5 Blood Vessels and Blood Flow Flashcards
Q: When the blood first the leaves the heart in systemic circulation, what carries it? They act as? Then the blood moves into? (2)
A: large, thick-walled, elastic arteries which act as dampening vessels
smaller arteries and arterioles
Q: What do arterioles and arteries contain? regulates? produces?
A: extensive smooth muscle in their walls which regulates their diameter and produces a resistance to blood flow
Q: Where does a lot of the pressure drop in arteries take place? (2)
A: small arteries and arterioles
Q: What’s the role of veins in systemic circulation? Why is this considered its role? What properties allow them to do this? (2)
A: veins are very stretchy and highly compliant so they act as a reservoir for blood volume
Much of the blood at any one point rests in the veins and venules hence they’re considered resevoirs
Q: What makes up the largest cross sectional area in the CVS? because?
A: capillaries
it has an exchange function
Q: What happens as a result of shifting the blood from the reservoir to the heart? (2) When does this occur? how?
A: produce more venous return and more cardiac output
if you need to exercise you get venoconstriction meaning that you decrease the amount of stored blood and move more blood back to the heart
Q: How is the fluid circuit of systemic circulation similar to an electric circuit? Laws? (2)
A: electrical circuit (Ohm’s law)
V= I x R
fluid circuit (Darcy's law) /\P = Q x R
(pressure difference= flow/CO x total peripheral resistance resistance)
Q: What can pressure difference be estimated as in /\P=QxR?
A: mean arterial blood pressure
Q: What is R in /\P=QxR? Considered? why? (3)
A: the resistance of all the vessels - also called peripheral vascular resistance
this is an approximation because it assumes a steady flow and assumed that the vessels are rigid and that right atrial pressure is negligible
Q: Physiologically how is the regulation of blood flow achieved? (2) Relies on? How can blood be directed to specific vascular beds? (2)
A: variation in resistance while blood pressure remains relatively constant (relies on mechanisms to detect blood pressure and feedback to keep it constant)
specific contraction and relaxation of the blood vessels that serve the particular vascular bed
Q: How does pressure change across circulation? due to? mostly? (2) On the right side?
A: Pressure falls across the circuit due to viscous (frictional) pressure losses
Small arteries and arterioles present most resistance to flow
pulmonary artery presents a resistance to flow as well
Q: What 3 variables does resistance to blood flow depend on? What is the main determinant of resistance and why?
A: 1. fluid viscosity (eta) = not fixed but in most physiological conditions is constant
- length of tubule (L) = fixed and remains constant
- inner radius of tube (r) = variable **
Q: What does the power function mean in terms of resistance? Example?
A: relatively small change in radius produces a large change in blood flow
Halving the radius would decrease blood flow 16 times
Q: When exercising, how much can we boost our CO by? What else can we do in terms of direction of blood flow? (2)
A: up to 25L/min
changing the radius of various vessels we can increase the blood flow to the working skeletal muscle - by constricting some vessels and dilating others to direct the blood to the place that needs it most
Q: What sort of flow occurs in vessels? Sounds?
A: Blood generally flows in stream lines which don’t tend to interfere with one another - it is laminated flow and hence laminar flow
You CAN NOT HEAR laminar flow
Q: How do you usually measure blood pressure with an inflation cuff? (4) continuation of deflation?
A: pump the cuff up on upper arm to obstruct blood flow = pressure exceeds arterial pressure
place stethoscope distal to the cuff (no sounds as blood flow is occluded)
let the cuff down -> eventually get to a point where the pressure in the cuff is just overcome by the pressure in the artery
blood starts to squirt through the occlusion and sets up turbulent flow - you hear a LIGHT TAPPING SOUND
(continue to reduce the pressure in the cuff, reach a point where you have no occlusion in the artery and so blood will start to flow in a laminar fashion = no sound)
Q: What characterises turbulent blood flow? Sounds? What can turbulent flow change? it can bring about?
A: whirlpool like regions and the velocity of the fluid is not constant
can hear (sounds of korotkoff) = soft tapping sound
the shear stress on the vessels
-pathophysiological changes
Q: Where is blood flow quickest in a vessel? slowest? Due to? Velocity increases as…?
A: quickest in the middle and slowest on the sides
This is because there are adhesive forces which attach the blood to the vessel walls
distance from wall increases
Q: What is shear rate?
A: velocity gradient that is established - the difference between the highest velocity blood in the middle of the lumen and the lowest velocity blood that adheres to the blood vessel walls
Q: How do you get shear stress? What does it do? (3) result? (2)
A: shear rate is MULTIPLIED by viscosity
disturbs endothelial function which is important for laminar flow and the production of various transmitter substances which give rise to vessel dilation and constriction
Q: Where is high shear stress found? promotes? result?
Where is low shear stress found? stimulates? cell behaviour? bearing on? (4)
Which is normal?
A: as found in laminar flow: promotes endothelial cell survival so the endothelial cells line up and produce substances normally**
(turbulent flow), ENDOTHELIAL PROLIFERATION is stimulated (cells are all mixed up and don’t behave in a normal way and don’t produce their substances normally)
has a bearing on vasoconstriction, coagulation, platelet aggregation and atheroma formation
Q: When measuring blood pressure with an inflating cuff, what does the appearance of sound represent? disappearance?
A: Sounds APPEARS = Systolic Blood Pressure
Sound DISAPPEARS = Diastolic Blood Pressure
Q: What is the difference between systolic and diastolic blood pressure?
A: pulse pressure
Q: How do you calculate mean blood pressure? Expectation?
A: Diastolic + 1/3 of pulse pressure (this is lower than you’d expect)
Q: How do ventricular and aortic pressure differ once aortic valves close? Why?
