arterioles and veins Flashcards
What does TPR control? (2)
what would an increase in resistane lead to for the same flow?
TPR controls blood flow and blood pressure
Increase in resistance means need to increase pressure to keep same flow
What controls TPR? (3)
Poiseuille’s law
Local control
Blood viscosity
TPR equations
Blood Flow (Cardiac output, CO) = Pa – CVP / TPR
therfore Blood Pressure (Pa) = CO x TPR
For the same blood flow hence CO, how does BP and flow change along arteries to capillaries?
how is this done?
It will go down a pressure gradient which is due to a pressure drop made by arterioles constricting which increases the resistance hence there is less flow as we go through the circulation -> useful for capillaries
For the same blood flow hence CO, how does BP and flow change along arteries to capillaries with LOW BP?
how is this different to before?
With LOW bp, the arterioles will dilate to decrease the tpr hence this will reduce resistance in arterioles, there is a lower pressure drop but more blood flow
For the same blood flow hence CO, how does BP and flow change along arteries to capillaries with HIGH BP?
how is this different to before?
with HIGH bp, the arterioles will constrict more to increase the TPR hence this will increase the resistance in arterioles, there is a bigger pressure drop and less blood flow
Why is hypertension and sepsis bad?
what do they both lead to? net effect of both?
hypertension leads to excessive constriction which can cut off the blood flow to some areas
sepsis leads to excessive dilation which can reduce the pressure gradient
hence both can reduce blood flow to end organs
How can tpr change resistance of arterioles and alter blood flow when we are sitting down?
what happens elswhere in the body?
We aren’t using our legs thefore arterioles to legs can vasoconstrict and reduce blood flow there and arterioles to the intestine can vasodilate and increase blood flow there for digestion
what controls resistance? poiseuille’s law
equation?
Resistance = 8n L / pi x r4 (conductance is reciprocal)
r = radius of vessel : n = blood viscosity : L = vessel length
combining darcy and poiseuille law
equation?
Blood Flow = Pa – CVP x pi x r4 / 8nL
arterial blood pressure x conductance determine blood flow
the r4 effect
what will effect of double radius be for resistance? blood flow?
what will happen upstream?
double radius will increase conductance by 16 or be 1/16 of resistance hence 16 times more blood flow to end organs but upstream there will be a pressure drop
what are the main vessels that control tpr?
why and how?
arterioles
Arteriole radius is tightly controlled by sympathetic nerves
Arterioles have largest pressure drop of 40-50 mmHg
amongst vessels as are able to provide the greatest chnage in R
Capillaries have a much smaller radius than arterioles
so why does do arterioles control TPR and not capillaries?
3 reasons
Radius, R4
No sympathetic innervation/smooth muscle in capillaries -> So cannot alter radius
Pressure drop (P1-P2) Less pressure drop across capillaries (20-30mmHg) than arterioles (40-50 mmHg) -> Due to less resistance to blood flow in capillaries
Length, L
Individual capillaries are short compared to arterioles (L, see Poiseulle’s law)
Why less resistance in capillaries?
what kind of flow? effect of this?
how are capillaries arranged? effect of this?
comapred to arterioles?
Bolus flow reduces viscosity (η, see Poiseulle’s law) This means there is reduced laminar flow and less interaction/friction hence less resistance
Capillaries are arranged in parallel,
So have a low total resistance as RTotal = 1/R1 + 1/R2 etc.
In contrast, arterioles are in series with arteries, arterioles, capillaries
RTotal = R1 + R2 etc – total resistance is greater
what controls local blood flow?
Local blood flow through individual organs/tissues is mainly controlled by changes in radius of arterioles supplying a given organ/tissue
control mechanism or areriole radius
instrinic controls? example
extrinsic controls? example
Intrinsic
Factors entirely within an organ or tissue - local hormones like histamine
Extrinsic
Factors outside the organ or tissue - horomones or neuronal (sympathetic)
what is viscosity?
Viscosity is a measure of internal friction opposing the separation of the lamina
what factors affect blood viscosity?
3 things
Velocity of blood
Vessel diameter
Haematocrit
Haematocrit and clinical implications
%?
typical blood viscosity relative to water?
condition for high rbc? effect on tpr, bp and bf?
condtion for low rbc? effect on tpr, bp? hr?
