S2: Haemodynamics II: Arterioles and Veins Flashcards
Give different equations for blood flow
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.
Equation for resistance (TPR)
Resistance (TPR) = 1 / Conductance (G)
Increase in resistance means decrease in conductance and vice versa
Resistance = 8 nL/ Pi r4
Factors controlling total peripheral resistance
- Poiseulle’s law
- Myogenic response
- Blood viscosity
Describe how TPR affects blood flow and pressure
Use hypertension as an example
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 does blood flow change in response to need?
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.
Describe Poiseuille’s law
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
Combines Darcy’s and Poiseulle Law
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
What is the r4 effect?
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
What is TPR controlled by?
- Radius 4
- Pressure difference across vessels - P1-P2
- Length L
Explain pressure drop in arterioles
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.
Compare radius of arterioles and capillaries
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.
Compare pressure drop of arterioles and capillaries
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.
Compare resistance of arterioles and capillaries
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)
Compare length of arterioles and capillaries
Capillaries are shorter
What controls local blood flow?
Local blood flow through individual organs/tissues is mainly controlled by the changes in radius of arterioles supplying a given organ/tissue
Name some factors controlling arteriole radius
Intrinsic: Factors entirely within an organ or tissue
- Tissue metabolites
- Local hormones
- Myogenic
- Endothelial factors
Extrinsic: Factors outside the organ or tissue
- Neural
- Humoral
Describe intrinsic control of TPR - Bayliss myogenic response
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.
What is viscosity?
Viscosity is a measure of internal friction (concentric shells of blood interacting with walls) opposing the separation of the lamina.
Factors affecting blood viscosity
- 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
Functions of venous blood pressure and veins
▪ 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)
Describe structure of veins
▪ Thin-walled, collapsible, voluminous vessels
▪ Contain 2/3ths of blood volume
▪ Contractile – contain smooth muscle, innervated by sympathetic nerves
▪Control radius
Describe pressure volume curve of veins
▪ 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
How does blood return to the heart (factors affecting venous return)?
- 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)
Explain how Bernoulli’s law explains blood flow with little pressure difference
Give examples when standing
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
