Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

CVS- Cardiovascular > S2: Haemodynamics II: Arterioles and Veins > Flashcards

S2: Haemodynamics II: Arterioles and Veins Flashcards

1
Q

Give different equations for blood flow

A

Darcy’s law = Blood flow (CO) = Pa - CVP / TPR

Blood flow = Pa - CVP x G

G = conductance

  • TPR controls blood flow and blood pressure
  • Increase in resistance means need to increase pressure to keep same flow.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Equation for resistance (TPR)

A

Resistance (TPR) = 1 / Conductance (G)

Increase in resistance means decrease in conductance and vice versa

Resistance = 8 nL/ Pi r4

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Factors controlling total peripheral resistance

A
  • Poiseulle’s law
  • Myogenic response
  • Blood viscosity
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Describe how TPR affects blood flow and pressure

Use hypertension as an example

A

Constriction causes an increase in blood pressure upstream from the constriction and downstream from the constriction will have a decrease in blood pressure which decreases flow.

Hypertension (high blood pressure) leads to over constriction of arterioles leads to reduction in blood flow which is harmful and causes end organ damage.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

How does blood flow change in response to need?

A

Sitting can constrict blood vessels to legs, increasing TPR and decreasing flow to legs. This means more blood can be increased to intestines to aid with food digestion. The opposite occurs when running.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Describe Poiseuille’s law

A

Interaction of concentric cells in blood flow determines viscosity of blood

Blood flow = Pa - CVP x G

so Poiseuille’s law describes the parameter that govern TPR

Conductance (G) = Pi r4 / 8nl

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Combines Darcy’s and Poiseulle Law

A

Resistance and conductance have an inverse relationship. The conductance can therefore be substituted into Darcy’s Law.

Blood flow = Pa - CVP x G

so

Blood flow = Pa - CVP x pi r4/ 8nL

n: blood viscocity
L= vessel length

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is the r4 effect?

A

By doubling radius of vessel, there is a 16 fold change of blood flow (2^4=16). Small changes of radius in vessels can drastically change blood flow - this is seen using poiseulle’s law on TPR

Increasing the radius increases blood flow

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What is TPR controlled by?

A
  • Radius 4
  • Pressure difference across vessels - P1-P2
  • Length L
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Explain pressure drop in arterioles

A

Arteriole radius is tightly controlled by sympathetic nerves. They have a natural tone to the that allows them to dilate and constrict.

Arterioles have largest pressure drop of 40-50 mmHg amongst vessels because they have high TPR so downstream there is a big pressure drop.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Compare radius of arterioles and capillaries

A

ARTERIOLES
Walls contain smooth muscle and are innervated. Radius can be altered.

CAPILLARIES
No sympathetic innervation/smooth muscle in capillaries and they are made of endothelial cells. The radius of capillaries cannot be altered.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Compare pressure drop of arterioles and capillaries

A

ARTERIOLES
Pressure drop of (40-50 mmHg) which is more of a pressure drop than capillaries (compared to ventricular/arterial pressure)

CAPILLARIES
Less pressure drop in capillaries (20-30 mmHg) due to less resistance to blood flow in capillaries.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Compare resistance of arterioles and capillaries

A

ARTERIOLES
In contrast, arterioles are in series with arteries, arterioles, capillaries

RTotal = R1 + R2 etc – total resistance is greater

CAPILLARIES
Bolus flow reduces viscosity (η, see Poiseulle’s law)

Capillaries are arranged in parallel,
So have a low total resistance as RTotal = 1/R1 + 1/R2 etc (blood spreads out in large capillary network)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Compare length of arterioles and capillaries

A

Capillaries are shorter

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What controls local blood flow?

A

Local blood flow through individual organs/tissues is mainly controlled by the changes in radius of arterioles supplying a given organ/tissue

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Name some factors controlling arteriole radius

A

Intrinsic: Factors entirely within an organ or tissue

  • Tissue metabolites
  • Local hormones
  • Myogenic
  • Endothelial factors

Extrinsic: Factors outside the organ or tissue

  • Neural
  • Humoral
17
Q

Describe intrinsic control of TPR - Bayliss myogenic response

A

Bayliss myogenic response maintains local blood flow during changes in local blood pressures and is important in renal, coronary and cerebral circulation. It is a protective mechanism as when BP drops there is still good flow and if BP is high there is less flow/damage.

