4 - Cardiovascular mechanics 3

Question 1

How is the circulation designed?

Answer 1

2 circulations - to the body and to the lungs with individual pumps (left and right)
Heart pumps blood and creates pressure gradient.
Also used to regulate temperature
Diffusion through capillaries to cells - highly branches capillary structure.

Question 2

What order does blood flow through blood vessels?

Answer 2

Artery
Arteriole
Capillary
Venule
Vein

Question 3

Which blood vessel has the greatest total surface area?

Answer 3

Capillaries

Question 4

Which blood vessel contains the largest total volume of blood? Why?

Answer 4

Veins

Very compliant and stretchy. Allow a reservoir of blood to stay there.

Question 5

Which blood vessel has the greatest mean pressure and where is the biggest drop in pressure?

Answer 5

Arteries have the greatest pressure. Biggest drop is in arterioles because of frictional pressure losses.

Question 6

Why does blood flow?

Answer 6

Blood pressure produced by the heart.

Question 7

How is voltage measured in an electrical circuit and how does this relate to pressure in the heart? What law is this?

Answer 7

Voltage = Current x Resistance

Darcy’s Law
Pressure = Blood flow x Resistance

Question 8

How does (Pressure = Blood flow x Resistance) link to mean arterial pressure? What are the assumptions

Answer 8

Blood pressure (MAP) = Cardiac output - Resistance
Assumes:
Steady flow
Rigid vessels
Right atrial pressure negligible

Question 9

How does mean blood pressure remain constant?

Answer 9

Resistance in vessels changes to regulate flow.

Question 10

What 3 variables does resistance depend on?

Answer 10

1. Viscosity (blood doesn’t really change) n
2. Length of vessel (doesn’t change either) L
3. RADIUS of vessel r

Question 11

What is the equation to find resistance and what is this simplified?

Answer 11

R = (8Ln)/(Pi * r^4)
R = 1 / r^4

e.g. halving the radius decreases the flow by 16 times.
Small change in radius = big change in flow

Question 12

What is cardiac output during rest and exercise?

Answer 12

Rest - 5L/min

Exercise - 20L/min

Question 13

What is laminar flow?

Answer 13

Velocity is constant
Blood flows fastest in centre of lumen
Flows in layers - linear

Question 14

What is turbulent flow?

Answer 14

Blood flows erratically, forming eddys (loops) and prone to pooling.
Associated with pathophysiological changes to endothelial lining of blood vessel.

Question 15

What is the shear rate?

Answer 15

The gradient of the velocity profile (faster in middle of lumen) at a point. Uphill line.

Question 16

What is shear stress?

Answer 16

Shear stress (T) = Shear rate x viscocity

Question 17

How does HIGH shear stress affect endothelial cells?

Answer 17

High shear stress is a result of laminar flow.
Promotes endothelial cell survival and inactivity
Cells aligned in direction of flow
Secretions promote vasodilation and anticoagulation.

Question 18

How does LOW shear stress affect endothelial cells?

Answer 18

Low shear stress is caused by turbulent flow.
Promotes endothelial proliferation, apoptosis and shape change.
Secretions promote vasoconstriction, coagulation and platelet aggregation.

Question 19

How is turbulent flow used to measure blood pressure?

Answer 19

Cuff placed on upper arm inflates to just higher than your systolic blood pressure (120)
This means no blood can pass through
Cuff is slowly deflates
Blood flow is turbulent as is passes through in ‘pulses’ which can be counted as turbulent flow can be heard.
Diastolic blood pressure is measured when no more sound can be heard as laminar flow is silent.

Question 20

What is pulse pressure?

Answer 20

Pulse pressure (PP) = Systolic blood pressure - Diastolic blood pressure

Question 21

What is mean arterial pressure?

Answer 21

MAP = DBP + 1/3PP

Because it spends 1/3 of cardiac cycle in systolic phase

Question 22

Why does aortic pressure remain more constant than ventricular blood pressure?

Answer 22

Aorta is very elastic.
As blood leaves, the lumen gets smaller in order to maintain the high pressure.
Elasticity relates to compliance - Windkessel effect (stretchy tube maintains pressure)

Question 23

Explain Windkessel effect in relation to arteriole compliance and pulse pressure.
How does age/stiffer arteries effect this?

Answer 23

Blood enters the aorta faster than in leaves.
Due to recoil of elastic arteries, pressure only falls slowly (during diastole)
With age/stiffening of arteries, compliance decreases the Windkessel effect is REDUCED and pulse pressure increases.

Question 24

What is the pressure gradient?

Answer 24

Pressure at first point - Pressure at second point

Question 25

What is circumferential stress?

Answer 25

Stress on the circumference of the blood vessel itself, caused by transmural pressure (from blood flowing through the vessel)

Question 26

What is transmural pressure?

Answer 26

Pressure from the blood flowing through the vessel. Makes a tension force on the wall: T = P x r

Question 27

How is circumferential stress measured?

Answer 27

𝛿 = Tension force (T) / Wall thickness (h)

Question 28

What can persistent high circumferential stress cause?

Answer 28

Vessel distention (extending)

Question 29

What is an aneurysm?

Answer 29

Over time, vessel walls weaken and have a ballon-like distention. Increased radius of vessel Law of LaPlace - for the same pressure the tension force must increase. BUT - muscles are weakened and can't produce this force so aneurysm expands until it ruptures.

Question 30

What is diverticuli in the gut?

Answer 30

Same principal as aneurysm - ballon-like distention in the gut.

Question 31

Which vessels have more compliance, arteries or veins? Why?

Answer 31

Veins - Compliance is the relationship between transmural pressure and vessel volume. Veins must be compliant as they store the majority of the blood. Increasing smooth muscle contraction decreases the vessel volume and increases the pressure. Volume can be changed very easily.

Question 32

How does standing up affect blood pressure?

Answer 32

Standing up increases hydrostatic pressure in the legs due to gravity. Vein compliance means blood can pool in the legs. Therefore less venous return to heart, less cardiac output. Fainting may occur. Called postural/orthostatic hypotension

Question 33

What is the skeletal muscle pump?

Answer 33

When muscles are activated (e.g. wiggling toes) pushes blood back through veins upwards the heart. Valves prevent backflow.

Question 34

What is the respiratory pump?

Answer 34

As you breathe in diaphragm drops, causes negative intrathoracic pressure. Allows blood to flow back to heart more easily.

Question 35

What can incompetent veins cause?

Answer 35

Varicose veins - pooling of blood in veins. | Dilated superficial veins in legs.

Question 36

What can prolonged elevation if venous pressure cause?

Answer 36

Oedema in feet