Circuitry of cardiovascular system
and structure-function relationship
Dr. Shafali
Learning Objectives
• Describe the organization of the circulatory
system and its function
• Explain how the systemic and pulmonary
circulations are linked physically and
physiologically
• Understand the relationship between flow,
velocity, and cross-sectional area
Learning Objectives
• Understand the relationship between
pressure, flow, and resistance in the
vasculature .
• Define resistance and conductance.
Understand the effects of adding resistance
in series vs. in parallel on total resistance and
flow.
Functions of circulation
• Supply the tissues with nutrients
• Removal of waste product of tissue
metabolism
• Control blood flow to the skin and limbs to
regulate heat loss
• Aids in body’s defence mechanisms by
delivering antibodies ,platelets and leucocytes
to the affected area of the body.
General features of the cardiovascular system
120
Pulmonary and systemic circulations
• Cardiac output and heart rate of the two circuits
are equal, so stroke volumes are the same.
• Despite this, all pressures are higher in the
systemic (peripheral) circuit. This shows that the
vessels of the circuits are very different. The
systemic circuit has much higher resistance and
much lower compliance than the pulmonary
circuit.
• The lower pressures mean that the work of the
right ventricle is much lower.
• In addition, the lower capillary pressure protects
against the development of pulmonary edema
NORMAL BLOOD PRESSURE IN DIFFERENT PORTIONS OF
CIRCULATORY SYSTEM
• Local arteriolar dilation decreases
arteriolar resistance, which increases
flow and pressure downstream (more
pressure and more flow get
downstream).
• Local arteriolar constriction increases
arteriolar resistance, and flow and
pressure decrease downstream
Types of blood vessels
1. Windkessel vessels/Distensible vessels- aorta
,pulmonary artery and their large branches.
2. Resistance vessels– arterioles ,metarterioles
and pre capillary sphincter.
3. Exchange vessels—capillaries
4. Capacitance vessels- venules and veins
5. Shunt vessels or Thoroughfare vessels/A-V
shunts
COMPLIANCE OF BLOOD VESSELS
• The compliance or capacitance of a blood vessel
describes the volume of blood the vessel can hold
at a given pressure. Compliance is related to
distensibility and is given by the following equation:
where C ,Compliance (mL/mm Hg) ,V Volume (mL), P
Pressure (mm Hg)
• The equation for compliance states that the higher
the compliance of a vessel, the more volume it can
hold at a given pressure.
• Compliance is essentially how easily a vessel is
stretched;(and remain so)
• If a vessel is easily stretched, it is considered
very compliant. The opposite is noncompliant
or stiff.
• Elasticity is the inverse of compliance. A vessel
that has high elasticity (a large tendency to
rebound from a stretch) has low compliance.
a. 15:1
b. 10:1
c. 1:1
d. 1:10
e. 1:20
15
750
Which one of the following values is greater in
the pulmonary circulation than in the systemic
circulation?
a. The mean arterial pressure
b. The arterial resistance
c. The vascular compliance
d. The blood flow
e. The sympathetic tone
BLOOD DISTRIBUTION IN DIFFERENT PARTS OF
CIRCULATORY SYSTEM
Blood Volume
• The largest blood volume in the
cardiovascular system is in the systemic
veins.
• The second largest blood volume is in the
pulmonary system.
• Both represent major blood reservoirs.
• The systemic veins and the pulmonary
vessels have very high compliance compared
to the systemic arteries; this is primarily
responsible for the distribution of blood
volume.
CHARACTERISTICS OF SYSTEMIC VEINS
• Systemic veins are about 20 times more
compliant than systemic arteries.
• Veins also contain about 70% of the systemic
blood volume and thus represent the major
blood reservoir.
• In the venous system, then, a small change in
pressure causes a large change in venous
volume
Example-Hemorrhage
• Cause venous pressure to decreases.
• Because veins are very compliant vessels, this
loss of distending pressure causes a significant
passive constriction of the veins and a
decrease in blood stored in those veins.
• The blood removed from the veins will now
contribute to the circulating blood volume
(cardiac output), a compensation for the
consequences of hemorrhage.
Volume loading (infusion of fluid)
• Increases venous pressure. The increased
pressure distends the veins; this is a passive
dilation.
