Topic 4B: Vascular Mechanics

We'll look at the mechanical properties and function of vasculature

What does the vasculature consist of a complex system of

arteries, arterioles, capillaries, venules and veins

What does the arterial system serves as a

conduit through which blood is transported from the heart to the capillary networks throughout the body

What is the arterial system split into

The arterial system is divide into two subsystems pulmonic and systemic arterial structure

What does the biomechanical behaviour depend on

strongly on specific location within the vasculature

What in different arteries/veins share some common substructures called…

Tunicae

• Tunica intima/interna

• Tunica media

• Tunica adventitia/externa

What is shown in pink

This slide is stained with Verhoeff’s stain to visualise the elastic fibers, and with eosin to show the cellular structures

What is the intima

Typically consisting of a monolayer of endothelial cells and an underlying thin basal lamina

What shape are endothelial cells

flat and elongated in the direction of blood flow

What is the lamina

fenestrated sheet of elastin that allows the transport of H2O nutrients and electrolytes across the wall as well as direct transmural cell to cell communication

What is the media

The media contains smooth muscle cells that are embedded in an extracellular piexus of elastin and collagen (primarily types I, III and V) as well as an aqeuous ground substance PG matrix

What are vascular smooth muscle cells (SMCs)

spindle-shaped cells that are typically 100um long and about 5um in diameter except near the nucleus where they are slightly thicker

Why does the orientation of vascular smooth muscles cells matter

Although the orientation and distribution of the medial constituents vary, vascular smooth muscle tends to be oriented helically, albeit nearly circumferentially in many vessels.

This preferential orientation is due to the primary role of SMC to contract and thus modify the distensibility of the large arteries or regulate the luminal diameter in medium and small arteries

What are elastic arteries made up of

• the medial smooth muscle is organize into 5 to 15um thick concentric layers that are separated by thin fenestrated sheets of elastin

• the outermost sheet of elastin is called the external elastic lamina: it separates the media and adventitia but is often considered to belong to the former

• There may be as many as 40 to 70 concentric layers of smooth muscle in a thick elastic artery such as the human aorta.

What is the adventitia

The outermost layer of the wall, consists primarily of a dense network of type I collagen fibres with admixed elastin, nerves, fibroblasts and the vasa vasorum. The adventitial collagen fibres tend to have an axial orientation in most arteries.

Although the adventitia comprises only 10% and 50% of the arterial wall in most arteries, respectively, it is thought to limit acute overdistension in all vessels

What is the vasa vasorum

an intramural network of arterioles, capillaries and venules that serves the outer portion of the wall in arteries that are too thick for sufficient transport of O2, CO2, nutrients and metabolites from the intimal surface.

What order does the layers in the artery vascular wall

adventitia, media, intima and lumen

What does the blue, red and white represent in this image

The red is mostly the media with smooth muscle staining red. The blue is the thick collagen deposits in the adventitia with spare amounts in the media. The white is the lumen

What are the two general types of arteries

elastic and muscular

types of elastic arteries

aorta, main pulmonary artery, common carotids and common iliacs

types of muscular arteries

coronaries, cerebrals, femorals and renals

What size do elastic arteries tend to be

elastic arteries tend to be larger diameter vessels located closer to the heart

What size do muscular arteries tend to be

muscular arteries are smaller diameter vessels located closer to the arterioles

Elastic Arteries

are large arteries carrying a high volume of blood away from the heart. With its high content of elastin and few SMCs, the walls of elastic arteries are extremely resilient and can withstand the pulsatile flow and pressure fluctuations exerted during the cardiac cycle.

Elastic arteries distribute blood to muscular arteries, which supply the body’s skeletal muscles and internal organs. Smooth muscle contraction and relaxation allows the muscular arteries to control regional blood pressure and blood flow by actively changing the lumen size.

