Pressure and flow in arteries and veins Flashcards
What are the ways to measure BP?
Arterial line
Auscultation using Korotkoff sounds (turbulent blood flow)
Oscillatory BP measurements
How is arterial line BP measured?
Line inserted into radial or brachial artery and BP measured continuously
What are the advantages or disadvantages of measuring BP by arterial line?
Ad - Continuously measures BP accurately
Dis - Invasive so only for acutely unwell
How is Korotkoff sounds measured?
Using a sphygmomanometer (inflatable cuff and pump) and a stethoscope
What are the advantages and disadvantages of measuring BP by Korotkoff sounds?
Advantages: non-invasive, cheap, quick and easy to use.
Disadvantages: Discontinuous and takes time, accuracy (calibration of device, size of cuff for patient, doctor judgement), needs care and skill
Korotkoff sounds - what do you hear when the cuff is not inflated
No sounds as no turbulent blood flow
Korotkoff sounds - what do you hear when pressure is increased above systolic pressure?
Hear nothing as completely occlude the vessel, no blood going through it
Korotkoff sounds - What do you hear when pressure in cuff falls below systolic?
Blood starts to flow back through artery, very turbulent, heart as a tapping sound = first Korotkoff sound
Korotkoff sounds - What doe you hear as pressure is slowly released?
Finally falls below diastolic BP laminar flow returns and you hear nothing
Korotkoff sounds - when do you hear first tapping?
Systolic BP
Korotkoff sounds - when do you stop hearing sounds?
Diastolic BP
How is oscillatory BP measured?
Turbulent blood flow sets up vibrations (oscillations) in the blood vessel wall.
Transducer monitors vibrations.
Maximum vibration occur at mean arterial pressure.
Algorithm estimates diastolic and systolic pressures.
What are some advantages and disadvantages of oscillatory BP?
Disadvantages: discontinuous, accuracy, needs care.
Advantages = non-invasive, cheap, easier to use, bit quicker.
Why does fluid/blood flow through a tube/vessels?
There’s high pressure at one end and low pressure at the other
Fluid flows down the pressure gradient
What happens in the LV in terms of pressure?
Big pressure changes
High in systole (blood is ejected) and low in diastole (heart is filling)
What happens to pressure throughout the vascular tree?
Small drop in pressure from heart to arteries
Large drop through arterioles
Low pressure in capillaries
Small pressure difference pushing blood back through veins
What is the drop in BP from heart to arteries and why?
95 mmHg to 90 mmHg
Low resistant conduit and wide lumen means its easy for blood to flow through
What is the drop in BP from arteries to arterioles and why?
90 mmHg to 40 mmHg
Arterioles are resistance vessels as have narrow lumen and thick muscular walls.
How can arterioles change TPR?
Muscles contract = constricting arterioles = increasing TPR
Muscles relax = dilate arteries = decreases TPR
What happens to pressure in the capillaries and why is this good?
Very low as capillaries are very thin-walled and wouldn’t cope with high pressure
What is the pressure change between capillaries and veins and what is this called?
20 mmHg to 5 mmHg
Called systemic filling pressure
How does pulmonary circulation compare to systemic circulation?
Pulmonary = 1/5 systemic
What is velocity of blood related to?
Total cross sectional area
What is the speed of blood in aorta compared to capillaries?
Fastest in aorta
Slowest in capillaries
What is the cross section area related to aorta and capillaries?
Aorta = small total cross sectional area as big but only one vessels
Capillaries = large total cross sectional area as individually tiny but there are many of them
What happens when ventricles contract?
Ventricle contracts
Semi-lunar valve opens and blood is ejected from ventricles and flows into the arteries.
Aorta and arteries expand and store pressure in elastic walls.
What happens when ventricles relax?
Isovolumic ventricular relaxation
Semi-lunar valve shunts, preventing flow back into right ventricle
Elastic recoil of arteries sends blood forward into rest of circulatory system
What type of artery is the aorta?
Elastic artery
What does the aorta act as in systole?
A pressure reservoir and damps down pressure variation
What happens to pressure when blood is ejected?
Rises quickly and peaks at systolic pressure
What causes the dicrotic notch?
Elastic recoil of the aorta as aortic valve closes
What is the gradual fall in pressure due to?
Elasticity of the aorta, elastic recoil pushes on column of blood so pressure falls gradually
What is the pressure wave affected by?
Stroke volume
Velocity of ejection
Elasticity of arteries
Total peripheral resistance
How does SV affect the pressure wave?
Contractility increases because of increased sympathetic tone
Blood pumped out with more force
Rising phase will rise more rapidly
Larger systolic pressure
How does velocity of ejection affect the pressure wave?
Pumping blood out quicker
Rising phase is steeper
Bigger systolic pressure
What happens to diastolic pressure when velocity of contraction increases?
