ELASTIC PROPERTIES OF BLOOD VESSELS Flashcards
___________ which have only an intimal layer of endothelial cells resting on a basal lamina are the exception.
Capillaries
____________ form a single, continous layer that lines all vascular segments.
Endothelial cells
___________ keep the endothelial cells together in arteries but are less numerous in veins.
Junctional complexes
The glomus bodies in the skin and elsewhere are unusual in that their “endothelial cells” exist in multiple layers of cells called _____________.
Myoepithelioid cells
These ____________ control small arteriovenous shunts or anastomoses.
Glomus bodies
____________ are a rubber-like material that accounts for most of the stretch of vessels at normal pressures as well as the stretch of other tissues.
Elastic fibers
The elastin core consists of a highly insoluble polymer of _________, a protein rich in nonpolar amino acids (i.e glycine, alanine, valine, proline).
Elastin
After being secreted into the extracellular space, the elastin molecules remain in a ______________.
Random-coil configuration
The _____________, which are composed of glycoproteins and have a diameter of ~10 nm, are similar to those found in the extracellular matrix in other tissues.
Microfibrils
In ___________, elastic fibers are arranged as CONCENTRIC, CYLINDRICAL LAMELLAE.
Arteries
A network of elastic fibers is abundant everywhere except in the _________, in the __________ and in the aforementioned ______________.
True capillaries
Venules
Arteriovenous anastomoses
___________ constitute a jacket of far less extensible material than the elastic fibers, like the fabric woven inside the wall of a rubber hose.
Collagen fibers
Collagen can be stretched only _______% under physiological conditions.
3 to 4%
The basic unit of the collagen fibers in blood vessels is composed of ____________ collagen molecules which are two of the fibrillar collagens.
Type I and Type III
After being secreted, these triple-helical molecules assemble into fibrils that may be _________ nm in diameter; these in turn aggregate into collagen fibers that may be several microns in diameter and are visible with a light microscope.
10 to 300 nm
Collagen fibers are presemt throughout the circulation except in the __________.
Capillaries
True or False: Collagen fibers are usually attached to the other components of the vascular wall with some slack, so that they are normally under tension.
False: They are normally not under tension
Are also present in all vascular segments except the capillaries.
Vascular smooth-muscle cells (VSMCs)
In _________, VSMCs are arranged in spirals with pitch varying from nearly longitudinal to nearly transverse-circular.
Elastic arteries
In _____________, they are arranged either in concentric rings or as helices with a low pitch.
Muscular arteries
______________ do not contribute appreciably to the elastic tension of the vascular wall, which is mainly determined by the elastin and collagen fibers.
Relaxed VSMCs
VSMCs exert tension primarily by means of ____________.
Active contraction
Because of the elastic properties of vessels, the pressure-flow relationship of passive vascular beds is __________.
Nonlinear
_________ predicts a linear pressure-flow relationsip.
Poiseuille’s law
However, in reality, the pressure-flow relationship is markedly __________ in an in vivo preparation of a vascular bed.
Nonlinear
The reason is that INCREASE of the driving pressure also INCREASES the transmural pressure, causing the vessel to _________.
Distend
Because radius increases, resistance _____ and flow ______ more than it would in a rigid tube.
Falls
Rise
The ____________ of vessels are the major cause of such nonlinear pressure-flow relationships in vascular beds exhibiting little or no “active tension”.
Elastic properties
As driving pressire and thus transmural pressure increases, vessel radius increases as well, causing __________.
Resistance
Conversely, resistance increases toward _______ when driving pressure falls.
Infinity
In Poiseuille’s case of course, resistance would be ___________ regardless of the driving pressure.
Constant
Contraction of smooth muscle halts blood flow when driving pressure ______ below the critical closing pressure.
Falls
The ____________ occurs because of the combined action of elastic fibers and active tension from VSMCs.
Stoppage of flow
Graded increases in active tension produced for example by ______________ - shift the pressure-flow relationship to the right and decrease the slope which reflects an increase in resitance over the entire pressure range.
Sympathetic stimulation
The criticsl closing pressure also shifts upward with increasing degrees of _____________.
Vasomotor tone
This phenomenon is important in _____________, in which massive constriction occirs in an attempt to raise arterial pressure and critical closing pressures rise to 40 mmHg or more.
