Principles Of Heamodynamics Flashcards
What are the main factors that determines blood flow 5 points
The main factors determining blood flow include the
force that is controlled by cardiac contraction,
the
work done that is controlled by isovolumetric contraction and subsequent ejection,
the pressure
difference between the arteries and veins,
the compliance of arteries in terms of how much they
stretch, the resistance provided by arterioles,
and flow velocity in terms of how the blood slows
down when it reaches the capillaries, and then speeds up (although not to the same extent) when it
returns to the heart.
Describe the central venous system
• The CVS is a closed system with the majority of the blood in it in the venous system held at a low pressure in reservoirs.
• Changing the blood flow can alter the distribution of blood in different parts
of the body.
How can the blood reservoirs be mobilised
The blood reservoirs in the veins can be mobilised through innervation from the ANS or
through contracting muscle that constricts veins.
What is Darcys law
Darcy’s law defines the role of pressure in blood flow. Blood flow can be calculated through
subtracting venous pressure from arterial pressure to find the pressure difference and divide
resistance (R) from the value reached.
What is tpr
TPR is the total peripheral resistance of the blood vessels.
What is the flaw I’m darcys law
However, blood flow is not entirely governed by pressure and blood can in fact flow against a
pressure or when there is no pressure. Darcy’s law would not be able to explain this and therefore
How does Bernoulli’s law differe to darcys
Bernoulli’s law comes into play. This law considers kinetic and potential energies, as well as pressure.
The equation used is blood flow = pressure (P) + Kinetic (mV2/2) + potential (mgh).
Kinetic energy
takes momentum of blood into account whilst potential energy considers gravity.
Define blood flow and perfusion
Blood flow itself
can be defined as the volume of blood flowing in a given time (ml/min). Perfusion is described as
blood flow per given mass of tissue (ml/min/g).
What effects the velocity of blood
Velocity of blood flow (cm/s) is affected by cross-
sectional area. Blood flow velocity in the aorta is high. The branching of the arteries slows the
velocity as the cross-sectional area increases.
Where is cross sectional area highest
Cross-sectional area is highest in the capillaries and is
therefore slowest in these vessels.
What blood vessel is blood slowed the most
The arterioles are the main vessels where blood is slowed the
most. When blood passes through the capillaries, it collects in venules that join up into veins and
eventually the vena cava.
Why does the speed not regain after venues
The speed however does not return to original levels due to the loss of
pressure. During this process, blood flow velocity increases again due to reduced cross sectional
area.
The relationship between cross-sectional area and velocity can be demonstrated using
examples. If the area of a blood vessel is 10 cm2 during vasodilation, velocity is 1 cm/s. In
vasoconstriction when cross-sectional area is 1 cm2, velocity is 10 cm/s. In normal conditions when
area is 2 cm2, velocity is 5 cm/s.
What is the equation for velocity and cross sectional area
Cross-sectional area and velocity can be linked together in an
equation; volume flow = velocity x cross-sectional area.
Blood flow can flow through blood vessels in different patterns.
What is the first type of blood flow
The first type of flow is laminar flow.
This is smooth blood flow that is fastest at the centre due to the solvation shell at the walls of the
blood vessel that interacts with the blood slowing it down slightly (mainly through friction). RBCs in
this flow tend to move to the centre.
Where is laminar blood flow found
This type of blood flow is mainly found in arteries, arterioles,
venules and veins.
What is turbulent flow
Turbulent flow occurs when blood flows too quickly. The result is vortexes and
eddies are generated.
Why does the blood not flow linearly in turbulent flow - and where is this seen
Blood does not flow linearly and smoothly due to the increased velocity as
well as the increased pressure. This means this type of flow is not only seen in ventricles and the
aorta, but also sites of atheroma where there is an injection of pressure onto the blood flow. The
disturbed blood flow can be heard using a stethoscope.
Where is bolus flow found
The flow of blood through capillaries is
known as bolus flow.
How does bolus flow occur
This flow occurs due to RBCs having a larger diameter than the diameter of the
capillaries so they move in single file and deform slightly. Plasma columns are trapped between RBCs.
Why is the bolus flow the most efficient
This is the most efficient way blood flows due to minimal internal friction and resistance.
How can bolus blood flow be calculated
The type of
blood flow can be calculated using Reynold’s number (Re). This is the number above which blood
flow moves from laminar flow to turbulent flow. Blood flow can be calculated using the equation
described above.
What is the Reynolds number
Reynold’s number will vary between different vessels and is calculated using the
following equation; Re = Density x velocity x diameter / viscosity. In the arteries this number is
between 1000 and 2000.
Why does blood flow in terms of pressure vary
Blood flow varies in terms of pressure due to systole and diastole. In the ventricle, the difference in
systole (120 mmHg) and diastole (almost 0) is huge.
How is the difference in pressure minimised
The entire blood pressure however does not vary
this much as the arteries expand and contract to minimise the pressure difference. The blood in the
arteries also loses pressure with the pressure differences continuing to be smoothed out until there
is virtually no pressure difference found in blood flow in veins. There is also a large drop in pressure
caused by the arterioles.
The larger arteries and the aorta play a role in minimising the pressure difference.