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Summary haemodynamics Pt1
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Summary of haemodynamics Pt1
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Haemodynamics IKey Points:
Description of physical terms relating to haemodynamics
Description of the Darcy’s law – the basic law of flow
Relationship of mean arterial pressure to cardiac output and total peripheral
resistance
Flow: laminar, turbulent and bolus flow patterns
Pressures: factors involved in the generation of arterial blood pressure
Definition of ‘pulse pressure’ and its relation to stroke volume and arterial
compliance
Physiological and pathological factors affecting mean arterial pressure
Key Physical Principles in CVS
Haemodynamics is the relationship between blood flow, blood pressure and
resistance to flow.
Force = Strength of cardiac contraction
Work = energy put in for isovolumetric contraction and ejection (this produces ->)
Pressure - Pressure difference between aorta and veins
Compliance = Extent of arterial stretch (important in creating diastolic pressure)
Resistance - Produced by arterioles
Flow
Velocity - Changes depending on where in circulation, slows down in capillaries
The CVS is a closed system, what happens in one part of the CVS has a major impact
on other parts, but it is a net effect. Meaning something happening in one area will
affect something happening in another but you cannot say something happening in
area 1 will be happening in area 2. There is what happens at a local level and what
happens in a total body level.
Because it is a closed system, you need to balance the system, Starling’s law balances
(maintains) blood flow. It is also important to remember that the majority of blood is
in the venous system, which is a low pressure reservoir system. Most of the blood is
in the small veins and venules, if needed we can access this reservoir and get more
blood returning to the heart which will be balanced by Starlings law increasing CO.
Definitions of Blood flow
Blood flow = The volume of blood flowing in a given time (ml/min)
Perfusion = Blood flow per given mass of tissue (ml/min/g)
Velocity = Blood flow divided by the cross sectional area through which the blood
flows (cm/s)
Basic law of flow – Darcy’s law
Darcy’s law of flow states:
, Q = P1 – P2
R
Where
Q = Flow
P1-P2 = pressure difference
R = Resistance to flow
Relationship Between Velocity and Flow
To explore this relationship, we can use the diagram below –
By looking at this, we can see that the volume flow (Q) (velocity x area) remains
constant at 10ml/s throughout the length of the tube, even when it increases in area
and constricts.
However velocity (volume flow/area) changes, at first part we have an area of 2 and
flow of 10 so velocity = 5cm/s. Then area increases and we get a drop/much slower
velocity, when we go to the narrower vessel we get an increase in velocity.
This tells us that the larger the area the slower the velocity.
This is important for the CVS, because of a number of reasons. Velocity of blood flow
through the aorta is high, because there is a large volume flow of blood (CO) going
through a small cross-sectional area.
As it branches and we get further branching of blood vessels, we have the same CO
(volume flow), but we have the same CO being spread out over a larger surface area,
thus velocity slows.
This slowing in velocity as cross sectional area increases continues until we reach the
capillaries where velocity of blood flow is slowest. Here we have a HUGE surface area
which is what we want for increases exchange and as a result the low velocity which
also helps exchange.
Then velocity increases with the veins coming together, it is under a lower pressure
(so there is less drive to the heart) but velocity has still increased.
Three Patterns of Blood Flow
There are three types of blood flow in the body; laminar, turbulent and bolus.
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