Pressure and flow in arteries and veins Flashcards
What is systolic pressure?
The pressure in the aorta on contraction of the left ventricle
What is diastolic pressure?
Pressure in the Aorta just before the contraction of rhe left ventricle ( on relaxation and filling of the left ventricle)
What is the pulse pressure?
The difference between the systolic and diastolic pressure
What is the origin of Korotkoff sounds and what is their use?
The sound comes from turbulent blood.
The initial tapping sound is when the pressure of the cuff falls just below that of the systolic pressure.
Becomes a thumping sound when the pressure of the cuff falls so that it is between diastolic and systolic pressure, as the cuff pressure falls just above diastolic pressure it will sound muffled, and when it is silent again will be just below diastolic pressure.
What happens to aortic pressure as it passes through the vascular tree?
It falls slightly from the aorta to the arteries.
The biggest change in pressure then occurs from the arteries to the arterioles as the arterioles are the resistance vessles and so can increase or decrease the amount of blood flowing through.
The lowest pressure is then through the capillaries and the pressure then increases as the blood returns via the vena cava.
How does blood flow change throughout the vasculature and how does the cross sectional area of the vessels change throughout the vasculature?
Blood flow velocity is related to total cross section fastest in aorta and vena cava (lowest total cross sectional area), slowest in capillaries (largest cross sectional area). NB speed on return of vena cava is approx half that of the aorta leaving.
What factors affect pressure and flow in veins
Pressure wave is affected by
- stroke volume
- velocity of ejection
- elasticity of arteries
- total peripheral resistance
Arterial pressure (especially pulse pressure) increase with age
What mechanisms are in place to prevent blood clots in vessels?
Anti platelet aggrigation:
Prostacyclin and NO
The endothelium keeps the collagen away from the platelets
Anti thrombin:
Heparin (inactivates thrombin)
Thrombomodulin (binds to Thrombin and inactivates)
Tissue Factor Pathway Inhibitor (stops thrombin production)
Clot removal:
Tissue Plasminogen Activator (t-PA), changes plasminogen to plasmin and digests clot.
What affects the pressure wave?
- stroke volume
- velocity of ejection
- elasticity of arteries
- total peripheral resistance
How can we measure arterial pressure? What are their main advantages/disadvantages?
Auscultation of Korotkoff sounds using
a sphygmomanometer & stethoscope
Disadvantages
Accuracy
Discontinuous
Needs care
Advantages
Non-invasive
Cheap
OR
Oscillatory blood pressure measurement
Turbulent blood flow sets up vibrations (oscillations) in the blood vessel wall
A transducer monitors those vibrations
Maximum vibrations occur at the mean arterial pressure
An algorithm estimates diastolic and systolic pressures
Disadvantages = needs care, inaccurate, discontinuous
Advantages = non-invasive, cheap
What is “normal” arterial blood pressure?
120/80
What is the mean arterial pressure (MAP) in the aorta leaving the heart?
around 95mmHg
What hat is MAP leaving the heart?
Around 95mmHg
What does the MAP drop from and to when going through the arteries?
From around 95mmHg to about 90mmHg
MAP drop through arterioles?
From around 90 to about 40mmHg (resistance vessles)
Pressure change through capillaries
from 40 - aroudn20mmHg
What is the systemic filling pressure?
It is the pressure of the blood coming back to the heart via the venules and veins (from 20-5mmHg)
What is velocity related to in blood flow?
The TOTAL cross sectional area, eg aorta - only one, smaller cross sectional area therefore slower speed. Capillaries, much bigger cross sectional area therefore slower flow.
On a graph representing the arterial pressure, what is the aortic notch and what does it represent?
It represents when the aorta shuts and then the slight increase in pressure as the blood bounces back.
Why does MAP fall when you stand up?
Due to the effects of gravity.
Gravity itself doesn’t make it harder to pump blood back up to the heart (think about a copper U tube, water flows down and would flow right back up), but rather because the veins and venules are distensible and collapsible it means that the additional pressure (due to gravity) causes them to fill up, meaning less blood getting back to the heart, This then leads to a reduced end diastolic volume and then a reduced pre load, meaning reduced stroke volume and therefore cardiac output, which causes the MAP to fall.
How can venous pressure be “measured” ?
By the extent to which the jugular vein can be seen.
This is because when upright, the effects of gravity means that the hydrostatic pressure in the jugular vein is reduced and this leads to the pressure in the jugular vein being (at rest in healthy individual) below that of the pressure from the surrounding cells and tissues, leading it to collapse (not fully), however as it returns to the heart the pressure will increase and it will reopen. Central venous pressure can be estimated by the level at which the jugular vein reopens.
How does the resp pump work?
As you inhale and exhale you change the pressure in the thoracic cavity. On inhalation, the pressure decreases, which helps to pull blood back to the heart, but on exhalation, the pressure increases and the blood flow is prevented from going backwards by the valves.
In exercise resp rate increases and this helps with increasing venous return.
What is venomotor tone? How is it changed?
The tenseness of the muscles surrounding the veins (not as much as around the arteries). Stimulated by the sympathetic nervous system,
What is the major thing pushing blood back to the heart?
The systemic filling pressure
What additional 4 factors affect the pressure and flow in veins?
Skeletal muscle pump (muscle squashes out blood from veins - valves prevent backflow)
Respiratory pump
gravity (blood pooling)
venomotor tone
How are platelet plugs and therefore thrombus formed?
collagen exposed, platelets stick to it and releases chemicals that causes more platelets to stick to it. This forms the platelet plug.
The thrombus is then formed when Fibrin is formed from fibrinogen by the enzyme thrombin. and then forms a fibrin clot.
