Quiz 2 (Hemodynamics) Flashcards
(285 cards)
1
Q
define hemodynamics
A
basic principles of the dynamics of blood circulation
2
Q
are hemodynamics the same for arterial and venous circulation?
A
no
3
Q
what are the tiniest blood vessels?
A
capillaries
4
Q
what is the order of blood vessels starting at arteries to veins?
A
- arteries
- arterioles
- capillaries
- venules
- veins
5
Q
what is an artery called when it cant be seen with the naked eye anymore?
A
arteriole
6
Q
what is thicker in arteries?
A
tunica media
7
Q
why is the artery tunica media thicker than veins?
A
allows for greater elasticity
8
Q
arteries tunica media have an _________ amount of smooth muscle
A
increased
9
Q
what does the arteries deal with?
A
surges of oxygenated blood from heart
10
Q
do arteries have valves?
A
no
11
Q
do arteries have a high or low pressure system?
A
high pressure system
12
Q
are highly tensile vessels needed in veins?
A
no
13
Q
is venous system a high or low pressure system?
A
low
14
Q
do veins contain valves?
A
yes
15
Q
are more valves in veins located in the proximal or distal part of the body?
A
distal
16
Q
what is the structure of vein walls?
A
collapsible
larger
misshaped
17
Q
why are veins easier to extract blood?
A
thinner walls
larger diameters
18
Q
why is it less painful to get sticked with a needle from a vein?
A
The thinner the vessel, the less the innervation, so the less painful to stick with a needle
19
Q
why will blood not seep out of your vein when getting sticked with a needle?
A
veins have blood under less pressure within them
20
Q
is their more blood in you arteries or veins?
A
veins, they store blood
21
Q
where do veins store blood?
A
in the valves
22
Q
why do veins have valves?
A
helps blood get back to the heart
23
Q
is it slower or faster getting blood back to the heart?
A
slower
24
Q
when do valves prevent backflow?
A
during diastole
25
what does momentum do?
during systole it forces valves open
26
when do valves close?
when momentum decreases
27
what moves blood forward into the right atrium?
decrease in thoracic pressure
28
are veins deep or near the surface of the skin?
near the surface of the skin
29
what is the process referred to when getting blood back to the heart with your muscles?
ladder system
30
what veins typically have a pulsatile flow?
veins closer to the heart (hepatic veins)
31
what does spontaneous mean?
probe down on vein and there is instant flow
32
describe low resistant vascular bed
always needs blood flow (brain)
33
describe high resistant vascular bed
changes depending-can stop at times (legs)
34
when can blood be stored in arteries?
when they expand
35
what will happen if arteries cant expand?
blood pressure will rise
36
pressure gradient
measure of pressure the veins excert
37
what is the direction of pressure gradient?
high pressure to low pressure
38
what is phrenic vein?
diaphragm
39
what is azygos vein?
lateral veins descends into thoracic to right side of vertebral column
40
what does the azygos serve?
as a collateral if the SVC or IVC is occulted or blocked
41
what is vessel compliance?
ability to expand and contract
42
vascular musculature
relative to max. dilated state
43
ischemia
tissue death
44
what are the pumps in the arterial system?
- heart
- aortic pump
- muscular venous pump
- respiratory pump
- gravity
45
what is the principal force behind blood flow?
pumping action of the heart
46
what exists in the right atrium?
A partial vacuum exists in the right atrium during the right ventricular filling phase to suck venous blood from the IVC
47
what does the aorta act as?
subsidiary pump
48
Much of the energy resulting from cardiac contraction must be _____
stored
49
Much of the energy resulting from cardiac contraction must be stored?
is because of the holdup of blood flow that occurs in the capillary bed
50
The aorta and other elastic arteries near the heart serve as what?
store excess energy by stretching with each systole
51
what does the stretching of the aorta during peak systole allows the aorta to act as?