Hypotensive shock
__________ and __________ determine the distensibility and compliance of vessels.
Elastic and collagen fibers
Have a low volume capacity but can withstand large transmural pressure.
Arteries
Have a large volume capacity (and are thus able to act as blood reservoirs) but can withstand only small transmural pressure differences.
Veins
The _____________ expresses the elastic properties of blood vessels.
Volume distensibility
Is the change in volume for a macroscopic step change in pressure.
Absolute distensibility
Because the unstretched size varies among vessels, it is preferable to normalize the volume change to the initial, unstretched volume (Vo) and thus to use a _______________.
Normalized distensibility
The most useful index of distensibility is ___________ which is the slope of the tangent to any point along the pressure-volume diagram.
Compliance
The steeper the slope of a pressure-volume diagram, the ________ the compliance.
Greater
True or False: A compliance reading should always include the transmural pressure or the volume at which it was made.
True
Differences in compliance cause arteries to act as _________ and veins to act as ________.
Resistors
Capacitors
As they accomodate the volume of blood ejected with every heartbeat, large, muscular systemic arteries (femoral artery) increase in radius by about ______%.
10%
This change for a typical muscular artery is much smaller than the distention that one would observe for ________ which have fewer layers of smooth muscle and are not under nervous control.
Elastic arteries
The compliance of a relaxed elastic artery is substantial: increasing the transmural pressure from 0 to 100 mmHg (near the normal mean arterial pressure) increases relative volume by ____%
180%
With further increases in pressure and diameter, compliance _________ only modestly.
Decreases
Because muscular arteries have a rather stable resistance, they are sometimes referred to as ____________.
Resistance vessels
For a relaxed vein, a relatively small increase of transmural pressure from 0 to 10 mmHg increases volume by _______%.
200%
At pressures <6 to 9 mmHg, the vein’s cross section is ___________.
Ellipsoidal
A small rise in pressure caises the vein to become _________, without an increase in perimeter but with a greatly increased cross-sectional area.
Circular
Thus, in their normal pressure range, _______ can accept relatively large volumes of blood with little buildup of pressure.
Veins
Because they act as volume reservoirs, veins are sometimes referred to as ____________.
Capacitance vessels
The true distensibility or compliance of the venous wall - related to the increase in __________ produced by pressures >10 mmHg is rather poor.
Perimeter
When the heart ejects its stroke volume into the aorta, the intravascular pressure increases modestly, from _______ to _______ mmHg.
80 to 120 mmHg
This change in intravascular pressure also known as the ___________ is the same as the change in transmural pressure.
Pulse pressure
We have already seen that venous compliance is ____ in this arterial pressure range.
Low
Changes in the volume of vessels during the cardiac cycle are due to changes in ________ rather than length.
Radius
___________ describes how tension in the vessel wall increases with transmural pressure.
Laplace’s law
The simplest mechanical model of linearly elastic solid is a _______.
Spring
The elongation 🔼L is proportional to the force according to ____________ where k is constant.
Hooke’s law
If an elastic body requires a larger force to achieve a certain deformation, it is _________ or less compliant.
Stiffer
The largest stress that the material can withstand while remaining elastic is the ___________.
Elastic limit
_______ is the force per unit cross-sectional area (F/A).
Stress
Is the fractional increase in length.
Strain
The proportionality factor, the Young ___________ is the force per cross-sectional area required to stretch the material to twice its initial length and also the slope of the strain-stress diagram.
Elastic modulus
The stiffer a material is, the steeper the slope, and the ________ the elastic modulus.
Greater
Becaus the elastic and collagen fibers in blood vessels are not arranged as simple linear springs, the stresses and strains that arise during the pressurization or filling of a vessel occur along three axes:
- An elongation of the circumference
- An elongation of the axial length (x)
- A compression of the thickness of the vessel wall in the direction of the radius (r)
In fact, blood vessels actually change little in length during _______, and the thinning of the wall is usually not a major factor.
Distention
The __________ is the distending force that tends to increase the circumference of the vessel.
Transmural pressure
It is convenient to express this wall __________ as the force that must be applied to bring together the two edges of an imaginary slit, of unit length L, cut in the wall along the longitudinal axis of the vessel.
Tension
Note that in using tension (units: ______________) rather than stress (units:______________), we assume that the thickness of the vessel wall is constant.