Haematocrit (45%)
Typical blood viscosity is 4-5 relative to water
Polycythaemia (high N)
high TPR and BP, low BF
Anaemia (low N)
low TPR and BP, with high HR (baroreflex)
Vessel diamter and clinical implications
(Fahraeus-Lindqvist effect)
what happens to blood viscosity in small vessels?
why? what is bf like?
Blood viscosity falls in small vessels
(< 100 micro m vessels) – due to bolus flow
low resistance, high BF in microvessels
e.g. capillaries
rbc deformity and clinical implications
effect on bf?
High N, decrease BF, Sickle cell crises
velocity of blood and clinical implications
speed of venous flow in immbile legs? effect on viscosity?
what happens when you stand up for a long time? why? net effect?
Slow venous flow in immobile legs – increased viscosity
if you stand up for long time, poor blood flow to venous system therefore increase viscosity and increased R hence decrease venous flow
blood volume in venous system
function?
importance?
60% of blood volume at rest is in systemic veins and venules
Functions as blood reservoir
Blood can be diverted from it in times of need
Such as exercise, haemorrhage
structure and function of veins
what type of vessel is it? (3)
significance of this?
how is it contractile? significance of this?
Thin-walled, collapsible, voluminous vessels hence can hold lots of blood as thin wall expand easily
Contain 2/3ths of blood volume
Contractile – contain smooth muscle, innervated by sympathetic nerves that Control radius hence shift blood flow back to the heart
contractibility of veins
net effect of this?
Contraction of vessels – Expels blood into central veins
– Increases venous return/CVP/end-diastolic volume
– Increases stroke volume (Starling’s law) via stretch and increased pre-load and increased contraction
pressure in vein
from foot, cvp and heart level?
Typical venous pressures (mmHg)
Limb vein, heart level 5-10
Central venous pressure (entering heart) 0-5
Foot vein, standing 90
Hand below heart vs heart above heart
what will it look like below heart? above heart? why?
Hand below heart means gravity will pool blood in veins hence increased pressure and they are still walled so will expand easily
if above heart, under gravity, blood will return to heart easier hence veins collapse due to low venous pressure
what is the pressure gradient for venous return?
equation?
Pressure in venules/veins is between 10 (supine) – 90 (standing) mm Hg
IVC/SVC/right atrium < 5 mm Hg
Venous return = Venous Pressure – Pressure right atrium / Venous resistance
how does thoracic pump help with venous return?
what happens during inhalation? effect of this? (2)
Inhalation - thoracic cavity expands leading to high abdominal pressure (squeeze veins in abdomen to force blood back to heart, hence why more inhalation, more return to heart), forcing blood upward towards heart, high right ventricular SV
Blood flows faster with inhalation
how does skeletal muscle pump return blood flow to the heart?
what prevents backflow?
why can legs swell when in upright position?
hwo does it reduce swelling?
Contraction of leg muscles returns blood into right atrium
Retrograde flow is prevented by venous valves
Reduce high local venous pressures when in the upright position as blood can pool due to gravity and lead to swelling due the increased pressure causing fluid to move out into interstitial fluid
Reduces swelling of feet and ankles – lower venous pressures, lower capillary pressure, less filtration
High CVP and SV during exercise, contracting leg muscles
why can standing still for a long time lead to fainting?
3 things that lead to pooling? effect of this ?
why fainting?
Due to gravity, heat-induced vasodilatation,
lack of muscle use, which all cause pooling of blood
Volume in lower limbs/feet hence less return to heart therefore less stretch, less starling’s law, less SV, less CO and less BP
therefore less perfusion to the brain as brain against gravity and leads to fainting
what 2 other factors control blood flow
what does this explain when standing?
kinetic energy and potential difference (gravity) which explains why blood can flow from heart to feet when standing
explain how blood flows from heart to feet when standing?
what is against the flow?
what is for the flow?
how is the flow maintained?
what is the clinical importance?
-90 mmHg pressure gradient against flow but
Ejected blood - greater kinetic energy at heart than feet (more velocity, V)
+ greater potential energy than at heart than feet (more height, h)
Greater kinetic/potential energies overcome pressure gradient to maintain flow
However: flow to feet easily compromised – clinically important
explain how blood returns back to heart when standing?
what cancel each other out? what causes flow?
Pressure/Potential gradients cancel each other out – but there is kinetic energy as velocity increases in veins