“reservoir”,storing much of the blood expelled by the heart
52
what happens in the aortic pump during diastole?
the energy stored in the arterial walls dissipates the large volume of blood stored in the aorta by forcing blood forward in the arteries
53
what is the pump for the venous system?
muscular pump
54
what ensures venous return at rest when in a supine postion?
pressure gradient
55
what muscles in the legs help with the muscular pump?
soleus and gastrocnemius sinusoids
56
flow which is initiated by cardiac contraction is further assisted by what?
muscular contraction and venous valve action
57
what does the respiratory pump assist in?
venous return to the heart
58
pressure in the thorax is always ________ than atmospheric pressure?
lower
59
Inspiration causes an _______ in abdominal pressure and a ________ in thoracic pressure
increase in abdominal pressure and decrease in thoracic pressure
60
what happens with the blood flow during inspiration?
reduced flow in the abdomen and increased venous flow in the chest
61
during each phrase in respiration what is the blood flow in the upper and lower extremities?
venous flow in the upper extremities is opposite to flow in the lower extremities during each phrase of respiration
62
where does gravity play a role for venous flow in the upper and lower extremities?
- upper extremity, gravity aids venous return from head and neck (any body part placed higher than level of heart)
- lower extremity, gravity acts as a deterrent
63
what is the end of the smallest airways?
alveoli
64
what is the purpose of alveoli?
exchange of oxygen and carbon dioxide takes place
65
what chamber does the pulmonary arteries come off of?
off the base of the right ventricle
66
what is ventricular systole?
period of contraction of the heart
67
what is arterial systole?
occurs toward the end of the ventricular diastole and completes by filling of the ventricles
68
what is ventricular diastole?
period during which both ventricles are relaxing
69
what is atrial diastole?
period during which the atria are relaxing (filling)
70
do the atria contact at the same time?
yes
71
do the ventricles contract at the same time?
yes
72
where is the blood going during systole?
great vessels
73
where is the blood going during diastole?
ventricles
74
where does the azygos vein drain?
into SVC just prior to the RT atrium
75
which brachiocephalic vein is longer?
left
76
what influences blood flow?
- cardiac function
- vessel compliance
- peripheral resistance
- tone of vascular musculature
- blood viscosity
- pattern of branching vessels and collaterals
- vasoconstriction and vasodilation
- exercise
- autoregulation
77
what does arterial physiology depend on?
cardiac status
78
what is blood flow determined by?
blood pressure and peripheral vascular resistance
79
heart failure
cardiac output is insufficient to meet the needs of the body and lungs
80
valvular dysfunction
prolapsed, vegetations, regurgitation
81
Ischaemia
Ischaemia due to impaired flow to left ventricular muscle (CAD)
82
arrythmias
irregular heart beat
83
Cardiomyopathy
problems within the heart muscle-thickening and enlargement of chambers
84
heart beat
rhythmic contraction of the heart
85
heart rate
number of times that the heart contracts per minute
86
Internal pacemaker
sino-atrial and atrio-ventricular nodes that initiates the hearts own impulses to contract
87
what is our heart rate controlled by?
autonomic nervous system
88
what increases heart rate and force of cardiac contraction?
sympathetic nervous system
89
what decreases heart rate and force of cardiac contraction?
parasympathetic nervous system
90
what must exist in order for fluid in a closed system to move from one point to another?
an energy gradient
91
which side of the heart has higher pressure and why?
left side is higher pressure to create the necessary pressure gradient
92
what is blood pressure?
Force exerted by the blood against the arterial walls when the heart contracts
93
why do we measure blood pressure?
indicator of current cardiovascular function and a risk indicator of future cardiovascular morbidity and mortality
94
what does the capillary bed consist of?
arterioles, capillaries, and venules
95
in each heart beat, how much blood is ejected?
approximately 70cc
96
what builds at the beginning of each cardiac contraction as the pressure in left ventricle rapidly rises?
pressure builds
97
how does the aortic valve open?
when pressure in the left ventricle exceeds the pressure in the ascending aorta
98
where does the pressure wave go after it goes through the arterial system?
past the resistance vessels (arterioles) and into the venous system
99
why does the mean pressure gradually decline?
losses in total fluid energy, however the systolic portion of the pressure wave actually increases
100
wave propagation
This phenomenon is due to increasing stiffness of the vessel walls toward the periphery and the reflected waves which are enhanced by increasing peripheral resistance
101
in the circulatory system there is a ______ pressure, ______ energy arterial reservoir
high pressure,high energy
102
in the circulatory system there us a _____ pressure, ____ energy venous pool
low pressure,low energy
103
when is energy stored in the stretched elastic walls of the arteries?
during diastole
104
what propels blood forward through the peripheral resistance vessels?
energy stored in the stretched elastic walls of the arteries
105
when is pressure in the heart the highest?
during ventricular systole (contraction)
lowest during diastole (filling)
106
what is the normal pressure during the cardiac cycle?
