Vasculature: arterial blood flow, peripheral resistance

Question 1

What is the Poiseuille equation?

Answer 1

Flow is proportional to Delta p (pressure at each end of pipe) times radius of vessel to the power of four over viscosity times length

This is apparently simplified to Flow = delta p times radius of vessel cubed over K
or
Flow = delta p over resistance

Where Arterial pressure = cardiac output x total peripheral resistance

Question 2

How does fluid flow through the vasculature?

Answer 2

In a parabolic velocity profile (peak in centre as vessels are lined with endothelial cells - fluid molecules on this wall adhere and move more slowly - next layer slips over these - next layer over these…–middle most layers move the most rapid

Question 3

What is the impact of turbulence in fluid flow through vasculature?

Answer 3

Turbulence disrupts flow, increases resistance.
• Poiseuille’s law doesn’t hold true during turbulence

Reynold’s number (Re) is used to indicate whether flow is laminar or turbulent.

Question 4

What is Reynolds number

Answer 4

Reynold’s number (Re) is used to indicate whether flow is likely to be laminar or turbulent.
– 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 5

What implications does Reynolds number have on turbulence?

Answer 5

Turbulence is therefore likely with (because Re increases with): 
– High velocity flow
– Large diameter vessels
– Low blood viscosity
– Abnormal vessel wall

Question 6

What are thixotropic fluids?

Answer 6

Fluids that are solid or more viscous when static and less viscous when agitated

Question 7

As blood is a thixotropic fluid, how will it be affected by flow?

Answer 7

Flow affects viscosity

– Static blood has 100x the viscosity of flowing blood

Question 8

When may turbulence be used by physicians?

Answer 8

Korotkoff sounds

Artificially generated turbulence
– Ausculatory measurements using a sphygnomanometer cuff

Question 9

What is LaPlace’s Law

Answer 9

Distending pressure (P) produces an opposing force or tension (T) in the vessel wall, proportional to the radius (R) of the vessel:

T = PR

Question 10

What are the practical consequences of LaPlace’s Law?

Answer 10

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

2) Capillaries
– Can be extremely thin and still withstand the pressure
– Thin walls essential for exchange processes

3) Aneurysm

Question 11

Discuss how vasculature leading into capillaries can regulate blood flow

Answer 11

Arterioles
– Control regional distribution (Local and extrinsic controls)

Metarterioles
– Links arterioles to venules, discontinuous smooth muscle

Precapillary sphincters
– When a true capillary branches from a metarteriole
– Vasodilation produced by local factors

Question 12

How is regulation of tissue blood flow achieved?

Answer 12

Active and reactive hyperemia
– Local factors associated with metabolic activity of tissues

Flow autoregulation
– In response to changes in arterial pressure
• Arterial pressure ↑, arterioles constrict to reduce flow
• Arterial pressure ↓, arterioles dilate to increase flow
– Myogenic response
• Stretch-activated Ca2+ channels

Vasomotion
– Spontaneous oscillating contraction of blood vessels

Response to injury
– E.g. endothelin-1 released from endothelial cells
• Potent vasoconstriction

Question 13

What is the difference between active and reactive hyperaemia?

Answer 13

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

Question 14

What causes vasoconstriction of the arteriolar radius?

Answer 14

Neural cause - sympathetic nerves

Hormonal - Adrenaline, angiotensin II, vasopressin

Local - Myogenic response, endothelin-1

Question 15

What causes vasodilation of the arteriolar radius?

Answer 15

Neural cause - NO-releasing nerves

Hormonal - Adrenaline, Atrial-natriuretic peptide

Local - K+, CO2, H+, Adenosine, Nitric oxide, Bradykinin, Decrease in O2

Question 16

Discuss blood flow through the capillaries

Answer 16

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

There are acute and long term regulations of local blood flow

Question 17

Discuss the acute regulation of local blood flows through capillaries

Answer 17

– Rapid changes within seconds or minutes
– Vasodilator theory widely accepted (“local factors”)
• E.g. ↑PCO2, ↓PO2, ↑H+, ↑K+, ↑lactic acid, ↑adenosine, ↑ histamine

Question 18

Discuss the long term regulation of local blood flows through capillaries

Answer 18

– A change in physical size or number of blood vessels

Question 19

Discuss capillary flow and pressure

Answer 19

Large number of capillaries therefore large cross-sectional area
Velocity of blood flow through capillaries is the slowest
– Allows time for diffusion and exchange of nutrients and waste

Question 20

What are the main points of the basic principles of circulatory function?

Answer 20

The rate of blood flow to each tissue of the body is almost always precisely controlled in relation to the tissue need.
The cardiac output is controlled mainly by the sum of all the local tissue flows.
Arterial pressure regulation is generally independent of either local blood flow control or cardiac output control.

Question 21

How is capillary structure different in the skin?

Answer 21

There are capillary loops in the dermis that feed up to the epidermis. They feed back into veins which travel deep to the subcutaneous tissue where they join a venous plexus

This venous plexus also takes blood from arteries in the subcutaneous tissue via arteriovenous anastomosis

This is controlled in thermoregulation

Question 22

How is capillary structure different in the lungs?

Answer 22

Decreased alveolar O2 reduces local alveolar blood flow
– Opposite to effect observed in systemic vessels
– Mediator unknown

Question 23

How is capillary structure different in the kidneys?

Answer 23

See: glomerulus

Blood enters and leaves glomerular capillaries at high pressure to cause fluid to filter into the tubule (Going into bowman capsule arterioles are afferent, after they are efferent)

Question 24

Learning outcomes

Answer 24

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 25

Other than via endothelin-1, what is a way that endothelial cells can regulate vascular tone?

Answer 25

In an increase in sheer stress in a blood vessel, a receptor dependant activation can occur for endothelial(?) nitrous oxide synthase or eNOS. This produces nitrous oxide from O2 + L-Arginine

**NOTNECESARRY**JUSTGETTHEGIST**

Once produced in endothelial cells, NO diffuses across the vascular smooth muscle cell membranes and activates the enzyme soluble guanylate cyclase (sGC), which catalyzes the conversion of guanosine triphosphate (cGTP) into cyclic guanosine monophosphate (cGMP).

cGMP, in turn, activates protein kinase G (PKG), which promotes multiple phosphorylation of cellular targets lowering cellular Ca2+ concentrations and promoting vascular relaxation.

The only example of parasympathetic impact on vasculature - in genitals