CVS: Arteries, Arterioles and Veins Flashcards
Define haemodynamics
Relationship b/w blood flow, blood pressure and resistance to flow
Centres on: BP = CO x TPR and Blood flow = BP/TPR
What are the key factors in haemodynamics?
Work - Expend energy to produce cardiac contraction which creates isovoulmetric contraction and ejection
Pressure - Generated to drive bulk/convection transport
Compliance - Large artery stretch
Resistance - Arterioles
Blood flow - Vital parameter, end result
Velocity - Slowing down of blood flow in capillaries
Describe the 3 patterns of blood flow
Laminar:
- Most arteries, arterioles, venules, veins
- Concentric shells
- 0 velocity at walls (molecular interactions b/w blood and wall)
- Max velocity at centre
- Move RBCs towards centre
- Speeds up blood flow through narrow vessels
Turbulent:
- Ventricles (mixing), aorta (peak flow), atheroma (bruits)
- Blood doesn’t flow smoothly in adjacent layers, instead there are eddies/vortices/whirlpools. This occurs due to changes in velocity
Bolus capillaries:
- In many capillaries, RBCs travel singly, separated by segments of plasma (bolus flow)
- RBCs have larger diameter than capillaries - single file
- Plasma columns trapped b/w RBC
- Uniform velocity
- Little internal friction - very low resistance
What 2 factors is blood flow dependent on?
- Aterial BP = Blood pressure b/w heart and end organs
- TPR
The pressure exerted on vessel walls is ultimately generated by LV ejection, and is highest in the aorta (120 mmHg during systole)
In systemic circulation, as distance from LV increases, BP decreases
What is the role of the aorta in BP?
- Recoil of elastic fibres of aorta + large arteries help to propel blood into circulation during diastole
- During LV ejection
- 60 - 80% of SV is stored in aorta + arteries as these structures expand
- Energy stored in stretched elastin
- During LV diastole
- Energy is returned to the blood as the walls of the aorta and arteries contract
- This sustains diastolic blood pressure and blood flow when heart relaxed
What is pulse pressure?
Pressure changes in large arteries e.g. pulse at wrist (radial artery)
Pulse pressure = Systolic - Diastolic
How does pulse pressure correlate with SV?
Greater SV:
- Greater stretch of arteries
- Less compliant
- Greater pulse pressure due to increased systolic pressure
Pulse pressure is the difference b/w systolic and diastolic BP
How does pulse pressure relate to compliance?
Pulse pressure = SV/ Compliance
Stiffer arteries = Decreased compliance = Increase systolic BP
What is Darcy’s law of flow?
Arterial BP → End organ
Blood pressure difference that drives blood flow to end organs, but there’s also resistance to that blood flow
Blood flow (CO) = BP/TPR
BP = CO x TPR
TPR controls both Blood flow and BP
How do arterioles reduce and increase resistance?
Reduce resistance:
- Dilate blood vessel
- Increase blood flow
Increase resistance:
- Constrict blood vessel
- Reduce blood flow
What happens if there’s an excessive drop in TPR?
Excessive vasodilation:
- Reduces BP upstream to extent insufficient perfusion of organs
- Leads to organ/tissue damage
What happens if there’s an excessive increase in TPR?
Excessive vasoconstriction:
- Leads to increased BP upstream and reduction in blood flow downstram
- Causes end organ damage
Describe Poiseuille’s law
- Poiseuille’s law describes parameters that govern TPR
- Blood flow ∝ Blood vessel radius to power of 4
- If you increase radius, blood flow increases
- Blood flow ∝ 1/viscosity
- If you increase viscosity, blood flow decreases
What are the main vessels that control TPR?
- Arterioles - Radius tightly controlled by sympathetic nerves
- Arterioles have largest pressure drop of 40-50 mmHg amongst vessels
What is viscosity?
Measure of internal friction opposing separation of lamina
Blood flow ∝ 1/viscosity
If you increase viscosity, blood flow decreases
What does blood viscosity depend on?
- Haematocrit
- Velocity of blood
Describe blood viscosity and its clinical implications
High haematocrit - e.g. Polycthaemia - ⬆️TPR and BP, ⬇️BF
Low haematocrit - e.g. Anaemia - ⬇️TPR and BP (with ⬆️HR due to baroreflex compensation)
Increased viscosity in slow venous flow in immobile legs e.g. increased risk of DVT
What is the function of veins and venules at rest?
Function as blood reservoir
This blood can be returned to heart when:
- Need greater CO (e.g. exercise)
- Drop in CO and BP due to decrease blood volume (e.g. haemorrhage, dehydration)
Describe the features of the veins
- Thin walled, collapsible, voluminous
- Contains 2/3 blood volume
- Contractile - contain smooth muscle
- Innervated by sympathetic nerves, cause venoconstriction
What is the consequence of increased venoconstriction?
- Contraction of vessels - expels blood into central veins
- Increases venous return/CVP/EDV
- Increases stroke volume (Starling’s law)
Explain how blood returns to the heart
- Pressure gradient
- Pressure in venules/veins b/w 10 - 90mmHg, IVC/SVC/RA <5mmHg
- Venous return = Venous pressure - Pressure RA/ Venous resistance
- Thoracic pump
- Inhalation - thoracic cavity expands, ⬆️abdominal pressure, forces blood towards heart, ⬆️RV stroke volume
- Blood flows faster with inhalation
- Skeletal muscle pump
- Contraction of leg muscles returns blood into RA
- Retrograde flow prevented by venous valves
- Reduce high local venous pressures when in upright position
- Reduces swelling feet + ankles - lower venous pressures, lower capillary pressure, less filtration
- ⬆️CVP and SV during exercise, contracting leg muscles
- Standing still for long time can lead to fainting due to gravity + lack of muscle use, pooling of blood volume in veins of lower limbs
- Pressure gradient