120/80
120=systolic
80=diastolic
107
what happens to the pressure as blood flows toward the periphery?
gradually decreases
108
how is pressure and energy produced?
contraction of the heart and resultant ejection of blood into aorta and systemic vessels
109
in the circulatory system, energy is lost in the form of what?
heat
110
what is vascular resistance?
resistance to flow that must be overcome to push blood through the circulatory system and create flow
111
systemic vascular resistance(SVR)
the resistance offered by the peripheral circulation
112
what is net flow controlled by?
arterial and arteriolar resistance
113
what are the main resistance vessels?
arterioles
114
flow model
An increase in resistance in a large distributing artery because of atheroma must be compensated by a decrease in the resistance of the small arteries and arterioles
115
what does any increase in the degree of stenosis lead to?
reduction in flow
116
what is the major determinant of vascular resistance?
precapillary arterioles
117
autoregulatory vessels
they can dynamically change in diameter to increase or reduce blood flow Ex) exercise
118
what happens to blood flow for increased flow to muscles?
The arterioles will increase in diameter to allow for increased flow to the muscles-the peripheral flow then becomes low resistant with continuous flow throughout systole and diastole
119
what are sources of peripheral resistance?
- blood viscosity
- blood vessel diameter(radius)
- total vessel length (not changing)
120
what is blood viscosity?
measure of resistance of blood to flow (thickness and stickiness)
121
what does viscosity determine?
- friction against vessel walls
- rate of venous return
- work required to pump the heart
- how much oxygen is transported to tissues and organs
122
what is blood viscosity affected by?
- hypertension
- cholesterol
- diabetes
- obesity
- smoking
- male
- aging
- temp.
- anemia
123
hemoconcentrated
red blood cell concentration increases
124
how does blood viscosity increase and decrease?
Blood viscosity increases as blood is more hemoconcentrated and decreases as blood is more dilute
125
the greater the viscosity the larger the _______
resistance
126
__________ blood will flow more readily
Hemodilute(thinner)
127
________ blood will flow more steadily
Hemoconcentrated blood(thicker)
128
hematocrit
percentage of red blood cells in the total blood volume
129
what does hematocrit affect?
blood viscosity so therefore resistance to flow
130
when can hematocrit increase?
there are more red blood cells or less plasma in the blood and vise versa
131
what does blood viscosity strongly depend on?
hematocrit
132
Polycythemia
a condition of abnormal elevation in red cell hematocrit causing higher blood viscosity
133
what can polycythemia cause?
increases the resistance to flow,increases the work of the heart and can impair organ perfusion
134
what does anemia cause?
low hematocrit and reduced blood viscosity
135
what happens to red blood cells in low flow states?
red cells stick together which increases the blood viscosity
136
what happens if clotting mechanisms are stimulated in the blood?
platelet aggregation and interactions with plasma proteins occur
137
does blood get thicker when it is hot or cold?
cold
138
what type of relationship does temperature and viscosity have?
inverse (blood temp. decreases and viscosity increases)
139
There is _______ pressure change as blood flows from the Aorta to large arteries
very little
140
what are the main regulators of SVR (systemic vascular resistance)?
small arteries ad arterioles
141
what is the percentage of pressure drop in the small arteries and arterioles?
70%
142
arterioles are ________
autoregulatory
143
what does vessel diameter affect?
peripheral resistance
144
when vessel diameter decreased why is there more resistance?
a greater proportion of the fluid is in contact with the walls therefore resistance to flow is increased and pressure rises
145
larger diameter= _______ volume= _______ pressure
same volume=less pressure
146
smaller diameter=________ volume= _______ pressure
same volume=more pressure
147
what does total vessel length affect?
peripheral resistance
148
what does a longer vessel length do in terms of resistance and blood pressure?
the longer the vessel length, the greater the resistance and the greater the blood pressure
149
what is the relationship of vessel resistance to viscosity?
directly proportional
150
what is the vessel resistance in relationship to radius?
inversely proportional to the radius to the 4th power
151
what is the vessel resistance in relationship to the length of the vessel?
directly proportional
152
what 2 factors controls peripheral resistance?
