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Lesson 11:
Circulation and Hemodynamics
This lesson contains 29 slides
plus 2 multiple-choice
questions.
This lesson was derived
from pages 53 through 58 in
the textbook:
MAJOR SYSTEMS OF
CARDIOVASCULAR CIRCULATION
Cardiopulmonary
Systemic
The two major systems of cardiovascular circulation in the body are
cardiopulmonary and systemic. The cardiopulmonary system includes the
heart and lungs. The systemic (peripheral) system includes any vessel that lies
outside the thorax and abdomen. The peripheral vascular system includes any
vessel that lies outside the thorax and abdomen. These are all of the veins and
arteries in the body that are located in the head, neck, arms, legs and penis.
SYSTEMIC CIRCULATION
Systemic circulation is the movement of blood from the left side of the heart,
through the body, and back to the right side of the heart. It supplies oxygenrich blood to all body organs. The beating heart propels blood into large
arteries, then into successively smaller arteries, and then into the arterioles (the
smallest branch of the arterial system), which feed the capillaries, which are
tiny blood vessels. It is in the capillary beds that gas exchange occurs with the
tissues, and the oxygen-rich blood becomes oxygen-deficient. The capillary
beds are drained of the oxygen-depleted blood by venules, which, in turn,
empty into veins that finally empty into the vena cava, which enters the heart.
SYSTEMIC CIRCULATION
Systemic circulation is the movement of blood from the left side of the heart,
through the body, and back to the right side of the heart. It supplies oxygenrich blood to all body organs. The beating heart propels blood into large
arteries, then into successively smaller arteries, and then into the arterioles (the
smallest branch of the arterial system), which feed the capillaries, which are
tiny blood vessels. It is in the capillary beds that gas exchange occurs with the
tissues, and the oxygen-rich blood becomes oxygen-deficient. The capillary
beds are drained of the oxygen-depleted blood by venules, which, in turn,
empty into veins that finally empty into the vena cava, which enters the heart.
HEMODYNAMICS
Hemodynamics is the
study of the movements
of blood and the forces
concerned therein.
TOTAL PERIPHERAL RESISTANCE
Total peripheral resistance (TPR) is the resistance to
blood flow through the various tissues of the body.
The total peripheral resistance determines the rate at
which blood flows out of the arterial vessels through
the arterioles.
Hemodynamics of
Arterial and Venous
Circulation
ENERGY
Kinetic energy
Potential energy
Total fluid energy
Energy is the capacity for doing work and for overcoming resistance. The
concepts of energy important to hemodynamics are kinetic energy, potential
energy, and total fluid energy.
ENERGY
Kinetic Energy
Kinetic energy is the energy of something in motion and is related to the
principles of inertia. Inertia is the tendency for an object at rest to remain at
rest. It is also the tendency for a moving object to continue moving in the
direction in which it was going rather than modify its course. The two factors of
mass and velocity determine how much kinetic energy is present.
ENERGY
Potential energy
Potential energy is the energy of something at rest rather than in motion. In
hemodynamics, potential energy has several components. It is a combination
of intravascular pressure and gravitational potential energy. The pumping
action of the heart’s muscle is the dominant source of the intravascular
component of potential energy. The hydrostatic pressure is the pressure
exerted by a fluid within a closed system.
ENERGY
Potential energy
The hydrostatic pressure is the pressure exerted by a fluid within a closed
system. The static filling pressure is the pressure that exists because of the
relationship between the amount of blood in a vessel and the elasticity of the
vessel walls. Gravitational potential energy is the capacity of a quantity of
blood to do work, based on its position above a specified reference point (the
right atrium). Hydrostatic pressure and gravitational potential energy usually
cancel each other out.
TOTAL FLUID ENERGY
Combination of the kinetic
energy (blood flow) and the
potential energy (blood
pressure) present.
Total fluid energy is the combination of the kinetic energy (blood flow) and the
potential energy (blood pressure) present.
