Vasculature

Question 1

Describe the main features of laminar fluid flow.

Answer 1

– Fluid molecules touching wall adhere move slowly
– Next layer slips over these etc.
– Middle most layers move the most rapid (Parabolic velocity profile)

Question 2

Describe the main consequences of turbulence on blood flow.

Answer 2

Disrupts flow
Increases resistance
Poiseuille’s law doesn’t hold true

Question 3

Is turbulence pathological or physiological ?

Answer 3

Can be both.

Question 4

Give an example of physiological turbulence.

Answer 4

When ejecting blood at high force into aorta, will create some turbulent flow (expansion of aorta allows turbulence to be harnessed).

Question 5

Define Reynold’s number.

Answer 5

Used to indicate whether flow is laminar or turbulent (i.e. for a given system, there will be a “critical value” for Re, above which turbulence is highly likely.)

Re = ((velocity of flow) x (radius of vessel)) / viscosity

Question 6

What factors increase Re ? Explain why.

Answer 6

Factors which increase Re/make turbulence more likely:
– High velocity flow
– Large diameter vessels (Larger vessel, more layers we can get in there, more likely disruption where one of those layers get disrupted)
– Low blood viscosity
– Abnormal vessel wall

Question 7

Is the aorta more likely to have laminar or turbulent blood flow ? Why ?

Answer 7

Turbulent

Highest velocity flow, large radius so turbulent

Question 8

Describe the main features of the viscosity of blood.

Answer 8

Blood is a thixotropic fluid (increased flow results in reduced viscosity)

Question 9

How much more viscous is static blood relative to flowing blood ?

Answer 9

100x more viscous

Question 10

Graph relative viscosity of blood as a function of blood flow velocity.

Answer 10

Refer to slide 5, on lecture “Vasculature”

Question 11

Give a clinical application of turbulent flow.

Answer 11

When we take blood pressure, you create artificial turbulent flow using a sphygmomanometer cuff.

Question 12

Describe LaPlace’s Law.

Answer 12

Distending pressure (P) produces an opposing force or transmural tension (T) in the vessel wall (in order to withstand the pressure within it), proportional to the radius (R) of the vessel.
T=PR

Question 13

Describe the implication of LaPlace’s law on arterioles, the aorta, and capillaries.

Answer 13

Aorta: Large radius, has to generate a huge amount of tension otherwise it would rupture (properties of it evolved to withstand it). Hence, thick walls.

Arterioles: small radius (than aorta) and lower amount of pressure (than aorta) which means it is easy to generate enough tension to stop vessel rupturing. Arterioles can have smooth muscle inside walls which can exert increase in tension which can further squeeze radius.

Capillaries: Very small radius and very little pressure, so walls are one cell thick only (tension required is small). This enables the exchange process to take place.

Question 14

Identify the practical consequences of LaPlace’s law on the control of blood flow.

Answer 14

1) CONTROL OF BLOOD FLOW
– Low tension required to oppose blood pressure in arterioles
– Smooth muscle control of arteriole and precapillary sphincters are the sites of tissue blood flow regulation (can squeeze radius)

Question 15

Explain how aneurysms happened, keeping LaPlace’s law in mind.

Answer 15

• Bulge caused by mechanical weakening of affected vessel
• Radius therefore changes (in the bulge, radius greater)
• Consequently, tension has to be greater in the bulge, but that’s also the part that is mechanically weakened, so it keeps expanding until it ruptures (because weakened part of the wall means you can’t withstand the pressure within it and the tension just gives way)

Question 16

Which vessels are the most responsible for blood flow regulation? Identify the main functions of each.

Answer 16

Arterioles
– Control regional distribution (Local (may either constrict or dilate) and extrinsic
(only constrict) controls)

Meta-arterioles (links arterioles to capillaries, discontinuous smooth muscle)
-May either constrict or dilate

Precapillary sphincters (not vessels, where a true capillary branches from a metarteriole)
-May either vasodilate (Local factors) or constrict

Question 17

Describe an example of specialized blood flow.

Answer 17

BLOOD FLOW TO THE SKIN
Capillaries fed by arteries draining into veins which branch off from artery.

1) Allows directing blood flow to artery at the surface to make sure cells there get flow + a way to radiate heat if direct blood towards surface
2) Allows directing blood away from skin to maintain heat (into the arterioveinous anastomosis). If these vessels are dilated, blood will be directed into them, then into venous plexus and finally back into the veins.

Question 18

Describe the main ways in which tissue blood flow is regulated.