A: Once the aortic valve closes, ventricular pressure falls rapidly but aortic pressure only falls slowly in diastole
This is explained by the elasticity of the aorta which buffers changes in pressure and so it doesn’t drop to zero like the ventricular pressure - the pressure is maintained by the elasticity of the vessel
Q: How does the elasticity of the aorta change? What is it related to?
A: this elasticity changes through life
compliance
Q: What is the Dichrotic Notch? What causes it?
A: second small bump on aortic pressure line (following large one)
When the aortic valve closes, the ejection of blood stops but there is a recoil because the arteries and the aorta are very elastic which produces the dichrotic notch
Q: What is the equation for resistance in a tube?
A: R= 8L(eta)
———–
pi r^4
pi x radius^4
Q: How does pressure change downstream of the aorta? showing? (2)
A: falls slowly downstream of the aorta hence showing that the elasticity allows it to act as a buffer
Q: What is the damping effect sometimes called?
A: WINDKESSEL effect
Q: What can cause arterial compliance to decrease? How can this affect pulse pressure and in what way?
A: age
the damping effect of the Windkessel is reduced and the PULSE PRESSURE WILL INCREASE
Q: What is the pressure inside the vessel called? What does it determine? What does pressure inside the vessel cause?
A: transmural pressure
-distension of the vessel wall
tension force (T) in the wall
Q: What is the relationship between pressure inside the vessel wall and wall tension determined by? Equation?
A: Laplace’s law
T=P x r
Q: What does circumferential stress depend on? What is the equation for circumferential stress? Including Laplace’s law?
A: vessel wall thickness
sigma = T
———-
h (vessel wall thickness)
including Laplace’s law
= P x r
——-
h
Q: What is the relationship between transmural pressure and vessel volume called? What is it dependent on?
A: COMPLIANCE
vessel elasticity
Q: Summarise Laplace’s law. Apart from aneurysms, where else does it apply?
A: The larger the vessel radius, the greater the wall tension required to withstand a given internal fluid pressure
formation of diverticuli (small bulges) in the gut wall
Q: What can occur if vessel walls weaken (over a prolonged period)? They can form as a result of?
A: causing a balloon like distension
Aneurysms form as a result of Laplace’s law
Q: What can happen if an aneurysm forms in a blood vessel? But if the muscle fibre is weakened and the compliance isn’t great? result?
A: for the same internal pressure, the inward force exerted by the muscular wall must also increase
the force needed to withstand the internal pressure cannot be produced and so the aneurysm will continue to expand
Q: Draw compliance curves comparing arteries and veins. How does venous compliance compare to arterial at low pressures?
A: pressure= X and volume= Y
vein = r shape artery= shallow
venous compliance= 10 to 20 times greater
Q: How can venous pressure affect veins?
A: Relatively small changes in venous pressure distends veins and increases the volume of blood stored in them
Q: For the same pressure, what holds more blood; veins or arteries?
A: veins hold larger volume of blood
Q: Draw various compliance curves for veins showing how smooth muscle contraction affects it. (2) What causes the change in smooth muscle contraction? What does this allow?
A: pressure= X and volume= Y
r shape getting shallower
top to bottom= increasing smooth muscle contraction
when you change the nervous supply to the smooth muscle causing contraction, you decrease the venous volume and increase venous pressure
allows you to change the volume of blood in the reservoir
Q: What happens when you stand up quickly? due to? What does this cause? can result in? What is this called?
A: gravity makes the blood pool in the legs which is due to the venous volume/capacitance
When it pools in the legs, this reduces venous return to the heart which means that cardiac output falls and you get less blood going to the brain
called postural/transient hypotension
Q: Describe the mechanism to compensate for postural hypotension.
A: get venous constriction which means that more blood is returned to the heart and cardiac output is increased
Q: Draw a graph showing how mean transmural pressure changes across circulation system (for both ankle and upper arm= heart level).
At any particular location, what is maintained? so?
A: X= large arteries, microcirculation, veins
-------
\
-------------
but more rounded
-ankle above upper arm
-the gradient of pressure from large artery to capillary is MAINTAINED so flow always occurs the normal way
Q: What is the major effect of gravity on in the circulation system? (2)
A: distensible veins in the leg and the volume of blood contained in them
Q: What does standing cause the activation of (to prevent fainting)? how does this affect veins? arteries? to? (2)
What also happens when we stand that prevents fainting? (2) allows?
A: Standing causes activation of the sympathetic nervous system -> stiffens and constricts veins + arteries are constricted to increase total peripheral resistance and maintain blood pressure
there may be a slight increase in heart rate and an increase in the force of contraction which allows more blood to return to the brain
Q: What can the failure of the mechanisms that prevent fainting when standing lead to? (2)
A: fainting (syncope)
hypovolaemia - you may become thirsty and your blood volume may drop a bit
Q: What is the effect of the skeletal pump? Assists? (2)
A: the contraction of the muscle squeezes blood back through the veins to the heart
This assists the movement of blood back to the heart and decreases venous capacitance
Q: Describe the respiratory pump. What does it allow? (2)
A: as we breathe in, we expand our chest and our intrathoracic pressure decreases
allows blood to come back to the right atrium and increase venous return
Q: What two simple mechanisms allow us to be able to stand up for long periods of time without fainting?
A: skeletal pump and respiratory pump
Q: What happens if you have incompetent valves in terms of standing up? Where is it often seen? (2)
A: could lead to VARICOSE VEINS = large swollen veins
in older people who have more incompetent valves
if you have been standing up for a long period of time
Q: What can happen after prolonged standing? (2)
A: have prolonged elevation of venous pressure = you get oedema in your feet
