4. Blood Vessels and Blood Flow Flashcards by Kieron Young

What are the features of arteries?

Large, thick-walled and elastic

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What are the features of small arteries and arterioles?

Extensive smooth muscle, used to produce resistance to blood flow

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Where does most of the pressure drop in the arteries occur?

Small arteries and arterioles

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What are the features of veins?

Very stretchy and highly complient

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What is the main function of veins?

Reservoir for blood volume

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Which blood vessel makes up the largest cross sectional area in the CVS?

Capillaries - has exchange function

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What happens to the store of blood during exercise?

Vasoconstriction, the store of blood decreases and more blood moves back to the heart.

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What effects does a decreased volume of stored blood have?

Increased venous return so more cardiac output

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Mean Arterial Blood Pressure =

Cardiac output x Resistance

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What can the pressure difference be estimated as?

Mean arterial blood pressure

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Define resistance?

The resistance of all the vessels, also called peripheral vascular resistance

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What is the equation for the mean blood pressure?

Delta P = QR

MBP = Cardiac output (CO) x Resistance (PVR)

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How is the regulation of flow achieved?

Variation in resistance while the blood pressure remains relatively constant

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Why does pressure fall across the circulation?

This is due to viscous (frictional) pressure losses

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What vessels present the most resistance to flow?

small arteries and arterioles

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What three variables affect the resistance to blood flow?

Fluid viscosity
Length of tube
Inner radius of tube

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What is the main variable that affects the resistance to blood flow?

Inner radius of tube

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How does halving the radius affect blood flow?

It would decrease blood flow 16 times.

R=1/r^4

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When exercising what can we boost our cardiac output to?

25L/min

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How can we increase the blood flow to working skeletal muscle

Vasoconstriction and vasodilation

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What is laminar flow?

Blood flowing in stream lines

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What is turbulent flow?

Blood flowing in whirlpool like flow. Sounds of korotkoff when using a pressure cuff and stethoscope

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Describe the speed of blood flowing in a blood vessel

Blood flows quickest in the middle and slowest on the sides

Why is there varying speed of blood flow within a blood vessel?

This is due to adhesive forces which attach blood to the vessel walls. Velocity increases as distance from the wall increases

Define shear rate

The 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

What is the equation for shear stree?

Shear stress = Shear rate x Viscosity

What happens when there is high shear stress?

Promotion of endothelial survival. They line up and produce substances normally - found in laminar flow

What happens when there is low, disturbed or changed shear stress?

Endothelial proliferation affecting vasoconstriction, coagulation, platelet aggregation and atheroma formation - found in turbulent flow

What is high shear stress the same as?

Normal shear stress

Measuring Blood Pressure - sound appears?

Systolic blood pressure

Measuring Blood Pressure - sound disappears?

Diastolic blood pressure

What is pulse pressure?

Systolic BP - Diastolic BP

Mean blood pressure =

Diastolic BP + 1/3 of Pulse pressure

Describe the pressure changes in the aorta and ventricles once the aortic valve closes

The ventricular pressure falls rapidly but the aortic pressure only falls slowly

Why do these pressure changed occur in the aorta and ventricles once the aortic valve closes?

The aorta is elastic so it buffers pressure changes preventing the pressure to dropping to zero

Define dichrotic notch

This is a secondary upstroke in aortic pressure after blood has left the heart. When blood enters the aorta faster than it leaves the aorta, about 40% of the stroke volume is stored by the elastic arteries. There is a recoil from the elastic aorta and arteries producing the dichrotic notch

What is the name of the dampening effect on blood pressure?

Windkessel effect

What happens to the pulse pressure with increasing age?

The arterial compliance decreases with age decreasing the dampening effect of windkessel, increasing pulse pressure

What is the name of the pressure inside the vessel?

Transmural pressure

What is the effect of pressure inside the vessel?

It causes distension of the vessel wall

What is Laplace's Law?

T = P x r

What does circumferential stress depend on?

Vessel wall thickness | CS = (P x r)/h h = wall thickness

What describes compliance?

The relationship between the transmural pressure and vessel volume

What is compliance depended on?

Vessel elasticity

Summarise Laplace's law

The larger the vessel radius, the greater the wall tension required to withstand a given internal fluid pressure

What is the venous compliance compared to arterial compliance?

10 to 20 times greater

What is the cause of postural hypotension?

When you stand up quickly, 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 A mechanism to compensate for the postural hypotension is that you get venous constriction which means that more blood is returned to the heart and cardiac output is increased

What does standing cause the activation of?

Sympathetic nervous system

In terms of standing, what does the sympathetic nervous system do?

Stiffens and constricts veins

What happens when you stand?

1) Arteries constrict to increase total peripheral resistance and maintain blood pressure 2) Slight increase in heart rate and increase in force contraction - increases blood to brain 3) Veins stiffen and constrict

What happens when the compensatory mechanisms for standing fail?

1) Fainting - syncope | 2) Hypovolaemia - you may become thirsty and your blood volume may drop

What types of circulatory pumps exist?

Skeletal muscle pump | Respiratory pump

How does the skeletal pump work?

Through the contraction of the muscle it squeezes blood back through the veins to the heart

What is the point of the skeletal pump?

It assist the movement of blood back to the heart ad decreases venous capacitance

How does the respiratory pump work?

As we breathe in, we expand our chest so our intrathoracic pressure decreases. This allows blood to come back into the right atrium and increase venous return

What causes varicose veins?

Incompetent valves

What occurs with prolonged elevation of venous pressure?

Oedema in your feet