Biomechanics Flashcards
(42 cards)
What is the meaning of randomization and replication in study design?
Randomization is using methods to randomly assign study units to control and treatment group. Randomization helps avoid selection bias. Replication is when assigning each treatment to many study units, meaning to perform a variation of a study with the intention to achieve the same result, where some aspects in study has been changed.
What is verification and what is validation?
Verification is testing the correctness of mathematical equations implementation, also test for analytical errors. It refers to the process of confirming the accuracy. Validation is testing that the model predict the desired feature, meaning to evaluate and determine if the model or data meets the intended purpose.
How is the hydraulic resistance of blood vessels distributed over the cardiovascular system?
The resistance is related to the difference in pressure and the. flow. Since the flow is considered as constant, the resistant will increase when increasing the pressure difference. A severe pressure drop appears when blood flows from arteries into the capillaries. The pressure drops due to the branching into smaller vessels with higher resistance. The pressure is thereafter not changing significantly when flowing from capillaries to the veins, where the resistance is lower.
What determines the transport across the wall of micro-vessels?
The osmotic pressure: is the pressure required to prevent net flow. This pressure determines the movement across the wall. It govern the direction and magnitude of the osmotic flow. In osmosis, molecules spontaneously moves from an area of higher concentration to a lower (diffusion) to equalize the concentration on both sides of the membrane.
Hydrostatic pressure: pressure difference between the lumen and interstitium (in and outside the vessel wall)
diffusion and advection
trancytosis
Compute the linear vector transformation Aa. Give the index notation
Aij*ai = Aai
How is the second Piola-Kirchhoff stress defined? Explain all parameters.
Piola-Kirchhoss stress is a stress used to describe the response of material under deformation. It is defined in context on finite deformation theory and is represented be the deformed material. It is divided into first and second. First: P=JsigmaF^-T, and Second: S=JF^-1sigmaF^-T. Here, F denoted the deformation gradient, sigma the stress matrix and J is the volume ratio (detF = J).
What is the characteristic equation of the eigenvalue problem?
det(λI - A)= 0, where A is the matrix where we are searching for the eigenvalues. The eigenvectors are then found through (λI -A)x=0 where x is the eigenvector.
Use Voigt notation and provide Hooke’s law in 3D of an isotropic material
see notes
Compute the matrix representation of the right and the left Cauchy-Green strains for
simple tension and equi-biaxial tension of an incompressible material.
see notes
Describe the mechanical loading that acts at arteries in the body.
Blood pressure, wall shear stress, strain and stretches
What is the hemolysis index?
The hemolysis is a blood-damage mechanism where mechanical shear stress is able to rupture or damage the membrane of erythrocytes (red blood cells). The hemolysis index (HI) is the ratio between the plasma-free hemoglobin and the total hemoglobin in the blood.
Describe the basic mechanism that results in the non-linear stress-strain properties of
vascular tissue. Draw a sketch that indicates the role of collagen and elastin.
Due to how collagen is arranged in “loose” fiber clusters, the first stage of the loading will only refer to the stiffness of elastin (linear). When the collagen fibers are “straighten” the vascular tissue becomes stiffer and acts more non-linear due to that the collagen is dominating
What is the role of residual strain in the vessel wall?
To obtain the same shape in the vessels when unloaded. Minimize the stress load on the wall when normal conditions.
Specify the deformation gradient F and the right Cauchy-Green strain C for simple shear
kinematics.
F = [1 γ 0, 010, 001] and C= F^TF = [1+γ^2 γ 0, γ 1 0, 0 0 1]
Explain the basic structure and physiological function of the adventitial layer of conduit
arteries.
Structure:
ECM (extra cellular matrix)
- Dense network of collagen fibers (60-80%).
- Less clearly organized than in the media.
- Other connective tissue (10-25%)
Physiological:
- Anchoring blood vessels to surrounding tissues
- Nervous connection to smooth muscle cells in the media
- Synthesis of collagen by fibroblasts
- Protects the media from overextension
How is the rate of deformation tensor defined, and what is its meaning? Explain all
parameters.
d = 1/2(F^-TdF^T+dFF^-1) = 1/2(l^T+l), where l=dFF^-1, gradv
What is artherosclerosis? Explain the disease’s progression.
Slowly progressing disease if the intima that leads to formation of intimal plaque. It forms at hemodynamic complex segments (low WWS [lower than 0.2 Pa]). High stresses can rupture the plaque cap anf then trigger a thrombo-embolythic event, which can cause cardiac complications.
What determines the viscosity of blood and why? Sketch how the blood viscosity depends
on the shear rate.
Hematocrit level, which is the parentage of erythrocytes (red blood cells) in the blood. Red blood cells are the major cellular component of blood, therefore does the viscosity relates to the amount of red blood cells. At low shear rates, the viscosity is higher (increases with higher hemotocrit level)
What are principal stresses?
The principal stresses is the stresses at the stress state where all shear stress components are equal to zero.
What are smooth muscle cells?
Smooth muscle cells is muscle cells in the vessel wall. It appears in spindle-shaped configuration. They can switch between contractile and synthetic phenotypes.
How is the blood pressure distributed in the cardiovascular system?
The pressure is related to the resistance and the flow. It is higher when leaving the heart in the arteries, decrease when entering the capillaries and then stays at a low pressure in the veins back to the heart.
Explain methods to estimate arterial compliance.
Pressure decay method
C = Δt/(Rᐧln(p0/p1)), where Δt is the t1-t0, p0 is the pressure at time t0, p1 is the pressure at time t1, R is the resistance. The pressures p0 and p1 are commonly taken at time t0 right after the dicrotic notch, as well as at the time t1 at the end of the diastolic phase.
right and the left
Cauchy-Green, Euler-Almansi, Green-Lagrange
Right: C = F^TF
Left: b = FF^T
Euler-Almansi: e = 1/2 (I-b^-1)
Green-Lagrange: E =1/2 (C-I)
Explain the basic structure and physiological function of the medial layer of large arteries.
Structure:
The media consist of 30-60% of the smooth muscle cells (SMC). extracellular matrix (ECM) components such as elastin (5-35%), collagen (15-40%), and other connective tissue (15-25%) like proteoglycan (PG).
Physiological:
- Regulate flow by vasconstriction/vasodilation
- Key for the physiological properties, which influences the cardiovascular system function
- Synthesis of connective tissue (collagen and elastin)
- Regulates wall stress by thickness adaption