POISEUILLE’S LAW
gradient: the difference
in pressure (pressure
drop) between the two
ends of a vessel or the
difference in pressure
across a valve
To move blood from one point to another requires an energy gradient. As
blood moves toward the periphery, energy is dissipated, but is continually
restored by the heart’s pumping action. Poiseuille’s Law provides an equation
that is analogous to Ohm’s Law by relating the flow rate with the pressure,
viscosity, vessel radius and length. The use of Poiseuille’s Law assumes that
the vessel is a small artery or arteriole.
PRESSURE GRADIENT
P
 P is the pressure gradient. It is normally expressed as mm Hg.
FLOW
Q
Flow (Q) is the volume of blood passing through a vessel per unit of time.
Blood flow is always from an area of higher pressure to an area of lower
pressure. The contraction of the heart imparts pressure to the blood, but
because of resistance (due to frictional losses), the pressure decreases as
blood flows through a vessel. The rate of flow is determined by the gradient
between the two ends of a vessel, rather than the absolute pressures. Volume
flow (Q) is normally expressed in cc/sec or mL/sec.
RESISTANCE
R
The resistance (R) is a measure of the hindrance to blood flow through a vessel
caused by friction between the moving fluid and the stationary vascular walls.
The resistance to blood flow depends on the viscosity (ƞ) of the blood, the
length (L) of the vessel, and the radius (r) of the vessel. Generally, resistance
and flow are opposite factors, that is, when one increases the other decreases.
RESISTANCE
R
Since vessel lengths remain constant in the body, they are not a factor in the
control of resistance. However, since resistance is directly proportional to
vessel length, a longer vessel would produce a greater resistance to flow. The
radius is the most important determinant of changes in resistance. According
to Poiseuille’s equation, the resistance increases to the fourth power as the
radius decreases.
STROKE VOLUME
SV
Stroke volume (SV) is the volume of blood pumped out of a ventricle with each
heartbeat. It is the difference between the final ventricle filling volume (end
diastolic volume) and the final ventricle emptying volume (end systolic volume).
The typical resting stroke volume is about 70 mL.
VISCOCITY
VISCOSITY
(Internal friction)
The viscosity (ƞ) is the internal friction existing between the contiguous layers
of a fluid, developed between its molecules as they slide over each other. As
the layers of red blood cells rub against each other, energy is lost in the form of
heat. The greater the viscosity, the greater the resistance to flow. The
hematocrit (volume of blood cells) and the concentration of plasma proteins
determine the viscosity of blood. Increasing hematocrit increases blood
viscosity.
VASCULAR RESISTANCE
RESISTANCE IN THE VASCULAR SYSTEM
Aorta
Large Arteries
Main Branches
Terminal branches
Arterioles
Capillaries
Total venous
4%
5%
10%
6%
41%
27%
7%
Most of the resistance in the vascular system comes from the arterioles. These
vessels have a muscular lining and can vasoconstrict and vasodilate and
therefore regulate resistance to flow.
FLOW PATTERNS
• plug
•laminar (parabolic)
• disturbed
• turbulent
Basic types of flow patterns that can be demonstrated in blood vessels include
plug flow, laminar flow, disturbed flow, and turbulent flow.
PLUG FLOW
• occurs during systole in large vessels
During systole in the large vessels, blood is pushed like a large plug with only
the boundary layer remaining stationary. The layers of blood accelerate at the
same rate with a relatively flat velocity profile. With this plug flow, the center
50% of a vessel may contain one very large layer of blood, with all blood cells
moving at the same velocity.
LAMINAR FLOW
• thought to exist in the majority of
vessels
During laminar flow, blood moves smoothly at a constant rate, sliding over itself
in concentric, orderly layers. Each layer increases slightly in velocity from the
wall of a vessel (where there are greater frictional forces) to its center (where
there are lower frictional forces). The velocity of blood is the actual linear
motion of the blood cells within the blood vessel. This is due to red blood cells
migrating toward center stream while leaving the less viscous plasma along the
wall. The orderly flow conserves greatly the kinetic energy contained in the
moving blood. Laminar flow, which is thought to exist in the majority of vessels
of uniform diameter, can be described as having a parabolic velocity profile.