  • Increase distension of vessel –> Constriction
  • Decreased distension of vessel –> Dilatation

Increase in pressure increases flow. However, if the vessel diameter is small (constricted) an increase in pressure may not increase blood flow that much due to an increase in TPR.

18
Q

What is viscosity?

A

Viscosity is a measure of internal friction (concentric shells of blood interacting with walls) opposing the separation of the lamina.

19
Q

Factors affecting blood viscosity

A
  • Velocity of blood
    Slow venous blood increases viscosity
  • Vessel diameter
    Blood viscosity falls in narrow tubes. Laminar flow changes to bolus flow so decreased resistance causes increase in blood flow in microvessel
  • Haemotocrit (number of cells)
    High number of cells increases viscosity
  • Red cell deformability
    Increases viscosity and decreases BF
20
Q

Functions of venous blood pressure and veins

A

▪ 60% of blood volume at rest is in systemic veins and venules
▪ Functions as blood reservoir
▪Blood can be diverted from it back to the heart in times of need e.g. Exercise (more CO needed, starlings law by shifting more blood back to the heart), haemorrhage

Veins are contractile.

Veins expel blood into central veins:

  • increases venous return/CVP/EDV
  • increases SV (starling’s law)
21
Q

Describe structure of veins

A

▪ Thin-walled, collapsible, voluminous vessels
▪ Contain 2/3ths of blood volume
▪ Contractile – contain smooth muscle, innervated by sympathetic nerves
▪Control radius

22
Q

Describe pressure volume curve of veins

A 
▪ Stimulation of sympathetic nerves causes venoconstriction
▪ Shifts blood centrally
▪ Increases venous return/CVP/end-diastolic pressure
Increased SV (Starling’s law) and greater CO

Increased pressure distends veins and increases volume (above heart decreases pressure, below heart increases pressure due to gravity)
Contracting veins decreases its blood volume as more blood is shifted to the heart
- Larger diameter of vein, higher venous pressure and more volume

23
Q

How does blood return to the heart (factors affecting venous return)?

A
  • Pressure gradient
    Pressure in venules/veins is always higher than IVC/SVC/RA
    Venous return = Venous pressure - pressure RA/venous resistance
  • Thoracic Pump
    Inhalation - thoracic cavity expands leading to ­increased abdominal pressure, forcing blood upward towards heart, ­ increased right ventricular SV
    Blood flows faster with inhalation
  • Skeletal muscle pump
    Contraction of leg muscles returns blood into right atrium. Retrograde flow is prevented by venous valves
    Blood naturally pools in our lower limbs due to gravity.

The skeletal muscle pump:
Reduce high local venous pressures when in the upright position
Reduces swelling of feet and ankles – lower venous pressures, lower capillary pressure, less filtration
Increases­ CVP and SV during exercise

In contrast, standing still (soldier on parade) for a long time can lead to fainting - due to gravity blood pools, heat-induced vasodilatation, lack of muscle use (skeletal muscle pump not working)

24
Q

Explain how Bernoulli’s law explains blood flow with little pressure difference

Give examples when standing

A

Flow cannot be determined by pressure alone as blood can flow from the heart to feet and blood can fill ventricles when CVP is low.

  • Gravity must therefore have a role

Bernoulli theory – mechanical energy of flow is determined by
pressure, kinetic, potential energies
Flow = Pressure (PV) + Kinetic (ρV2/2) + potential (ρgh)
ρ = fluid mass
Potential = amount of energy caused by gravity

Standing
-90 mmHg pressure gradient against flow
Ejected blood - greater kinetic energy at heart than feet (more velocity, V)
+ greater potential energy than at heart than feet (more height, h)
Greater kinetic/potential energies overcome pressure gradient to maintain flow
But: flow to feet easily compromised – clinically important

Returning blood to heart - no pressure gradient but kinetic energy

CVS- Cardiovascular (24 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions