4. Blood Vessels and Blood Flow Flashcards
Put the following in order in circulation: resistance, reservoir, elastic arteries, exchange
(pump) 1) Elastic arteries 2) Resistance 3) Exchange 4) Reservoir (pump)
Outline the structure of the systemic circulation
- Blood leaves heart through large, thick-walled, elastic arteries (dampening vessels)
- Smaller arteries and arterioles with extensive smooth muscle (regulates diameter/resistance)
- Capillaries make up the largest cross-sectional area - exchange function
- Veins are stretchy and compliant - reservoir for blood volume (highest blood volume out of all vessles)
What happens to the veins during exercise?
- Venoconstriction
- Decreased stored blood
- More blood moves back to the heart
- i.e. more venous return and more cardiac output
How is the fluid circuit similar to an electrical circuit?
• Electrical: V=IR
current x resistance
• Fluid: ΔP = QR
or Pressure difference = volumetric flow x resistance
(or mean arterial blood pressure = cardiac output x resistance)
What does resistance (peripheral vascular resistance) assume in ΔP = QR?
- Steady flow
- Rigid vessels
- Negligible right atrial pressure
- Therefore it is an approximation
How is blood flow regulated with reference to resistance and pressure?
- Regulation of flow achieved by variation in resistance
- Constant blood pressure using feedback mechanisms
- Blood directed by specific contraction and relaxation of blood vessels
How does pressure change throughout circulation?
- High pressure in large arteries, and falls across the circuit due to viscous (frictional) pressure losses
- Small arteries and arterioles - most resistance
- Pressure slightly rises in the right side of the heart
What 3 variables does the resistance of a tube to flow depend on?
1) Fluid viscosity (η) - not fixed but in most physiological conditions is constant
2) Length of tube (L) - fixed
3) Inner radius (r) - variable, main determinant of resistance
What is Poiseuille’s equation?
R = (8Lη) / (πr^(4))
• Emphasises the importance of arterial diameter as a determinant of resistance
• Small changes in vascular tine (vasoconstriction/dilation) can produce marked changes in flow
• 1/2 radius decreases the flow 16x
How does blood flow to the heart and brain change when exercising?
It doesn’t
What is laminar flow?
- Blood flowing in stream lines
- No interference
- No sound
Why do the sounds of korotkoff disappear after further dropping of the cuff pressure?
- Vessel is no longer occluded
* Blood starts to flow in a laminar fashion
What is turbulent flow and what can it result in?
- Whirlpool like regions
- No constant velocity
- Can bring about pathophysiological changes
- Can change the shear stress on the vessels - damage the lining - atheroma
What is the shear rate?
- The velocity gradient that is established
- Difference between the highest velocity of blood in the middle of the lumen and the lowest velocity blood that adheres to the walls
What is shear stress?
- Sheer rate x viscosity
- Disturbs endothelial function - important for laminar flow and production of various transmitter substances which gives rise to vessel dilation and constriction
What happens when shear stress is high or low?
- High/laminar - promotes endothelial cell survival, quiescence, cell alignment and secretion of substances that promote vasodilation and anticoagulation
- Low, (disturbed or changed/turbulent) - promotes endothelial proliferation - bearing on vasoconstriction, coagulation, platelet aggregation and atheroma formation
How do you take blood pressure and explain the steps?
1) Cuff around the upper arm, and increase the pressure until cuff pressure > arterial pressure
2) Place stethoscope distal to the cuff - initially no sound as blood flow is occluded
3) Arterial pressure (just) > cuff pressure - blood starts to squirt through the occlusion - turbulent flow - light tapping sound (sounds of korotkoff) - SBP
4) Continue to reduce pressure - no occlusion - laminar flow - no sound - DBP
(difference between SBP and DBP = pulse pressure)
(mean blood pressure = DBP + 1/3 pulse pressure)
Why do ventricular and aortic pressure differ?
- Aortic valve closes - ventricular pressure falls rapidly, aortic pressure falls slowly (diastole)
- Explained by the elasticity of the aorta - buffers changes in pressure - doesn’t drop to zero like the ventricular pressure
How does the elasticity of the aorta play a part in causing the dichrotic notch?
- Blood enters the aorta faster than it leaves
- About 40% of the stroke volume is stored by the elastic arteries
- Aortic valve closes - ejection of blood stops but recoil due to elasticity produces dichrotic notch
What is the Windkessel effect?
- The dampening effect of the aorta
- Elastic - buffer
- Blood flow becomes smoother and constant
What happens to the Windkessel effect and pulse pressure when arterial compliance decreases (with age)?
- Damping effect of the Windkessel is reduced
* Pulse pressure increases
What is transmural pressure?
Pressure inside the vessle (difference in pressure between 2 sides of the wall)
How can Transmural pressure be described by Laplace’s relationship?
- Transmural pressure causes a tension force (T) in the wall, so
- T = P x r
How do you calculate circumferential stress?
- circumferential stress = tension force / wall thickness
- σ = T / h
- σ = (P x r) / h
What does a maintained high circumferential stress cause?
Vessel distension (dilation)
What is the relationship between the transmural pressure vessel volume called?
- Compliance
* Dependent on vessel elasticity
How do aneurysms form?
- Vessel walls can leak over a prolonged period of time
- Balloon like distension forms
- Result of Laplace’s Law
- If they form, for the same internal pressure, the inward force exerted by the muscular wall must increase
- If the muscle fibre is weakened and compliance isn’t great, force cannot be produced - aneurysm expands further
- Laplace’s Law also applies to diverticuli formation in the gut wall
What are the compliance properties of the vein?
- Small changes in venous pressure distends veins and increases the volume
- For the same pressure, veins can therefore hold a much larger volume of blood (than arteries)
- Nervous supply => smooth muscle => contraction => decrease venous volume => increase venous pressure
What is postural hypotension?
- Standing up quickly
- Gravity makes blood pool in the legs - venous volume/capacitance
- Reduced venous return to the heart
- Cardiac output falls
- Less blood to brain
• Venous constriction increase blood return and cardiac output
Why don’t we faint when standing and what happens when these mechanism fail?
- Activation of sympathetic nervous system
- Stiffens and constricts veins
- Arteries constrict - increase total peripheral resistance and maintain BP
- Slight increase in heart rate and an increase in the force of contraction - more blood return to the brain
- Failure => fainting (syncope) or hypovolaemia (decreased blood volume, salt depletion - thirst)
What is the skeletal pump?
- The contraction of the muscles squeezing blood back through the veins to the heart
- Decreases venous capacitance
- Helps with standing for long periods of time without fainting
What is the respiratory pump?
- Breathing in => chest expands & intrathoracic pressure decreases
- Blood allowed back into the right atrium
- Increased venous return
- Helps with standing for long periods of time without fainting
What happens to people with incompetent venous valves?
- Could lead to varicose veins
- Often seen in older people
- Standing for a long period of time - prolonged elevation of venous pressure - oedema in feet