Dynes per centimeter
Dynes per square centimeter
The ___________ is adapted to withstand wall tension, not transmural pressure.
Vascular wall
Laplace’s law:
T = 🔼P x r
Thus, stretching the radius of the aorta causes a considerable _______ in wall tension, which reflects the aorta’s moderate compliance.
Rise
The _____________ fills over a wide range of radii before any wall tension develops, which reflects the shape change that makes it appear to be very compliant at the low pressure that are physiological for a vein.
vena cava
However, further stretching of the vein’s radius results in a steep _______ in wall tension, which reflects the inherently limited compliance of veins.
Rise
According to Laplace’s law, exactly one pressure -___________ or 150 mmHg satisfies this combination of r and T.
200,000 dyne / cm2
Thus, comparing vessels of similar size, ____________ tells us that a high wall tension is required to withstand a high pressure.
Laplace’s law
Comparing two vessels of very different size reveals a disparity between ___________ and _________.
Wall tension and pressure
A large vein such as the vena cava must resist only _________ in transmural pressure but is equipped with a fair amount of elastic tissue.
10 mmHg
A capillary which must resist a transmural pressure of _____ mmHg does not have any elastic tissue at all.
25 mmHg
The key concept is that what the vessel really has to withstand is not pressure but ___________.
Wall tension
According to Laplace’s law, ___________ is the product of transmural pressure and radius.
Wall tension
With increasing stretch, the vessel wall resists additional deformation more; that is the slope of the relationship becomes ____________.
Steeper
The increasing slope (increasing elastic modulus) of the radius-tension diagram of blood vessel is due to the ____________ of the elastic material of the vascular wall.
Heterogeneity
We can quanititate the separate contributions of elastic and collagen fibers by _______________.
Chemical dissection
After selective digestion of elastin with elastase, which unmasks the behavior of collagen fibers, the length-tension relationship is ________ and ________ to the linear-relationship expected from the Hooke’s law.
Very steep and closer
After selective digestion of collagen fibers with formic acid, which unmasks the behavior of elastic fibers, the length-tension relationship is __________.
Fairly flat
True or False:
The collagen is steeper than the elastin because collagen is stiffer than elastin.
True
In a normal vessel, modest degrees of stretch elongate primarily the elastin fibers along a relatively _______ slope.
Flat
Progressively greater degrees of stretch recruit collagen fibers, resulting in a ______ slope.
Steeper
__________ reduces the distensibility of arteries.
Aging
The most obvious difference in aortic _____________ with increasing age is that the curves shift to progressively higher volumes which reflects an increase in diameter.
pressure-volume curves
True or false: The compliance of the aorta first rises during growth and development to early adulthood and then falls during later life.
True
_______________ reduce the vessel’s compliance per se
Arteriosclerotic changes
Thus during ______________, a normal-sized increase in aortic volume in a young adult produces a relatively small pulse pressure in the aorta.
ventricular ejection
Because blood pressure frequently rises with age, the older person operates on a _______ portion of the pressure-volume curve where the compliance is even lower than at lower pressures.
Flatter
The second approach for assessment of the effects of age on the elastic properties of blood vessels is to examine the ______________.
Radius-tension diagram
Because of the progressive, diffuse fibrosis of vessel walls with age, and because of an increase in the amount of collagen, the maximal slope of the radius-tension diagram __________ with age.
Increases
In addition, with age these curves start to bend ________ at lower radii, as the same degree of stretch recruits a larger number of collagen fibers.
Upward
True or False: Active tension from smooth-muscle activity adds to the elastic tension of vessels.
True
The __________ from VSMCs also contributes to wall tension.
Active tension
Stimulation of VSMCs can reduce the internal radius of muscular feed arteries by __________%
20-50%
Laplace’s law tells us that as the VSMC shortens- thereby reducing vessel radius against a constant transmural pressure- there is a _________ in the tension that the muscle must exert to maintain that new, smaller radius.
Decrease
______________ helps stabilize vessels under vasomotor control.
Elastic tension
As we inject more fluid into the vessel, the radius ________ (i.e the vessel is more “STRESSED”)
Increases
Are characterized by poor elastic tissue and abundant myoepitheliod cells.
Arteriovenous anastomoses
Exposure to cold leads to extreme constriction of arterioles in the extremities particularly the fingers.
Raynaud disease