- sympathetic nerve innervation
| - autoregulation (vasoconstriction and vasodilation)
153
what happens to the blood flow and pressure within a vessel during vasoconstriction?
When an artery constricts, the flow of blood decreases and the pressure within the vessel rises
154
what does vasoconstriction and vasodilation regulate?
body temperature and blood pressure
155
what happens to blood flow during vasodilation?
When a blood vessel dilates, blood is able to flow through the vessel with less resistance
156
where does vasodilation occur?
at the arteriole and capillary bed level
157
what are the "stopcocks" of the vascular tree?
arterioles
158
where does blood pressure take its biggest drop?
arterioles
159
reactive hyperemia
marked increase in blood flow which follows restoration of arterial inflow to a previously ischemic limb
160
what is an example of reactive hyperemia?
during exercise induced increased heart rate and vasodilation of arterioles to allow more flow to muscular branches
161
what increases SVR?
vasoconstriction (vise versa for vasodilation)
162
what is the amount of blood entering the arterial tree determined by?
cardiac output
163
what is the amount of blood exiting the arterial tree determined by?
arterial pressure and total peripheral resistance(dependant on status of microcirculation)
164
what is the driving pressure for flow within a vessel determined by?
- potential energy
| - kinetic energy
165
what is potential energy generated by?
the left ventricular contraction that distends the vessel wall
166
what is the pulsatile arterial flow signal a summation of?
forward flow from the left ventricle and reverse flow from the tidal reflection
167
how is energy lost in the flow of blood?
friction of blood at the vessel wall and between adjacent layers of blood
168
where are energy losses the highest?
in the region of stenosis due to considerable friction during turbulent flow and vortex type motion
169
what will increase within a stenosis?
there will be an increase in blood kinetic energy associated with the increase in blood velocity
170
bernouilli's principle
there is a corresponding fall in blood pressure immediately after the stenosis
171
what is compliance?
the ability of a vessel to distend and increase volume with increasing transmural pressure
172
how much larger is venous compliance than arteries?
30x
173
what is blood pressure affected by?
peripheral resistance and blood vessel elasticity
174
what does a healthy elastic artery do during systolic pressure?
expands,absorbing the shock of systolic pressure
175
what maintains the continued flow during diastole?
elastic recoil
176
what does a arteriosclerosis artery do during systolic pressure?
become calcified and rigid,so they can’t expand when the pulse wave of systolic pressure passes through them
177
plug flow
when flow enters a vessel there are no leaders in velocity (velocity is the same)
178
parabolic (laminar) flow
as blood moves along a straight tube it becomes parabolic
179
in parabolic flow, where is the flow the fastest?
in the center of the vessel, slowest at the walls
180
why does laminar flow occur?
This profile is determined by frictional and inertial forces between the layers of blood and changes throughout the pulse cycle
181
when does blood velocity increase uniformly across a vessel?
early systole as a fluid motion begins
182
when does turbulent flow occur?
very high velocities
183
what happens with energy with turbulent flow?
significant irreversible loss of energy
184
what are the essential flow states?
turbulent
| laminar
185
what does disturbed flow show as?
The velocities of the blood cells will be more random and displays itself as “spectral broadening”,but will not show an increase in velocity in a normal vessel
186
when does disturbed flow occur?
sites of arterial dialation, curvature, branching or bifurcation
187
in normal circulation, flow is mainly ________
laminar
188
where in the vessel does disturbed or turbulent flow occur?
post stenotic region
189
what does turbulent and disturbed flow give rise to?
spectral broadening
190
The pressure drop across a stenosis is __________ as a result of _________in the poststenotic region
high, energy loss
191
critical stenosis
narrowing of the arterial lumen resulting in a hemodynamically significant reduction in volume,pressure and flow
192
what happens to pressure before a stenosis?
pressure decreases before a stenosis to allow fluid to accelerate into the stenosis and decelerate out of it
193
what is the magnitude dependant on?
degree of stenosis
194
where is arterial pressure high and low at a site of narrowing?
higher proximal to the site and lower distally because of the loss of kinetic energy
195
Anastomosis
branches form between adjacent blood vessels
196
what happens due to atherosclerosis to prevent perfusion?