DISTURBED FLOW
• caused by high peak velocities, curving,
branching, and divergence
• often produces bruits
Disturbed flow is a deviation from laminar flow consisting of oscillatory
variations in direction or the formation of vortices. Vortices are elements of
rotational flow often seen with flow separations and disturbance, and often
comprising a wide range of velocities aligned in two directions along a line
passing through its center. High peak velocities may cause disturbance of
blood flow. Disturbance may also be caused by curving, branching, and
divergence of vessels, or by projections into the vessel lumen.
TURBULENT FLOW
• often at the location of a stenosis
• significant pressure gradients are
present
antegrade: moving or
extending forward in the
direction of blood flow
or with the normal
direction of blood flow
retrograde: backward
flow, backward filling, or
against the normal
direction of flow
stenosis: the narrowing
or constriction of a
blood vessel
When turbulent flow is present, blood is moving in a swirling fashion. It may
contain blood moving in a direction opposite to that of normal flow, or it may be
disorganized, with blood flow chaotically oriented in many directions
simultaneously both antegrade and retrograde. Turbulence often appears at
the exit point of a stenosis. The increased friction and inertia losses
associated with turbulence cause significant pressure gradients across the
stenosis. Turbulence accounts for great reductions in the energy of moving
blood.
BERNOULLI EFFECT
The Bernoulli Effect describes the
relationship between changes in
fluid flow and changes in
pressure energy.
Q=VxA
(flow = velocity x area)
The Bernoulli Effect, when applied to peripheral vessels, is the reduction in
pressure that accompanies an increase in velocity of fluid flow. It explains the
large drop in pressure across stenotic vessels, which occurs due to the change
in the direction of flow caused by the turbulence.
BERNOULLI EFFECT
A reduction in pressure
accompanies an increase
in the velocity of fluid
flow.
According to the “Bernoulli Principle”, if the flow rate is constant, a decrease in
area causes an increase in velocity. As velocity increases, there is a
corresponding increase in kinetic energy and a decrease in pressure. The loss
of energy due to the Bernoulli Effect is responsible for the lower pressure
downstream from the stenosis.
CRITICAL STENOSIS
• causes a significant
reduction in the amount
of blood flow distal to the
location of the stenosis
A critical stenosis is an area of disease within a vessel, which causes a
significant reduction in the amount of blood flow distal to the location of the
stenosis. The degree of stenosis characterized as significant varies from one
blood vessel to another.
CRITICAL STENOSIS
abdominal aorta  90% area reduction
carotid artery  75% area reduction
In the abdominal aorta, a 90% reduction (10% remaining) in area is required
before the stenosis is critical, while in the carotid artery a 75% reduction (25%
remaining) in area is characterized as critical. A 75% area reduction is
equivalent to a 50% diameter reduction, often called a 50% stenosis.
HYDROSTATIC PRESSURE
Hydrostatic pressure is the variation in the body, which occurs when one part of
the body is at a different elevation than another. This variation arises because
of the gravitational potential energy of the blood. The blood has a higher
potential energy when it is at a higher elevation. The higher elevation blood
pushes down on the blood at lower elevations. An individual in a standing
position has a high hydrostatic venous pressure compared to when he or she is
lying down. When pressure measurements are performed, the patient is using
lying down, often permitting hydrostatic pressures to be ignored.
The venous system is responsible for the return of oxygen deficient blood to the
heart. Normally, the average pressure in the veins is only 2 mm Hg, compared
to the much higher average arterial pressure of approximately 100 mm Hg.
The low pressure is insufficient for venous return of blood to the heart. This is
particularly true for venous return from the lower extremities. Instead, venous
return is regulated by contraction of the surrounding skeletal muscles.
Contractions of muscles that encase the calf form a “calf muscle pump” that
propels venous blood upward, while lowering venous pressures and volumes in
the extremity.
Answers to the following
TWO practice questions
were derived from
material in the textbook:
Question 1
The two MAJOR systems of cardiovascular circulation are:
pulmonary and circulatory
cardiopulmonary and pulmonic
systemic and diastolic
cardiopulmonary and systemic
Page 53
Question 2
The MOST important determinant of changes in a vessel’s
resistance is:
elasticity
radius
viscosity
length
Pages 54 and 55
END OF LESSON 11