arterioles and veins

Question 1

What does TPR control? (2)

what would an increase in resistane lead to for the same flow?

Answer 1

TPR controls blood flow and blood pressure

Increase in resistance means need to increase pressure to keep same flow

Question 2

What controls TPR? (3)

Answer 2

Poiseuille’s law
Local control
Blood viscosity

Question 3

TPR equations

Answer 3

Blood Flow (Cardiac output, CO) = Pa – CVP / TPR

therfore Blood Pressure (Pa) = CO x TPR

Question 4

For the same blood flow hence CO, how does BP and flow change along arteries to capillaries?

how is this done?

Answer 4

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

Question 5

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?

Answer 5

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

Question 6

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?

Answer 6

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

Question 7

Why is hypertension and sepsis bad?

what do they both lead to? net effect of both?

Answer 7

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

Question 8

How can tpr change resistance of arterioles and alter blood flow when we are sitting down?

what happens elswhere in the body?

Answer 8

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

Question 9

what controls resistance? poiseuille’s law

equation?

Answer 9

Resistance = 8n L / pi x r4 (conductance is reciprocal)

r = radius of vessel : n = blood viscosity : L = vessel length

Question 10

combining darcy and poiseuille law

equation?

Answer 10

Blood Flow = Pa – CVP x pi x r4 / 8nL

arterial blood pressure x conductance determine blood flow

Question 11

the r4 effect

what will effect of double radius be for resistance? blood flow?

what will happen upstream?

Answer 11

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

Question 12

what are the main vessels that control tpr?

why and how?

Answer 12

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

Question 13

Capillaries have a much smaller radius than arterioles
so why does do arterioles control TPR and not capillaries?

3 reasons

Answer 13

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)

Question 14

Why less resistance in capillaries?

what kind of flow? effect of this?
how are capillaries arranged? effect of this?
comapred to arterioles?

Answer 14

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

Question 15

what controls local blood flow?

Answer 15

Local blood flow through individual organs/tissues is mainly controlled by changes in radius of arterioles supplying a given organ/tissue

Question 16

control mechanism or areriole radius

instrinic controls? example
extrinsic controls? example

Answer 16

Intrinsic
Factors entirely within an organ or tissue - local hormones like histamine

Extrinsic
Factors outside the organ or tissue - horomones or neuronal (sympathetic)

Question 17

what is viscosity?

Answer 17

Viscosity is a measure of internal friction opposing the separation of the lamina

Question 18

what factors affect blood viscosity?

3 things

Answer 18

Velocity of blood
Vessel diameter
Haematocrit

Question 19

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?

Answer 19

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)

Question 20

Vessel diamter and clinical implications
(Fahraeus-Lindqvist effect)

what happens to blood viscosity in small vessels?
why? what is bf like?

Answer 20

Blood viscosity falls in small vessels
(< 100 micro m vessels) – due to bolus flow
low resistance, high BF in microvessels
e.g. capillaries

Question 21

rbc deformity and clinical implications

effect on bf?

Answer 21

High N, decrease BF, Sickle cell crises

Question 22

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?

Answer 22

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

Question 23

blood volume in venous system

function?
importance?

Answer 23

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

Question 24

structure and function of veins

what type of vessel is it? (3)
significance of this?
how is it contractile? significance of this?

Answer 24

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

Question 25

contractibility of veins

net effect of this?

Answer 25

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

Question 26

pressure in vein

from foot, cvp and heart level?

Answer 26

Typical venous pressures (mmHg)
Limb vein, heart level 5-10
Central venous pressure (entering heart) 0-5
Foot vein, standing 90

Question 27

Hand below heart vs heart above heart

what will it look like below heart? above heart? why?

Answer 27

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

Question 28

what is the pressure gradient for venous return?

equation?

Answer 28

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

Question 29

how does thoracic pump help with venous return?

what happens during inhalation? effect of this? (2)

Answer 29

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

Question 30

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?

Answer 30

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

Question 31

why can standing still for a long time lead to fainting?

3 things that lead to pooling? effect of this ?
why fainting?

Answer 31

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

Question 32

what 2 other factors control blood flow

what does this explain when standing?

Answer 32

kinetic energy and potential difference (gravity) which explains why blood can flow from heart to feet when standing

Question 33

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?

Answer 33

-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

Question 34

explain how blood returns back to heart when standing?

what cancel each other out? what causes flow?

Answer 34

Pressure/Potential gradients cancel each other out – but there is kinetic energy as velocity increases in veins