- development of a collateral circulation
- a degree of local dilation of the affected arterial segment
- An increase in the extraction efficiency of oxygen from blood
197
damped and trickle flow
Very high degrees of stenosis are accompanied by a low flow rate and low velocities
198
what are the three types of flow waves?
- monophasic
- biphasic
- monophasic
199
what are the 2 types of vascular beds?
- low resistant bed (brain)
| - high resistant bed (muscles at rest)
200
low resistant beds
Their metabolic process needs continuous
forward flow throughout the cardiac cycle
(kidneys, liver, brain)
201
High resistant beds
peripheral circulation (small intestine not during digestion)
202
how is high resistant beds characterized?
low, absent, or reversed flow during diastole
203
where would you find triphasic waveforms?
The triphasic waveform is normally found in arteries supplying a high resistance peripheral vascular bed
204
what happens in the periphery during systole?
forward flow throughout
205
why does temporary flow reversal occur during diastole?
a negative pressure gradient caused by peripheral resistance to forward flow
206
when does flow reversal decrease?
increasing vasodilation as in exercise, body heating, and stenosis
207
when does flow reversal increase?
vasoconstriction
208
what are the 3 components of flow?
- forward flow in systole
- diastolic flow in reversal
- forward flow in late diastole
209
describe appearance of biphasic flow?
loss of 3rd component in late diastole
210
describe appearance of monophasic waveform?
flow is all above baseline
211
when is monophasic waveform seen?
low resistance beds
212
what are factors determining the stenosis effect?
- Length and diameter of the narrowed segment
- Roughness of the epithelial surface
- Degree of irregularity of the narrowing and its shape(abrupt or gradual)
- Ratio of the cross-sectional area of the stenotic zone to that of the normal vessel
- The rate of flow
- The arteriovenous pressure gradient
- Peripheral resistance beyond the stenosis
213
tandem lesions
2 or more stenosis
214
why may a vessel appear normal when there is a stenosis?
collaterals happen so we measure systolic pressure
215
what resistance will blood flow have proximal to a stenosis or occlusion?
higher resistant pattern due to the impending resistance to flow up ahead
216
what is the resistance after a stenosis?
pressure wave is more damped than normal due to the pressure drop across the stenosis
217
what happens to flow reversal distal to a stenosis?
disappears
218
when do monophasic signals only occur?
during systole
219
The disappearance of reversed flow distal to the stenosis results from a combination of which factors:
1-The maintenance of a relatively high level of forward flow throughout the cardiac cycle
2-Resistance to reverse flow created by the stenotic lesion
3-A decrease in peripheral resistance as a result of ischemia
4-Damping of the pressure wave by the lesion,resulting in attenuated pressure pulses
220
in normal arteries when does blood flow velocity increase?
during early systole
221
in normal arteries when does blood flow velocity decrease?
during early diastole
222
what reflects pressure gradient?
The shape of the resulting pulse velocity wave
223
over normal peripheral arteries what do the sounds represent?
second sound represents the diastolic flow reversal (biphasic)
third sound represents the second forward component (triphasic)
224
what is the main parameter for evaluating the severity of carotid stenosis?
flow velocity
225
where do we measure blood flow to evaluate the significance of the occlusive disease?
blood flow at and distal to arterial obstructions
226
what are the effects of arterial stenosis?
- Widening or dispersal of the band of systolic velocity
- Spectral broadening(loss of clear window below systolic portion)
- Complete filling- in of the spectral tracing
- Reversal of blood flow due to eddies
227
what is the pressure remaining in the veins after the blood has went through capillaries for a subject in a supine position?
pressure is low
228
Alterations in venous hemodynamics occur with :
- changes in posture
- important consequence of competence or incompetence of venous valves
- effects of venous obstruction
229
what are the 3 components of the venous pressure of the lower limbs?
- hydrostatic (gravitational pressure)
- residual pressure
- muscular and respiratory pressure
230
in lower limbs what is the main resistance to flow?
microcirculation
231
residual pressure
At the venous end of the capillaries,the venous pressure supplied by the arterial pressure
232
how does the cardiac pump bring blood back to the heart?
contraction (systole) and relaxation (diastole) brings flow back to the right atrium
233
what do the volume in the veins of the thorax do with inspiration?
volume increases
234
what happens to lower limb flow in veins during inspiration?
blood flow decreases
235
what happens to upper limbs in veins during expiration?
blood flow decreases
236
what are the venous flow characteristics?
