Lecture 6 - Vasculature and Vascular Mechanics

What are the three main structures in terms of vascular structure?

• veins

• arteries

• microcirculation structures: arterioles, capillaries, venules

Describe the characteristics of veins

• Lowest pressure 

• Thin muscled walls 

• Highly distensible 

  • Capable of stretching, expanding etc. 

• High resistance 

  • As blood flows through the system, blood ends up through veins, pressure drops, increasing resistance (resistance is highest in arterioles though)

Describe the characteristics of arteries

• Highest pressure 

• Strong, muscular walls 

• Provides low resistance to flow

Describe the characteristics of microcirculation structures

• arterioles → main site of vascular resistance

• capillaries and venules → sites for gas and nutrient exchange

what is compliance?

• how readily a structure deforms/expands when force is applied 

• Reciprocal of compliance is stiffness 

  • The more its able to stretch, the less stiff it is 

• Reflected by increase in volume in response to increasing transmural pressure 

• Compliance = delta V / delta P

• Ex. high compliance in vein when a high volume of blood enters vein, but small change in pressure caused

What is elasticity?

• how readily a structure returns to its original volume/size after removal of external force 

  • Deformed elastic structures store elastic potential energy 

how compliant are veins?

• very compliant, large changes in volume cause small changes in P 

  • Able to accommodate for huge changes in volume, but cause small changes in p 

  • As volume increases in graph, pressure slowly increases (graph)

how compliant are arteries?

less compliant then veins, will still expand in response to increase in pressure (in contrast to a rigid pipe)

describe the P-V graph in terms of vascular compliance and the compliance formula

???

How elastic are arteries?

• Arteries are highly elastic but not compliant 

How is arterial BP maintained?

• Due to high elasticity of arteries, 

  • During systole, blood enters aorta, stretching of aortic walls stores elastic potential energy 

  • Heart pumps blood into aorta faster than it drains it out 

  • If aorta was not compliant, SBP would shoot up 

  • SBP is kept from increasing further as aorta expands to hold excess blood 

  • During diastole, aortic walls recoil, squeezing blood, sustaining aortic pressure 

  • Required to maintain constant blood flow 

How does left ventricle pressure decrease to near 0 in diastole, while aortic pressures do not? Why?

• LV pressure drops after heart finishes contraction (so all blood is ejected into aorta) and the ventricle relaxes

• aorta is now full with blood, so Paorta > Pventricle and aortic elasticity (recoil) maintains constant blood flow out of aorta

Why does the pressure oscillation decrease along the arterial system? 

1. Elasticity

   1. Aortic recoil preserves pressure, decreasing magnitude of oscillation

2. Vascular resistance

   1. Blood encounters resistance as it flows resulting in p

   2. As blood moves away from the heart and flows through circulatory system, resistance begins to increase, dropping pressure 

what is the pulse pressure equation, and when is it at its peak? when does it drop?

• Pulse pressure = SBP - DBP 

• SBP in aorta is similar to SBP in LV 

• Peak aortic pressure during mid systole (rapid ejection), drops as ejection decreases and blood drains out of aorta

Left ventricle pressure drops lower than aorta, what happens?

• Aortic valve closes → dicrotic notch

  • Aortic elasticity (recoil) causes momentary rise in pressure

• Aorta continues to recoil during diastole, p drops to 80 mmHg 

pulse p magnitude is determined by?

• stroke volume (positive effect)

• arterial compliance (negative effect)

• Compliance dec → aortic distension dec (inc. SBP) → dec elastic recoil (dec DBP)

What conditions alter pulse pressure waves?

1. arteriosclerosis (specifically, artherosclerosis)

2. aortic valve stenosis

how does atherosclerosis affect pulse pressure?

• Lesions develop in aortic walls (plaque)

• Arteries become less compliant (increased stiffness) (hardened arteries)

• SBP increases due to less compliance **

• DBP decreases (sometimes or unchanged ) due to less elasticity (less recoil)

• Increased pulse pressure (amplitude)

  • Based on graph, pressure increase from peak to dicrotic notch, and curve would decrease more than graph around diastolic decline 

how does aortic valve stenosis affect pulse pressure?

• Incomplete opening of aortic valve 

  • Diameter of aortic valve opening is reduced

• Increases resistance to flow into aorta 

  • Inc. resistance results in pressure building up in LV and pressure drop in aorta dec SV (dec. SBP)

  • SBP decreases 

• Can be a small dec. in recoil (dec or unchanged DBP) 

• Smoothened or absent dicrotic notch 

• Dec. pulse pressure (amplitude)

Define internal intravascular pressure (Pi)

• force of blood against artery walls 

define external pressure (Pt)

• pressure exerted on walls from external tissue and interstitial fluid 

Define transmural pressure

(delta P) = Pi-Pt

Define tension

• internal pulling force within wall that resists the net distending pressure across the wall to prevent tear 

  • Tension = delta P x radius/wall thickness 

Tension and larger arteries

• Larger arteries with greater radius experience greater tension in vessel wall 

  • Must have stronger, thicker walls reinforced with collagen, elastic tissue and smooth muscle 

Hypertension

• Chronic increase in Pi causes inc in tnesion

• Hypertrophic remodelling increases vessel wall thickness and decrease radius to decrease tension, prevents tear under high pressure 

describe venous compliance

• Veins distend easily which helps keep the pressure in the venous system low

• The venous system can accommodate large changes in blood volume with only small changes in pressure (blood loss, transfusion, etc.)

• The venous system is a blood reservoir and by contracting or relaxing, can adjust how much blood is available in active circulation

• Veins distend easily which helps keep the pressure in the venous system low

• The venous system can accommodate large changes in blood volume with only small changes in pressure (blood loss, transfusion, etc.)

• During sympathetic stimulation, veins squeeze blood out of veins and into heart to ↑ venous return, ↑ SV and ↑ CO

• During sympathetic inhibition, veins relax, allowing blood to pool, reducing venous return to heart

describe the venous blood reservoir

• Veins hold about 65% of total volume of blood

• By regulating venous tone (constriction or dilation), blood in the reservoir can be redistributed to perfuse more/fewer capillary beds as necessary

  • With sympathetic stimulation, ↑venous constriction, ↑venous return, ↑SV, ↑ CO and ↑ blood to microcirculation

Even with ↑ venous return, total blood volume in arteries changes very little. Why?

• ↓ compliance, cannot accommodate much larger volumes

• Arteries are high, flow conduits rather than reservoirs (flow conduit = fast)

How are venous pressures affected by gravity?

Right atrial pressure (central venous pressure) is normally very low (0 mmHg) → there is no “sucking power” 

• Blood in veins can only return to heart if pressure in veins is greater than 0

• When lying, veins are located at same level to heart, therefore pressure from heartbeat pushes blood 

• When standing, hydrostatic pressure (pressure exerted by fluid due to gravity) is exerted 

  • Increases with fluid height 

• With height, there is more downwards pressure (hydrostatic pressure) causing blood to pool in legs 

• To overcome this, skeletal muscles contract by squeezing veins to push blood upward to the heart 

• When muscles relax, blood may pool back down → to prevent this, one-way valves lock blood in place, to prevent backflow and venous pooling 

• Gravity alters venous pressure based on changes in venous return/position of the vessel