter across a vessel with laminar flow?
Velocity gradient from middle to edges of the vessel with blood travelling faster at the centre and stationary at the edges
What is turbulent flow?
As mean velocity increases turbulent flow begins whereby the velocity gradient breaks down, fluid tumbles over and resistance increases
List the factors effecting mean velocity
Viscosity and radius of vessel
What is the relationship between mean velocity, and viscosity and radius?
- Mean velocity is inversely proportional to viscosity
- Mean velocity is proportional to cross-sectional area
List the factors effecting flow resistance
Resistance increases as viscosity increases
Resistance decreases with fourth power of radius
Outline the key relationships between pressure, resistance and flow
Fixed pressure- increasing resistance decreases flow
Fixed flow- increasing resistance increases pressure
How does resistance differ between vessels in series and those in parallel?
Series - resistance of vessels adds
Parallel - effective resistance is less (RT = R1 x R2/ R1 + R2)
At which points of the systemic circulation will resistance be low and why?
- Arteries and veins due ti small drop in pressure
- Capillaries as despite high individual resistance, they are connected in parallel creating low overall resistance
Why is pressure within arteries so high?
Pressure must be high enough to pump cardiac output into arterioles and overcome total peripheral resistance
What is transmural pressure?
Pressure gradient between the inside and outside of a vessel that stretches walls of distensible vessels
How would a decrease in pressure effect blood flow through distensible vessels?
Walls of the vessel would collapse causing blood flow to cease before driving pressure reaches zero
How are distensible vessels capable of capacitance?
As vessels widen due to an increase in pressure, more blood transiently flows in than flows out which is stored
How is mean arterial pressure calculated and why are these proportions used?
1/3 systolic pressure + 2/3 diastolic pressure
Diastole is longer in duration than systole
What is pulse pressure?
Difference between systolic and diastolic pressure, approximately 40 mmHg
Define total peripheral resistance
Resistance of vessels of the systemic circulation that opposes blood flow
How does the elastic nature of arteries reduce the difference in pressure between systole and diastole
- Walls stretch in systole allowing more blood to flow in than out so pressure doesn’t rise as much
- Arteries then recoil in diastole
How would arterial pressure differ during systole and diastole if artery walls were rigid opposed to elastic?
Pressure would rise to a maximum in systole to force the stroke volume through the total peripheral resistance and would fall to zero in diastole
List the factors effecting systole and how they differ from diastole
How hard heart pumps, total peripheral resistance, stretchiness of arteries
Diastolic pressure is unaffected by stretchiness of arteries
What is a pulse wave?
Wave produced from contraction of ventricles which propagates along the arteries faster than blood
How do arterioles act as resistance vessels?
Narrow lumen, narrowed by tonic contraction of smooth muscle, generates high resistance
What would be the resistance vessels found in skeletal muscle?
Pre-capillary sphincters
Define vasomotor tone
Tonic contraction of smooth muscle in the walls of vasculature
What is reactive hyperaemia?
- Removal of blood supply
- Build up of local vasodilator metabolites
- Maximum dilation of arterioles
- Blood flow returns
- Enormous increase initially due to low resistance generating high flow
- High flow washes away metabolites
- Smooth muscle constricts again
How does vasodilatation differ from vasodilation?
- Vasodilation is when blood vessels become fully relaxed
- Vasodilatation is reduced vasoconstriction
Outline the process of blood flow autoregulation
- Change in supply pressure
- Blood flow changes
- Metabolite concentration alters
- Alter resistance of arterioles
- Blood flow returns to appropriate level in line with metabolism
How is pressure in the veins determined?
Determined by volume of blood they contain which is dependent on balance between the inflow and outflow of the heart
What is central venous pressure and what does it depend on?
Pressure in the great veins
Depends on return of blood from the body, pumping of the heart, gravity and ‘muscle pumping’
