Vasculature – Arterial Blood Flow, Peripheral Resistance

Question 1

Leaning outcomes

Answer 1

• To explain how physical factors affecting the cardiovascular system (e.g. flow, pressure, tension, vessel radius, blood viscosity, and the velocity of flow) are interrelated and the practical haemodynamic implications of these.
• To describe how blood flow through the microcirculation is regulated at the tissue level, via both short-term (acute) and long-term mechanisms.
• To identify systems with specialised blood flow requirements and describe the function of these specialised flows.

Question 2

Where does laminar flow occur?

Describe laminar flow

Answer 2

• Laminar flow occurs in vessels lined with endothelial cells
• With laminar flow, the fluid molecules touching the wall move slowly, with the next layer inwards slipping over this layer
• This results in the middle layer moving the most rapid

Question 3

What does turbulence do to blood flow?

How is Poiseuille’s law affected by turbulence?

Answer 3

• Turbulence disrupts flow by increasing resistance through collisions of layers of flow
• Poiseuille’s law doesn’t hold true during turbulence

Question 4

What are 3 examples of where turbulence can occur?

Answer 4

• Examples of where turbulence occurs:
  1) Aorta – large ejections of blood at high force creates turbulence, but the elastic nature of the aorta harnesses this energy to push blood forward
  2) Bifurcation of vessels
  3) Obstructions

Question 5

What is Reynolds number?

Answer 5

• Reynolds number (Re) is used to indicate whether flow will be laminar or turbulent in a specific vessel
• For a given system, there will be a critical number for Re, above which turbulence is highly likely

Question 6

What are 4 examples of when turbulence is likely?

Answer 6

• Turbulence is likely (because Re increases with):
  1) High velocity flow – Increased potential for collision events
  2) Large diameter vessels – increased number of laminar flow layers leads to an increase likelihood of collisions
  3) Low blood viscosity – Less transference of friction, more opportunity for high-speed collisions and disarray
  4) Abnormal vessel wall

Question 7

What is a thixotropic fluid?

What is an example?

Answer 7

• A thixotropic fluid is a fluid that’s viscosity is affected by flow
• Blood is a thixotropic fluid, as static blood has 100x the viscosity of flowing blood

Question 8

What is an example of when we make artificially generated turbulence?

Answer 8

• We artificially generate turbulence when taking auscultatory measurements (Korotkoff sounds) using a sphygmomanometer cuff while measuring blood pressure

Question 9

What is LaPlace’s law?

What equation does this form?

What is this law good for explaining?

Answer 9

• LaPlace’s law states distending pressure (P) produces on opposing force of tension (T) in the vessel wall that is proportional to the radius (R) of the vessel
• This forms the equation T=PR
• LaPlace’s law is good for explaining what properties a vessel wall needs to stand the pressures within it, and also when things might go wrong
• It also shows how small amounts of smooth muscle can generate tension to overcome flow through some vessels by generating enough opposing tension e.g muscular arteries

Question 10

How do compliant/non-compliant vessels accommodate LaPlace’s law?

Why does an increase in Radius/pressure require greater tension in the vessel?

Answer 10

• Compliant vessels accommodate LaPlace’s law by stretching to increase tension and accommodate the pressure
• Non-compliant vessels accommodate LaPlace’s law by being rigid enough to overcome the pressure without breaking (e.g capillaries, arterioles, copper pipe)
• An increase in radius will mean there are more layers of laminar flow present, which increases the likelihood of collisions, therefore increasing tension required (e.g arteries veins)
• An increase in pressure will increase the number of collisions against the vessel wall, meaning more tension is required

Question 11

Why is low tension required to oppose blood pressure in arterioles?

Answer 11

• Low tension is required to oppose blood pressure in arterioles because arterioles have a thin layer of smooth muscle that can contract and generate the tension required to overcome the blood pressure

Question 12

Why is LaPlace’s law important for capillaries?

Answer 12

• LaPlace’s law is important to capillaries as capillaries need to be very thin walled for gas exchange, but still need to be able to withstand the pressure without rupturing
• This means there needs to be low blood pressure in the capillaries to avoid this

Question 13

What is an aneurysm?

How does LaPlace’s law apply to aneurysms?

Answer 13

• An aneurysm is a weakened part of the vessel wall
• When the vessel wall is weakened, it can not generate enough tension to overcome to blood pressure
• This leads to the blood vessel swelling, which increases the radius and further increases the tension required to overcome blood pressure

Question 14

How can the aneurysm be healed?

What happens if this does not happen?

Answer 14

• The aneurysm can be healed with fibrous material to help support the vessel
• If this does not happen, the aneurysm can rupture/burst

Question 15

What are 3 ways blood flow is regulated by vessels?

Answer 15

• Ways blood flow is regulated by vessels:

1) Arterioles
* Controls regional distribution (local and extrinsic controls)
* Can direct blood towards and away from tissues at a large scale
* Can also be used for local distribution

2) Metarterioles
* Links arterioles to venules and can shunt blood from arteries to veins without going through certain capillaries
* Uses discontinuous smooth muscle

3) Precapillary sphincters
* Found when a true capillary branches off a metarteriole
* Vasodilation produced by local factors, which are things that tissues release, ANS neurons, or hormones

Question 16

What are the 4 ways in which tissues blood flow is regulated?