Elastic arteries are large enough to sustain systemic hemodynamic forces with lumen diameters up to 2.5cm

Muscular arteries range in size from 4mm to 0.5mm in internal diameter while arterioles are 30um or less

What causes of elastic arteries allows them to expand

the abundance in elastic fibers

What would happen if artery walls were rigid and unable to expand and recoil

Their resistance to blood flow would greatly increase and blood pressure would rise to even higher levels. The elastic recoil of vascular wall helps to maintain the pressure gradient that drives the blood through the arterial system

What are the specific orientations of certain structural constituents within the arterial wall

• SMCs are oriented circumferentially/helically

• Collagen in the adventitia is oriented longitudinally

• Elastin is organised into thin concentric sheets

anisotropic

a physical property which has a different value when measured in different directions

Residual Stress

The stress that exists in a body in the absence of externally applied loads

In the artery when responding the radial cut what does the opening imply

It implies that the inner wall of the intact unloaded ring is in compression and the outer wall in tension.

The increased waviness in the internal elastic lamina in the unloaded ring is consistent with the existence of compressive residual stresses in the inner wall in this configuration

What is responsible for much of the residual strain in the normal arterial wall

elastin due to the selective digestion of different components of the arterial wall

What is the state of residual stress dependent on

The thickness and the composition of the artery. In fact, as arteries are remodelled in response to mechanical stress changes

what is the mark of the amount of residual stress

how much the blood vessel will open when cut. Since the blood vessel is under stress, when we cut the vessel, the stress holding the vessel together is removed and the blood vessel springs open

What is the effect of the remodelling (ligature was performed right below the diaphragm, to cause sudden increase in systolic pressure)

high effect on the aortic arch and descending aorta, where pressure was 25% above normal

lower effect below the diaphragm due to autoregulation

method for determining residual strains in an unloaded artery

optically measure the opening angle (a) of a cross-section taken from the artery

As the ring is cute, it opens the sector, on which the opening angle is defined as the angle between two radii, each drawn from the midpoint of the sector to the outer ends of the sector. Assuming no deformation in the axial direction, the circumferential stretch ratio can be calculated for a point in the vessel wall at a distance R from the centre

What is the equation of the circumferential stretch ratio

𝜃=pi r / O R where r and R are the radii of points in the wall in the unloaded and zero-stress states respectively and r and R the internal radii

Whats in compression the inner or outer wall

the inner wall is in compression and the outer wall is in tension in the intact unloaded configuration

however, by accounting of the presence of intramural pressure, these values change and the actual value of circumferential stretch are lower than the nominal ones and more even

What does a larger opening angle correlate to

a more uniformly distributed circumferential stress

What happens to deformation while applying stresses

while at low applied stresses arteries are very easily deformed the arterial response becomes much stiffer at higher applied stresses.

As many other biological tissues, arteries exhibit an important variability in their mechanical response, both across species, organs, location or inter-individual

What are the physiological parameters in the artery

Physiological parameters (pressure, tethering) involved in the forces and stresses acting on the vascular wall.

What are the physiological forces in the artery

Physiological forces acting on blood vessels resulted from the blood pressure and flow and surrounding tissues: radial force resulted from the blood pressure, longitudinal force resulted from the blood pressure, tangential force resulted from the blood flow, longitudinal force resulted from the tethering

What are the stresses in the artery

Stresses generated in the vascular wall from the physiological forces: circumferential stress, longitudinal stress (due to pressure and tethering) and shear stress

What are the relationships in the artery

Stresses relationships assuming incompressibility and uniform strain across the vascular wall

Uniaxial tensile testing

the artery is cut in either the circumferential or longitudinal direction and specimens are cut out of it and mounted on a standard tensile testing machine. this is to test the arterial mechanical properties

what are the important parameters for uniaxial tensile testing

• preconditioning (5 cycles)

• pre-load, to set a zero strain configuration

• displacement rate

• output (force, displacement/stretch/tissue strain if camera is available

• end of test: peak strain, failure

What happens macroscopically for uniaxial testing

non linear response (toe, heel, linear)

longitudinal direction is way stiffer than the circumferential direction

important variability in the mechanical response, both across species, organs, location, or inter-individual

What happens microscopically for uniaxial testing

Collagen bundles in the tunica adventitia transition from a crimped to a straightened state under loading (fibre recruitment) allowing them to bear load alongside softer tissue components

Adventitial collagen bundles realign with load direction, facilitated by the elastin network. In the media, elastin lamellae, collagen fibers, and smooth muscle cells also reorient under load, aligning with the load direction

Collagen engagement begins earlier