- phasicity
- spontaneity
- augmentation
- compressibility
237
where is flow most spontaneous?
closest to the heart-instant flow pattern
238
Augmentation
wrap around flow
239
Spontaneous flow
Flow is readily apparent
240
Valsalva manoever
Increased abdominal pressure by “bearing down” will cause the valve to close and the flow to cease -this proves the valve is competent
241
arterial hemodynamics?
- Waveform reflects the cardiac cycle
- Not affected by respirations
- High pressure system
- Pulsatility:Varies according to vascular bed it supplies
- Has a pulse
242
venous hemodynamics
- Waveform reflects the respiratory -movements:
- Phasicity
- Low pressure system
- Pulsatility:Varies according to proximity to the heart
- No pulse
243
what is the doppler shift equation?
fd=ft-fr
244
cos0
1
245
cos30
0.8
246
cos60
0.5
247
cos90
0
248
what parameters would affect the doppler equation?
- velocity
| - angle of insonation (cosine)
249
what angle does not help us?
90
250
how does the angle affect the flow?
Direction of blood
flow in relation to the
transducer-toward
or away
251
what angle should we be for doppler?
under 60 degrees
252
what do we do to improve doppler angle?
- heel and toe
- use angle correct
- change colour box angle
- change window of insonation
253
in spectral doppler where is the flow in relation to the baseline when it is toward the transducer?
above the baseline
254
where should our sample volume-gate be set in the vessel?
center of vessel and be as small as possible so we pick up a cleaner signal
255
why do we have to keep angle correct the same universally?
consistency within:
- entire exam
- recheck exams
- patient to patient
256
ideally what should the spectral trace be set?
2/3 of the display
257
what adjusts the scale on doppler?
PRF
258
what happens when spectral wall filter is set too low?
wall thump from arterial systole
259
what happens when spectral wall filter is set too high?
eliminates echoes within waveform
260
what causes aliasing?
PRF too low
| so we increase the velocity scale
261
what will happen If you adjust the colour bar?
will result in inadequate colour representation
262
what happens if you place the colour baseline too low?
only higher velocity flow will be depicted
263
what happens if you place the color baseline too high?
only lower velocities will be depicted
264
what direction does red indicate?
red indicates flow towards the transducer
265
what direction dies blue indicate?
flow away from the transducer
266
where is arterial flow in relation to the baseline in spectral doppler?
above the baseline
267
what does a colour box too big do to our image?
decreased frame rate and poor resolution of colour within the vessel
268
frame rate ________ as the box size increase
frame rate decreases as the box size increases
269
besides colour box what else influences frame rate?
PRF
270
increasing the PRF will ___________ the frame rate
increase
271
what decreases as beam is steered with a colour box?
sensitivity of transducer
272
when would we use a straight colour box?
vessel is angled quite steeply
273
what direction do we steer our colour box?
Steer the box so that it is
parallel to the flow,not
angled in the opposite direction
274
how do we angle our colour box when we get to a bifurcation?
angle to colour box twice to delineate each branch with the most clarity
as the vessels usually
are not parallel
275
what velocity range do carotids have?
carotid settings have a higher velocity range due to higher arterial flow
276
when could aliasing occur when thinking about velocity ranges?
Color aliasing will occur for example-if a venous setting is used to visualize
arterial flow
277
when wall filter is set too high and velocity close to the walls is not shown what can it give a false impression of?
wall thickening
278
what are guidelines for optimal doppler examination?
- adjust gain and filter
- adjust velocity scale and baseline
- doppler angle under 60 by steering and probe position
- colour box as small and superficial as possible
- sample volume size
- avoid transducer motion
279
advantages for power doppler?
- no aliasing
- angle independent
- detect low velocity flow
- imaging tortuous vessels
- grading stenosis
280
disadvantages to power doppler?
- no velocity of flow
- no direction of flow
- very motion sensitive
281
when is fluid flow laminar?
low velocity
282
what is the resistance offered by the peripheral circulation also known as?
systematic vascular resistance
283
what is the major regulator of vascular resistance?
vessel radius
284
Augmentation
Venous flow can be squeezed to observe a large bolus of blood flowing through a vein in an unobstructed vessel
285
what is blood flow determined by?
- blood pressure
| - peripheral vascular resistance