Answer 16

• Ways in which tissues blood flow is regulated:

1) Active and reactive hyperaemia
* Local factors associated with metabolic activity

2) Flow autoregulation
* In response to changes in arterial pressure
* Protective mechanism
* Can cause relaxation in blood vessels depending on local changes in pressures, to prevent damage in the local vessels
* Increase in arterial pressure, smooth muscle in arterioles constrict to reduce flow
* Arterial pressure decrease, smooth muscle in arterioles dilate to increase flow
* If resistance increases, either pressure must increase to maintain flow, or flow rate must reduce to maintain pressure
* This is a myogenic response, where stretch activated Ca2+ channels trigger the process of contraction

3) Vasomotion
* Spontaneous oscillating contraction of blood vessels e.g capillary sphincters relaxing and contracting

4) Response to injury
* E.g local factor endothelin-1 released from endothelial cells
* Can cause potent vasoconstriction to prevent blood loss

Question 17

What are the 2 types of hyperaemia?

When do they each occur?

Answer 17

• 2 types of hyperaemia:

1) Active hyperaemia

2) Reactive hyperaemia

Question 18

when does active hyperaemia and reactive hyperaemia occur?

Answer 18

1) Active hyperaemia
* Most tissue is regulated by this
* If tissue is highly active, the rate of flow will increase
* Metabolically active tissue produces break down products of ATP, like ADP, Pi, CO2 ,Protons (hydrogen ions)
* These contribute to whether a region is subject to flow or not
* E.g up to 20x more supply to skeletal muscle when exercising to ensure adequate perfusion

2) Reactive hyperaemia
* When blood supply is blocked (for a few s or h)
* Blood flow increase 4-7x normal
* E.g squeezing a finger

Question 19

What are the 3 steps in endothelial cells regulating vascular tone?

Answer 19

• Endothelial cells in regulating vascular tone:
  1) Shear stress activates receptors on endothelial cell
  2) This leads to an increase in production of nitric oxide (NO) via enzyme eNOS
  3) Nitric oxide diffuses out of endothelial cells, causing dilation in order to relieve stress in the vasculature

Question 20

What are examples of the neural, hormonal, and local vasoconstrictors and vasodilators

Answer 20

• Neural, hormonal, and local vasoconstrictors and vasodilators

Question 21

What is the acute regulation of blood flow?

What is the long-term regulation of blood flow?

Answer 21

1) Acute regulation of blood flow
* Rapid changes within seconds or minutes
* Vasodilator theory widely accepted e.g Increase in PCO2, H+, K+, lactic acid, adenosine, histamine, and a decrease PO2 leads to vasodilation to those areas

2) Long term regulation of local blood flow
* Changes in physical size or number of blood vessels

Question 22

When does blood flow through capillaries?

How many capillaries are filled at rest?

Answer 22

• Blood flow through capillaries is intermittent, turning on/off every few seconds or minutes
• At rest, only about 5% of total cardiac output is in the capillaries

Question 23

What is the velocity of flow through the capillaries like?

What makes it this way?

Why is this important?

Answer 23

• Velocity of blood flow through the capillaries is the slowest
• This is because we are past the point of high resistance and there are many capillaries, which gives a large cross-sectional area, causing the velocity to be lower
• This low velocity and capillaries being one cell thick gives time for diffusion and exchange of nutrients and waste

Question 24

Why is there no increase in pressure, but an increase in flow rate when we go from capillaries to venules and veins?

Answer 24

• Capillaries drain into fewer vessels as we go through the vessel tree (lots of capillaries, fewer venules, fewer small veins)
• This pools blood into a smaller overall diameter, with a high compliance and low resistance, leading an increase in flow rate, but no pressure increase

Question 25

Describe the specialise blood flow in the skin.

What is the function of this specialised flow?

Answer 25

• Specialized blood flow in the skin
• Skin has specialised regions called venous plexuses and arteriovenous anastomosis beneath the surface that allows us to redirect blood flow towards or away from the skin surface through bypassing circuits
• The function of this specialise flow is to control how much heat is radiated from the skin
• More blood flow to skin surface = more heat radiated
• Less blood flow to skin = less heat radiated

Question 26

Describe the specialise blood flow in lungs.

Why does this happen?

What is the mediator of this?

What does this specialise flow make difficult?

Answer 26

• Specialised blood flow in lungs
• Decreased alveolar O2 reduces local alveolar blood flow, which is the opposite effect to what is observed in systemic circulation
• We don’t want to diffuse a region of the lung that is not going to contribute significantly to the oxygenation of the blood
• The mediator of this is unknown
• This specialise flow makes it difficult to get medication to the regions where there is reduced blood flow

Question 27

Describe the specialised blood flow in the kidneys.

What is the purpose of specialised blood flow?

Answer 27

• Specialise blood flow in the kidneys
• At the glomeruli of the kidneys, there is an arteriole, into capillary, into arteriole connection
• This allows us to have 2 muscled regions either side of the kidney where pressure can be controlled