Mechanics 3 Flashcards
What is the function of the circulation?
transport blood around body (deliver O2, nutrients, hormones, clear CO2/metabolites)
regulate temperature
What is the physiological function of the heart?
muscular pump that generates pressure gradient to propel blood through blood vessels
What are the adaptations of capillaries?
diffusion is effective over short distances
capillary is 10 micrometres from each cell
highly branched structure
reservoir for blood volume
largest SA in CVS
What are the functions of veins?
reservoir during exercise as cardiac output increases
veins have a large capacitance for blood
venules and veins are highly compliant
How do small arteries/arterioles regulate diameters and resistance to blood flow?
elastic
have smooth muscle in walls
What is blood pressure physiologically?
force that drives circulation by a pressure difference
What is the function of small arteries and arterioles?
present most resistance to flow
alter perfusion to the vascular bed (gatekeeping)
useful during exercise - redirect flow to working muscles
Why does pressure fall across the circuit?
due to viscous pressure losses
i.e. frictional pressure losses
% of circulation to pulmonary?
17%
% of circulation to heart?
9%
% of circulation to arterioles and capillaries?
7%
% of circulation to arteries ?
11%
% of circulation to venules and veins?
61%
What is Darcy’s Law?
defines blood flow between particular resistances and pressures P = Q X R Q = volumetric flow R = resistance P= pressure difference
Formula for MAP?
cardiac output x resistance
= Q x R
Why is the formula for MAP only an approximation?
assumes steady flow (heart is pulsatile)
assumes vessels are rigid (resistance may change)
assumes right arterial pressures is negligible (it may change)
What happens in reality?
flow is regulated by changing R to keep the BP constant
Q = P/R
Q is inversely proportional to resistance
What is Poiseuille’s Equation?
R = 8Ln/πr4
Resistance to blood flow depends on:
- fluid viscosity (n) that is usually constant
- length of tube (L) fixed
- inner radius of tube (r) that changes
What would a half in radius of a vessel do to flow?
reduce flow 16x
therefore small changes in vascular tone (vasodilation/vasoconstriction) produce large changes in flow
Why does blood flow distribution to organs differ?
according to metabolic needs
arterioles/arteries vasoconstrict/dilate to reroute blood supply to more active orgsans
How does CO change during exercise ?
5 –> 20 L (increase total blood supply - less venous reservoir)
skeletal muscles receive 30 fold increase in supply
decrease reservoir in veins to increase venous return
increase preload
increase stretch
increase CO via contractility
divert blood to skeletal muscles
Blood flow to digestion and kidney at rest?
1L
Blood flow to digestion and kidney at exercise?
0.75L
1L
Blood flow to heart at rest/exercise?
0.25L —> 1.25L
Blood flow to brain/skin at rest and exercise?
NO CHANGE
- 75L
- 25L
Blood flow to bone at rest and exercise?
0.15L —> 0.25L
Blood flow to skeletal muscle at rest and exercise?
0.75L —> 16L
What is the flow like in arteries?
pulsatile that becomes more continuous along the circulation
What is the normal circulatory flow described as?
laminar flow
blood flows in layers
velocity is constant throughout
flow faster at centre, slow near walls due to friction and adhesive forces between fluid and walls
What is the flow associated with pathophysiological changes (atheroma)?
turbulent flow
irregular
eddys form, prone to pooling
velocity not constant due to changes in SHEAR STRESS
What is the shear rate?
velocity gradient
difference between fastest and slowest velocity in lumen
s = difference in v/difference in r
What is the formula for shear stress?
T = s x n
governs how well endothelial cells work
What is high shear stress?
laminar flow
endothelial cells align normally
produce substances normally
What is low shear stress?
turbulent flow - associated with endothelial dysfunction
cells mix up
produce wrong substances
more granules
What is the velocity profile of laminar flow?
parabolic
What physical signs are typical of laminar flow?
not heard
high shear stress
promote endothelial cell quiescence and survival
cells aligned in direction of flow
secretions promote vasodilation and anticoagulation
What physical signs are typical of turbulent flow?
low shear stress
promote cell proliferation, apoptosis, shape change
secretions promote vasoconstriction, coagulation, platelet aggregation –> increase clotting and blood vessel occlusion
What do age related changes do to flow?
increase turbulent flow in carotid arteries
decline in endothelial function
What is blood pressure a measure of?
turbulent flow
Describe how BP is measured?
- Stop blood flow using a cuff - occlude arterial pressure
- Decrease cuff pressure so that at SBP blood flows through turbulently
- Hear using stethoscope on brachial artery
- When sound disappears flow is laminar –> DBP reached
Formula for PP
DBP - SBP
Formula for MAP
DBP + 1/3PP
Why is there a dichrotic notch when measuring SBP and DBP?
from closure of aortic valve
What does increases elasticity mean?
increased compliance
Why does pressure fall slowly in arteries? What effect is this?
elasticity buffers change in PP to sustain diastolic blood pressure
Windkessel effect - buffering
What happens to the Windkessel effect with age?
arterial compliance decreases
elasticity decreases
reduced buffering (WK effect)
increase PP
Describe how the WK effect works?
- ejection - blood enters aorta faster than it leaves (40% of SV stored by elastic arteries)
- AV valve closes
- elastic arteries recoil so pressure decreases slowly - they absorb pressure during systole
- Ensures diastolic flow is pulsatile and continuous in downstream circulation
Vascular Law of Laplace?
T = P x r
transmural pressure causes tension in the wall
Formula for circumferential stress?
s = P x r /h h = wall thickness
What determines flow
pressure difference between 2 points
transmural pressure
What is compliance?
relationship between TMP and vessel volume
depends on vessel elasticity
What is the consequence of maintained high circumferential stress?
vessel distension —> aneurysm
overtime vessel walls weaken
balloon like distension
Describe a vascular aneurysm?
increase vessel radius
for same internal pressure inward force exerted by muscle wall increases (increase circumferential stress)
if fibre weakened no force produced
aneurysm expands till rupture
How to cure an aneurysm?
mesh around it
What do compliance curves show?
propensity of vessel to stretch under pressure
veins compliant at low P
- small changes in venous pressure distend veins and blood volume stored there
- venous compliance 10-20x greater than arterial at low P
arteries compliant over range of P
How do decrease venous compliance?
increase smooth muscle contraction
decrease venous volume, increase pressure
What is the effect of venous compliance? Consequence of standing?
blood pooling
standing increase hydrostatic pressure in legs due to gravity
blood pools in veins with high compliance and reduces venous return
with time CO and BP decrease
fainting
How much does gravity increase hydrostatic pressure in veins by?
100mmHg
How can pooling be prevented?
activate SNS
constrict veins and arteries to increase total peripheral resistance
increases BP
standing HR increases and contractility increases to return more blood to heart
Hows does low pressure blood return to the heart?
skeletal muscle pump - contract to compress veins (positive pressure)
respiratory pump - inhalation causes negative intrathoracic pressure so blood returns to central veins quickly -> extravascular pressure decreases in thorax but increases in abdominal cavity -> pressure gradient drived flow
valves ensure unidirectional flow
What affects venous volume distribution?
venous tone (constriction)
gravity
skeletal muscle and respiratory pump
What are two issues with standing ?
- varicose veins
- dilated superficial veins
- incompetent valves - oedema
- prolonged elevated venous pressure
- failed venous valves - fainting due to blood pooling
Tropys of the heart?
Chronotropic (Heart rate) Dromotropic (Conduction velocity) Inotropic (Contractility) Bathmotropic (Excitability) Lusitropic (Relaxation)
How is ESPVR generated?
IVC occlusion
progressive fall in preload