Answer 18

1. ACTIVE AND REACTIVE HYPERAEMIA
   - Build up of local factors associated with metabolic activity of tissues (e.g. metabolic waste products being generated in tissues) will cause smooth muscle to relax, resulting in dilation to allow more blood to be directed into area of concern (Independent of innervation/hormonal control)
2. FLOW AUTOREGULATION (way to protect capillaries from high pressures upstream, not so much about delivering blood or stopping it going to areas)
   – In response to changes in arterial pressure
   • Arterial pressure↑, arterioles constrict to reduce flow
   • Arterial pressure↓, arterioles dilate to increase flow
   – Constriction due to auto-regulation is through myogenic response (↑ arterial pressure results in stretch-activated Ca2+ channels causing Calcium influx into smooth muscle, and thus depolarization of smooth muscle) (not so much dilation)
   – Constriction due to auto-regulation has also been hypothesized to be due to ↑arterial pressure increases O2 and “washes out” local factors (metabolic theory)
3. VASOMOTION
   – Spontaneous oscillating contraction of blood vessels (randomised pattern)
4. RESPONSE TO INJURY
   – E.g. endothelin-1 released from endothelial cells (potent vasoconstrictor, to stop blood loss and allow coagulation to occur)
5. SYMPATHETIC NERVOUS SYSTEM
   - Will result in vasoconstriction (e.g. in exercice, to direct less blood is non-essential areas in order for sk. muscle to get more blood flow)

Question 19

To what extent are constrictions and dilations resulting from the auto-regulations going to affect peripheral resistance and MABP ?

Answer 19

It depends. If these constrictions and dilations are in localised areas, they will barely affect total peripheral resistance and MABP.
If they are more widespread, larger effects.

Question 20

What is the key driver of blood flow to tissues ?

Answer 20

Metabolic demands of said tissue.

Can override sympathetic signal to constrict.

Question 21

Distinguish between active and reactive hyperaemia. Provide an example which involves both active and hyperactive hyperaemia.

Answer 21

ACTIVE HYPERAEMIA
– If tissue is highly active, the rate of flow will increase
– E.g.by up to 20x in skeletal muscle

REACTIVE HYPERAEMIA
– When blood supply blocked (few s to h)
– Blood flow increases to 4-7x normal (due to buildup of metabolic product)

During exercice, higher metabolic demands of skeletal muscle causing higher blood flow to it (active hyperaemia). In the meantime, non-essential areas might be deprived of blood flow, resulting in reactive hyperaemia later once metabolic products have accumulated in those areas.

Question 22

Represent active and reactive hyperaemia in graphical form.

Answer 22

Refer to graph on slide 12, lecture on “Vasculature”

Question 23

Describe the role of the endothelial cells in regulating vascular tone.

Answer 23

1. Secrete endothelin (potent vasoconstrictor)
2. Shear stress (e.g. due to viscous blood) results in conversion of O2 + L-Arginine into Nitric Oxide and waste product using an enzyme present in endothelial cells (eNOS). NO is a soluble gas so it diffuses easily through endothelial cell into smooth muscle beneath it and causes a process which causes smooth muscle to relax (VASODILATION)

Question 24

Identify the main hormonal, neural, and local vasodilators and vasoconstrictors.

Answer 24

VASOCONSTRICTORS

1. Neural: Sympathetic Nerves
2. Hormonal: Adrenaline (depending on tissue, may either vasoconstrict or vasodilate), Angiotensin II, Vasopressin
3. Local: Myogenic response, Endothelin-1

VASODILATORS

1. Neural: NO-releasing nerves
2. Hormonal: Adrenaline, Atrial-natriuretic peptide
3. Local: ↓Oxygen, Adenosine, NO, Bradykinin, K+, CO2, H+, Lactic acid, Histamine

Question 25

At rest, what proportion of total cardiac output is in the capillaries ?

Answer 25

5%

Question 26

Is there always blood flow in the capillaries ?

Answer 26

No, blood flow through capillaries is intermittent, turning on/off every few seconds or minutes

Question 27

Distinguish between acute and long term local blood flow.

Answer 27

ACUTE LOCAL BLOOD FLOW REGULATION
– Rapid changes within seconds or minutes
– Vasodilator theory widely accepted (“local factors”)

LONG TERM LOCAL BLOOD FLOW REGULATION
– Change in physical size or number of blood vessels (angiogenesis)

Question 28

Define the vasodilator theory for acute local blood flow regulation.

Answer 28

“Greater the rate of metabolism or the less the availability of oxygen or some other nutrients to a tissue, the greater the rate of formation of vasodilator substances in the tissue cells”

Question 29

Describe the velocity of blood flow at the capillary level. What is the significance of this ?

Answer 29

Velocity of blood flow through capillaries is the slowest.

Allows time for diffusion and exchange of nutrients and waste.

Question 30

List the main types of capillaries and where each if found.

Answer 30

1. Continuous Capillaries: very little gaps between cells, quite a few junctions so not a huge amount of cells or proteins traveling across membrane (Most tissues)
2. Fenestrated Capillaries: Smaller molecules can make it across holes in membrane (Skeletal muscle)
3. Sinusoid/Discontinuous Capillaries: larger molecules can make it across membrane (Liver)