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Circulatory System.
Hemodynamics
Functions of the Circulatory System
• A unicellular organism can provide for its own maintenance and
continuity by performing the wide variety of functions needed for life.
• By contrast, the complex human body is composed of specialized cells
that demonstrate a division of labor.
• The specialized cells of a multicellular organism depend on one
another for the very basics of their existence; since most are firmly
implanted in tissues, they must have their oxygen and nutrients
brought to them, and their waste products removed.
• Therefore, a highly effective means of transporting materials within
the body is needed.
Functions of the Circulatory System
• The blood serves this transportation function.
• An estimated 60,000 miles of vessels throughout the body of an adult
ensure that continued sustenance reaches each of the trillions of
living cells.
• In order to perform its various functions, the circulatory system works
together with the respiratory, urinary, digestive, endocrine, and
integumentary systems in maintaining homeostasis.
Functions of the Circulatory System
• Transportation:
• Respiratory:
• Transport 02 and C02.
• Nutritive:
• Carry absorbed digestion products to liver and to tissues.
• Excretory:
• Carry metabolic wastes to kidneys to be excreted.
Functions of the Circulatory System
(continued)
• Regulation:
• Hormonal:
• Carry hormones to target tissues to produce their effects.
• Temperature:
• Divert blood to cool or warm the body.
• Protection:
• Blood clotting.
• Immune:
• Leukocytes, cytokines and complement act against pathogens.
Components of Circulatory System
• Cardiovascular System (CV):
• Heart:
• Pumping action creates pressure head needed to push blood through
vessels.
• Blood vessels:
• Permits blood flow from heart to cells and back to the heart.
• Arteries, arterioles, capillaries, venules, veins.
• Lymphatic System:
• Lymphatic vessels transport interstitial fluid.
• Lymph nodes cleanse lymph prior to return in venous blood.
Pulmonary and Systemic Circulations
• Pulmonary circulation:
• Path of blood from right
ventricle through the lungs
and back to the heart.
• Systemic circulation:
• Oxygen-rich blood pumped
to all organ systems to
supply nutrients.
• Rate of blood flow through
systemic circulation = flow
rate through pulmonary
circulation.
Definition of Hemodynamics
Hemodynamics is concerned with the physical and physiological
principles governing the movement of blood through the
circulatory system.
• The forces involved with the movement of blood throughout
the human circulatory system include:
1. Kinetic and potential energy provided by the cardiac pump
2. Gravity
3. Hydrostatic Pressure
4. Pressure gradients, or differences, between two any points
• Properties of blood itself that affect its flow:
1. Viscosity
2. Inertial mass
3. Volume of blood to be moved
• Factors that affect the motion of blood through the
vascular conduits include:
1. Size of blood vessel
2. Condition of blood vessel
3. Smoothness of lumen
4. Elasticity of muscular layer (tunica media)
5. Destination of blood (distal vascular bed)
Definition of Physical Concepts
PRESSURE: the ratio of a force acting on a surface to the area of the surface
(force per unit area).
Units : Newtons/m², pascal (Pa), atmospheres(atm), mmHg.
FLOW RATE: Amount of fluid passing a given point over a given period of
time
Described as either flow volume or flow velocity.
Flow volume is measured in mI/mm or cm3/sec - defined by Poiseuille’s
law.
Flow velocity is measured in cm/sec or m/sec - described by Bernoulli’s
principle.
VISCOSITY: The internal friction between adjacent layers of fluid.
Blood is 1.5 times as viscous as water and its viscosity is directly
related to hct level
KINETIC ENERGY: active energy , the energy of motion.
In hemodynamics- described as the forward movement of blood.
POTENTIAL ENERGY: stored energy.
Kinetic energy is transferred into potential energy when it produces a lateral
pressure or stretching of vessel walls during systole.
The potential energy is converted back into kinetic energy when the arterial
walls rebound during diastole.
VELOCITY OF BLOOD FLOW
• The blood vessels of the cardiovascular system vary in terms of diameter
and cross-sectional area.
• These differences in diameter and area, in turn, have profound effects on
velocity of flow. The relationship between velocity, flow, and crosssectional area (which depends on vessel radius or diameter) is as follows:
where
V - Velocity of blood flow (cm/sec)
Q - Flow (mL/sec)
A - Cross-sectional area (cm2)
Velocity of blood flow (v) is linear velocity and refers to the rate of displacement of blood
per unit time. Thus, velocity is expressed in units of distance per unit time (e.g., cm/sec).
Flow (Q) is volume flow per unit time and is expressed in units of volume per unit time (e.g.,
mL/sec).
Area (A) is the cross-sectional area of a blood vessel (e.g., aorta) or a group of blood vessels
(e.g., all of the capillaries). Area is calculated as A = πr2, where r is the radius of a single
blood vessel (e.g., aorta) or the total radius of a group of blood vessels (e.g., all of the
capillaries).
Blood Flow, Vessel Diameter and Velocity
• As diameter of vessels increases, the total cross-sectional
area increases and velocity of blood flow decreases
• The total blood flow at each level
of blood vessels is the same and
is equal to the cardiac output.
