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.
What is the percentage of blood storage of stroke volume - and where is energy stored
During left
ventricular contraction and ejection, around 60-80% of stroke volume is stored in these vessels as
these structures expand. The energy is specifically stored in stretched elastin.
Where is the energy transferred at diastole
During diastole, this
energy is returned to the blood as the walls of the aorta and arteries contract. This sustains diastolic
blood pressure and flow when the heart is relaxed.
How is pulse pressure measured
Pulse pressure is measured at the wrist through
the radial artery. This pressure can demonstrate stroke volume and arterial compliance.
What is the lowest diastole value and at ejection at peak systole
During
diastole, the lowest value pulse pressure reaches is 80 mmHg. This pressure increases during ejection
and peaks at 120 mmHg during peak systole.
This increase is much more rapid than the decrease.
When does diastole systolic pressure begin to increase
The decrease begins with systolic decline before the aortic valve closes to begin diastole. During this
short period of time, pressure increases slightly. Pulse pressure then continues to decrease until it
reaches peak diastolic levels of 80 mmHg.
What is pulse pressure and value
The variation between peak systolic pressure and peak
diastolic pressure is known as pulse pressure and its value is usually around 40 mmHg.
How can pulse pressure also be calculated
Pulse pressure
can also be calculated by dividing arterial compliance from stroke volume. This means the more
compliant the arteries are, the lower the pulse pressure is.
There is a difference in arterial pulse pressure and stroke volume during rest
and exercise and this can be plotted on a graph (see right).
What happens to stroke volume during exercise
In exercise, stroke
volume is increased, the arteries are stretched to a greater extent, they are
less compliant to blood flow and there is relatively greater systolic pressure.
The graph on the right shows that bigger pulse pressure increases stroke
volume and vice versa. This compliance curve gets very steep as stroke
volume increases.
What happens to the compliance of Aorta to elderly
In the elderly, the compliance of the aorta as well as other
arteries decreases and this results in a disproportional increase in systolic
and pulse pressure. This means that at resting levels, stroke volumes and
pulse pressures would be similar between an elderly individual and a younger one.
Elderly person pulse pressure during exercise
However, during
exercise, the elderly individual will experience a much more disproportionate increase in pulse
pressure compared to stroke volume than the younger person.
What can happen if the pulse is disproportionate to stroke volume
This can be very damaging to the
individual that can cause an aneurism (damage to blood vessel), stroke or damage to the vasculature.
What does increased pressure also do
The increased pressure also increases afterload and leads to heart failure.
How can compliance of stroke volume and pressure be calculated
The compliance can be
calculated by change in volume/change in pressure.
Compare pulse in aorta and arterioles
Pulse pressure at the aorta is relatively small and can increase further down the arterial tree as the
arteries become less compliant. However, the aorta still has effects on pulse pressures of these
vessels so pulse pressure is not increased much.
What can age do to arteries and what is the effect
Age also increases stiffness of the arteries and
particularly the aorta. This increases pulse pressure in the arterial tree.
What is difference abiht the pulse pressure of arteriola
Pulse pressure in the
arterioles cannot be detected.
What governs the mean blood pressure
The mean blood pressure is governed by the area under a blood
pressure curve. However, this is too complicated to quantify so instead the rough estimate of
diastolic pressure plus a third of pulse pressure is used as a reliable measure of mean blood pressure.
At resting levels, this is around 93.3 mmHg.
What effects mean blood pressure
Mean blood pressure is controlled by a number of
factors. Age, disease, distance along the arterial tree (rough measure of blood vessel compliance),
blood volume (stroke volume and cardiac output), exercise, emotions (stress, anger, fear,
apprehension and pain that all initiate the ANS to constrict or dilate blood vessels), and whether an
individual is awake or asleep are all factors affecting mean blood pressure
