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Blood vessels

Course Code
Christopher Perumalla

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Lecture 27
Blood vessels, blood flow and blood pressure
1.Physical laws governing blood flow and blood pressure
Vasculature is an elaborate system of pipes that runs through the body so
fundamental laws of physics that describe the flow to any liquid through a
system of pipes also pertain to the blood flow in the cardiovascular system.
Whenever there is a difference in pressure between two locations, the
pressure gradient drives the flow from the region of high pressure to low
pressure, down the pressure gradient. The driving force for the bulk flow is
always the pressure gradient, the difference in pressure.
Flow rule
The cardiovascular system is a circulatory system, you end up where you
started. Pressure is defined by the force exerted by blood. Flow is defined as
the pressure gradient /resistance in the cardiovascular system. So flow is
directly proportional to the gradient and indirectly proportional to resistance.
If there is a great pressure gradient, the flow will go up while If you have a
great resistance, the flow will go down.
Pressure gradients in the Cardiovascular system
Pressure gradients drive flow from high pressure to low pressure. It is done
by the bulk flow and not diffusion. It is the job of the heart to create this
pressure gradient so that the blood can move by bulk flow. The heart’s
pumping action creates a pressure gradient that is used to drive the blood to
the body which eventually returns to the heart. So, a gradient must exist
throughout circulatory system to maintain blood flow.
Pressure gradient across systemic circuit
In the systemic circuit, the pressure gradient is the pressure in the aorta
minus the pressure in vena cava just before it empties into the right atrium.
The pressure in the aorta was the highest since it was used to deliver the
blood to the body and the pressure in the vena cava is the lowest (returns to
the heart). The pressure in the aorta is taken as a mean arterial pressure
(MAP) of about 90 mm Hg while the pressure in the vena cava is nonexistent
(CVP). The pressure gradient = MAP-CVP which is 90 mm Hg.

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Pressure gradient across pulmonary circuit
In the pulmonary circuit, the pressure gradient is the pressure in the
pulmonary arteries minus the pressure in the pulmonary veins. The
pulmonary arterial pressure in 15 mm Hs while the pulmonary venous
pressure is 0 mm Hg. The pressure gradient = 15-0=15 mm Hg.
Pressures of the pulmonary and systemic circuits
The largest drop in pressure is at the level of the arterioles because although
they are small in diameter, they are many of them. Because they are so
numerous, when all the arterioles are put together, their diameter is bigger
than that of the artery. The pressure drops in the arteries and the veins are
The pressure in the pulmonary circuit is lower than the pressure in the
systemic circuit. The right ventricle generates less pressure than the left

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Resistance in the cardiovascular system
A smaller diameter means that there is more resistance while a large
diameter means that there is less resistance. The resistance of blood vessels
is a measure of the degree to which the tube hinders or resists the flow of
liquid through it.
Factors affecting resistance to flow
The factors that affect the resistance of the flow is the radius of the vessel,
the length of the vessel and the viscosity of the fluid.
Radius of the vessel: Big diameter decreases resistance while small diameter
increases resistance.
Length of the vessel: the longer the vessel, the higher the resistance.
Viscosity: As viscosity increases, resistance increases.
However, in regulating flow, the radius of the vessel is the most important.
2. Overview of the vasculature
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