Diastolic pressure will be lower
- Less elastic energy stored up
- Less pressure to push back on column of blood
How does elasticity of arteries affect systolic and diastolic pressure?
If elasticity lost:
Systolic pressure rises
Diastolic pressure falls; less elastic recoil to maintain pressure on column of blood
What happens to diastolic pressure if arterioles are constricted?
TPR increases
Harder to push blood from arteries into arterioles
Falling phase is more gradual
**Higher diastolic pressure
What happens to diastolic pressure if arterioles are dilated?
Easier to push blood into them
Falling phase will be steeper
**Diastolic pressure will be lower
What is normal BP?
120/80
What happens to BP as we age?
BP increases due to elasticity in arteries
How does BP vary throughout the day?
Lower while asleep
Higher in the morning
Higher under pain and sex
What external influences affect flow?
Gravity
Skeletal muscle pump
Respiratory pump
Venomotor tone
Systemic filling pressure
What is the effect of gravity on veins?
Causes venous distension
Orthostatic hypotension
Venous collapse in the neck
What does gravity not affect?
Driving pressure from arteries to veins
- Since there’s the same pressure gradients pushing blood from arteries to veins and veins to right heart
- These cancel out and hence same driving pressure
What happens in venous distension?
Veins will distend and blood will pool in your legs.
Less blood returning to the heart
How does gravity affect the values?
Decreases EDV, preload, SV, CO, MAP
What is orthostatic (postural) hypotension?
Light headedness and even fainting when standing up
How does gravity cause venous collapse in the neck?
Negative pressure causes collapse, usually not visible but if CVP rises it will move up neck and may be visible.
What does a raised JVP mean?
Indication of various heart conditions increasing right sided heart failure
What happens to pressure in your feet and head when stood up?
Feet = increase by 80mmHg
Head = decrease by 40mmHg
How does skeletal muscle pump work?
When muscles in the leg contract they also get fatter. This compresses the veins and venules and pushes blood back up to heart.
What effect does skeletal pump have on values?
Increased venous return and thus EDV
What can lack of use of the skeletal muscle pump cause?
Deep vein thrombosis
- Increased venous pooling in legs and this static blood is more likely to clot
How can you prevent DVT?
Wear flight socks or hospital DVT stockings
These compress vessels in the legs and push blood more quickly back to the heart
How can the respiratory pump affect venous return?
Increases venous return and EDV by increasing respiratory rate and depth.
Decrease in thoracic pressure causes a large pressure gradient that pulls blood back to the heart and therefore increases venous return.
What is venomotor tone and how is it changed?
Is the state of contraction of the smooth muscle surrounding the venules and veins
Innervated by sympathetic nervous system which releases noradrenaline and adrenaline to bind to adrenergic receptors on smooth muscle causing them to contract.
How does venomotor tone affect venous return?
Squeezes capacitance back to heart and increases EDV
What is systemic filling pressure?
Pressure created by ventricles and transmitted through vascular tree to veins
How does systemic filling pressure change?
Increase in exercise
- Increased sympathetic tone and increased contractility
- Increased strength of contraction will cause MAP to increase
- Decrease TPR
- Higher pressure by the time blood gets to veins and venules
What can inappropriate clotting lead to?
Formation of a thrombus
- Can break off and embolise
- Leads to myocardial infarction, stroke of pulmonary embolism
How does a clot come about?
Formation of a platelet plug
Formation of a fibrin clot
What happens to platelets when damage occurs?
Platelets are activated and stick to the collagen in the basement membrane.
They then send out signals to bring in other platelets.
How does a fibrin clot form?
Large insoluble protein forms a mesh over the top of the platelet plug (binding platelets together)
How does fibrin form?
Fibrinogen converted to fibrin by enzyme thrombin
What are some anti-clotting mechanisms that the endothelium has developed? (6)
Stops blood contacting collagen
Produced prostacyclin and NO
Produces tissue factor pathway inhibitor (TFPI)
Expresses thrombomodulin
Expresses heparin
Secretes tissue plasminogen activator (t-PA)
How can stopping blood contacting collagen stop a clot?
No platelets aggravated
Acts as a physical barrier
How does prostacyclin and NO production stop clotting?
Both inhibit platelet aggression
Stopping platelets adding on and getting a larger platelet plug
How does producing tissue factor pathway inhibition (TFPI) stop clotting?
Stops thrombin production
- Less fibrinogen converted to fibrin so inhibits the fibrin clot
How does expressing thrombomodulin stop clotting?
Binds to thrombin and inactivates it
- Less fibrinogen converted to fibrin so inhibits the fibrin clot
How does inhibiting heparin stop clotting?
Inactivates thrombin
How does secreting tissue plasminogen activator (t-PA) stop clotting?
Plasminogen (inactive) > plasmin (active) and digests clots
If blood clot has formed
Most widely used as treatment for ischaemic strokes (clot blocking blood supply to the brain)
Also used to treat myocardial infarction and pulmonary embolism