• Because of the inverse
relationship between velocity and
total cross-sectional area, the
velocity of blood flow will be
highest in the aorta and lowest in
the capillaries.
• From the standpoint of capillary
function (i.e., exchange of
nutrients, solutes, and water), the
low velocity of blood flow is
advantageous: It maximizes the
time for exchange across the
capillary wall.
RELATIONSHIPS BETWEEN BLOOD FLOW,
PRESSURE, AND RESISTANCE
• Blood flow through a blood vessel or a series of blood vessels is
determined by two factors:
• the pressure difference between the two ends of the vessel (the
inlet and the outlet)
• and the resistance of the vessel to blood flow.
• The pressure difference is the driving force for blood flow, and the
resistance is an impediment to flow.
• The relationship of flow, pressure, and resistance is analogous to the
relationship of current (I), voltage (ΔV), and resistance (R) in electrical
circuits, as expressed by Ohm's law (Ohm's law states that ΔV = I × R
or I = ΔV/R).
• Blood flow is analogous to current flow, the pressure difference or
driving force is analogous to the voltage difference, and
hydrodynamic resistance is analogous to electrical resistance.
• The equation for blood flow is expressed as follows:
Q = ∆𝑃/R
where
Q - Flow (mL/min)
ΔP - Pressure difference (mm Hg)
R - Resistance (mm Hg/mL/min)
• The magnitude of blood flow (Q) is directly proportional to the size of
the pressure difference (ΔP) or pressure gradient.
• The direction of blood flow is determined by the direction of the
pressure gradient and always is from high to low pressure. For
example, during ventricular ejection, blood flows from the left
ventricle into the aorta and not in the other direction, because
pressure in the ventricle is higher than pressure in the aorta.
• For another example, blood flows from the vena cava to the right
atrium because pressure in the vena cava is slightly higher than in the
right atrium.
Blood Flow & Blood Pressure
• Blood flow (F) is directly proportional to the difference in blood pressure
(P) between two points in the circulation
• Furthermore, blood flow is inversely proportional to
resistance (R). Increasing resistance (e.g., by
vasoconstriction) decreases flow, and decreasing
resistance (e.g., by vasodilation) increases flow.
• The major mechanism for changing blood flow in the
cardiovascular system is by changing the resistance of
blood vessels, particularly the arterioles.
• The flow, pressure, and resistance relationship also can be rearranged to
determine resistance. If the blood flow and the pressure gradient are known,
the resistance is calculated as R = ΔP/Q.
• This relationship can be used to measure the resistance of the entire systemic
vasculature (i.e., total peripheral resistance), or it can be used to measure
resistance in a single organ or single blood vessel.
• Total peripheral resistance.
The resistance of the entire systemic vasculature is called the total peripheral
resistance (TPR) or the systemic vascular resistance (SVR).
TPR can be measured with the flow, pressure, and resistance relationship by
substituting cardiac output for flow (Q) and the difference in pressure
between the aorta and the vena cava for ΔP.
• Resistance in a single organ.
The flow, pressure, and resistance relationship also can be applied on a
smaller scale to determine the resistance of a single organ.
As illustrated in the following sample problem, the resistance of the renal
vasculature can be determined by substituting renal blood flow for flow (Q)
and the difference in pressure between the renal artery and the renal vein
for ΔP:
• Resistance - opposes blood flow because of the friction produced
by the vessel walls
• Factors that affect resistance (R)
• (1) resistance (R) is proportional to viscosity: V  R
• “thickness” of the blood
• e.g., dehydration, elevated plasma proteins, polycythemia (RBCs), leukemias
(WBCs)
• (2) resistance (R) is proportional to vessel length
• obesity increases the route lengths within connective tissue
• (3) resistance (R) is inversely proport. to vessel width
• decrease the radius by 1/2 and R increases by 16X
• most important in vessels that can change their size actively
• changes in diameter affect flow
• vessel wall drag – blood cells dragging against the wall
• laminar flow – layers of flow
The factors that determine the resistance of a blood vessel to
blood flow are expressed by the Poiseuille equation:
Where,
R - Resistance
η -Viscosity of blood
l - Length of blood vessel
r4 - Radius of blood vessel raised to the fourth power
LAMINAR or STREAMLINE
FLOW
P1
P2
P1 > P2
-Cone Shaped Velocity Profile
-Not Audible with a Stethoscope
Ideally, blood flow in the cardiovascular system is
laminar, or streamlined. In laminar flow, there is a
parabolic profile of velocity within a blood vessel, with
the velocity of blood flow highest in the center of the
vessel and lowest toward the vessel walls.
The parabolic profile develops because the layer of
blood next to the vessel wall adheres to the wall and,
essentially, does not move.
The next layer of blood (toward the center) slips past
the motionless layer and moves a bit faster.
Each successive layer of blood toward the center
moves faster yet, with less adherence to adjacent
layers.
Thus, the velocity of flow at the vessel wall is zero, and
the velocity at the center of the stream is maximal.
Laminar blood flow conforms to this orderly parabolic
profile.
When an irregularity occurs in a blood vessel (e.g., at the valves or at the site of a blood clot),
the laminar stream is disrupted, and blood flow may become turbulent.
In turbulent flow, the fluid streams do not remain in the parabolic profile but, instead, the
streams mix radially and axially.
Because energy is wasted in propelling blood radially and axially, more energy (pressure) is
required to drive turbulent blood flow than laminar blood flow.
Turbulent flow is often accompanied by audible vibrations called murmurs.
LAMINAR VS TURBULENT FLOW
THE REYNOLD’S NUMBER
LAMINAR
FLOW
TURBULENT
FLOW
Nr = pDv / n
laminar = 2000 or less
p = density
D = diameter
v = velocity
n = viscosity
The Reynold's number is a
dimensionless number that is
used to predict whether blood
flow will be laminar or
turbulent.
It considers a number of
factors, including diameter of
the blood vessel, mean velocity
of flow, and viscosity of the
blood.
If Reynold's number (NR) is less than 2000, blood flow will be laminar.
If Reynold's number is greater than 2000, there is increasing likelihood that
blood flow will be turbulent.
Values greater than 3000 always predict turbulent flow.
The major influences on
Reynold's number in the
LAMINAR VS TURBULENT FLOW
cardiovascular system are
THE REYNOLD’S NUMBER
changes in blood viscosity and
changes in the velocity of blood
flow.
Inspection of the equation shows
LAMINAR
TURBULENT
FLOW
that decreases in viscosity (e.g.,
FLOW
decreased hematocrit) cause an
p = density
increase in Reynold's number.
D = diameter Likewise, narrowing of a blood
Nr = pDv / n
v = velocity
vessel, which produces an
n = viscosity
laminar = 2000 or less
increase in velocity of blood flow,
causes an increase in Reynold's
number.
Blood Pressure (BP)
• Measure of force exerted by blood against the wall
• Force per unit area exerted on the wall of a blood
vessel by its contained blood
• Expressed in millimeters of mercury (mm Hg)
• Measured in reference to systemic arterial BP in large
arteries near the heart
• Blood moves through vessels because of blood
pressure
• The differences in BP within the vascular system
provide the driving force that keeps blood moving
from higher to lower pressure areas
Systemic Blood
Pressure
Arterial Blood Pressure
• Pulsatile in arteries due to the
pumping of the heart
• Systolic/diastolic values
• Pulse pressure = systolic (minus)
diastolic
Mean arterial pressure is the average
pressure in a complete cardiac cycle and is
calculated as follows:
Notice that mean arterial pressure is not the
simple mathematical average of diastolic and
systolic pressures.
This is because a greater fraction of each
cardiac cycle is spent in diastole than in
systole.
Thus, the calculation of mean arterial pressure
gives more weight to diastolic pressure than
systolic pressure.
Beginning in the small arteries,
arterial pressure decreases, with the
most significant decrease occurring in
the arterioles.
At the end of the arterioles, mean
pressure is approximately 30 mm Hg.
This dramatic decrease in pressure
occurs because the arterioles
constitute a high resistance to flow.
Since total blood flow is constant at all
levels of the cardiovascular system, as
resistance increases, downstream
pressure must necessarily decrease (Q
= ΔP/R, or ΔP = Q × R).
Systemic Blood Pressure
• Capillary Blood
Pressure
• relatively low blood
pressure
• low pressure is good
design for capillaries
because:
• capillaries are fragile high pressure would
tears them
• capillaries are very
permeable - high
pressure forces a lot of
fluid out
Systemic Blood Pressure
• Venous return
• the volume of blood
flowing back to heart
from systemic veins
• depends on pressure
difference from
beginning of venules
(16 mmHg) to heart (0
mmHg)
• any change in right
atrial (RA) pressure
changes venous return
Heart
• The heart is a hollow, muscular organ situated in the space between
lungs(mediastinum) , its about 12 cm in length & about 9 cm in width
Functional Anatomy of the Heart
Cardiac Muscle
• Characteristics
•
•
•
•
•
Striated
Short branched cells
Uninucleate
Intercalated discs
T-tubules larger and
over z-discs
Functional Anatomy of the Heart
Chambers
• 4 chambers
• 2 Atria
• 2 Ventricles
• 2 systems
• Pulmonary
• Systemic
Functional Anatomy of the Heart
Valves
• Function is to prevent backflow
• Atrioventricular Valves
• Prevent backflow to the atria
• Prolapse is prevented by the chordae tendinae
• Tensioned by the papillary muscles
• Semilunar Valves
• Prevent backflow into ventricles
Functional Anatomy of the Heart
Intrinsic Conduction System
• Consists of
“pacemaker” cells and
conduction pathways
• Coordinate the
contraction of the atria
and ventricles
Cardiac Output (CO)
• Is volume of blood pumped/min by each ventricle
• Heart Rate (HR) = 70 beats/min
• Stroke volume (SV) = blood pumped/beat by each
ventricle
• Average is 70-80 ml/beat
• CO = SV x HR
• Total blood volume is about 5.5L
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