• The volume of fluid stored in the veins
increases, which means that some of the
infused volume will not contribute to cardiac
output.
• The large volume and compliant nature of the
veins act to buffer changes in venous return
and cardiac output.
• Because of the high compliance of veins, large
increases of pressure occur mainly with
substantial increases of volume, as in congestive
heart failure, or with massive sympathetic activity
that reduces compliance.
• Similarly, substantial decreases of central venous
pressure occur with large loss of volume.
• An exception is the effect of posture, which can
lower central venous pressure, even though
blood volume has not changed.
• This is because gravity causes blood to pool in the
dependent veins.
The actual venous return to the heart is determined
by the venous pressure gradient.
Total Cross-Sectional Area
Velocity of the Bloodstream
• Velocity, as relates to fluid movement, is the
distance that a particle of fluid travels with
respect to time, and it is expressed in units of
distance per unit time (e.g., cm/sec).
• Flow, is the rate of displacement of a volume
of fluid, and it is expressed in units of volume
per unit time (e.g., cm3/sec).
• In a rigid tube, velocity (v) and flow (Q) are
related to one another by the cross-sectional
area (A) of the tube
Total cross
sectional area
• The extent to which velocity is increased by
stenosis is defined by the equation of
continuity:
Factors influencing velocity
• Cross sectional area of segment
• Phase – Systolic phase ↑ velocity
Diastolic phase ↓ velocity
Viscosity - ↑viscosity
↓ velocity
↓ viscosity
↑ velocity
Applied physiology
Velocity decreases in heart failure.
Velocity of
circulation
• In most arterial locations, the dynamic
component will be a negligible fraction of the
total pressure.
• However, at sites of an arterial constriction or
obstruction, the high flow velocity is
associated with a large kinetic energy, and
therefore the dynamic pressure component
may increase significantly.
• Hence, the pressure would be reduced and
perfusion of distal segments will be
correspondingly decreased.
Q. The greatest pressure decrease in the
circulation occurs across the arterioles because
(A) they have the greatest surface area
(B) they have the greatest cross-sectional area
(C) the velocity of blood flow through them is
the highest
(D) the velocity of blood flow through them is
the lowest
(E) they have the greatest resistance
Q. A 25 year old graduate student while going for
her lectures on her power bike skids off the road
and sustains a fracture to her right leg. The
fractured leg is bleeding profusely. At the ER, her
blood pressure is determined to be low.
Homeostatic mechanisms in stabilizing the
blood pressure will include increases in total
peripheral resistance. The site of highest
resistance in the vasculature is in the;
A. Arterioles
B. Venules
C. Capillaries
D. Large arteries
E. Veins
Q. A healthy 32-year-old woman participates in a clinical
study. Her blood volume is 5,200mL. Images are
obtained to determine the volume of blood in various
vessels in various body positions at rest and during
exercise. While lying supine, which of the following
vascular structures will most likely contain the largest
portion of the total blood volume in this woman?
A. The left ventricle
B. The right ventricle
C. The pulmonary vasculature
D. Veins and venules
E. Vena cavae
F. Capillaries
G. Arterioles
A MD 2 student is performing experiments on
blood flow in various vessels. She came to the
conclusion that the velocity of blood flow is
slowest in the capillaries. The most likely reason
for this is:
A.Capillaries have the smallest cross-sectional area
B. Capillaries have the largest cross-sectional area
C. Decreased in blood viscosity in the capillaries
D.Single stream of blood flow
E. Decreased in turbulence
• In order to maintain constant flow through a tube
with varying diameters, which of the following
would be true( where A1 and A2 represent cross
sectional areas, and V1 and V2 represents the
corresponding flow velocities)?
a.
b.
c.
d.
e.
V1=V2
V1=A1× V2
A2= A1 ×V1/V2
V1=A1 ×A2/V2
V1 ×A2=V2 ×A1
BLOOD FLOW
• Quantity of blood that passes a given point of
circulation in a given period of time.
Units= ml/min
• Normal blood flow is – streamline or
laminar(Silent)
• Random flow in a vessel - Turbulent flow
• In laminar flow , the velocity of flow is greater in
the center than the outer edges .
DEMONSTRATION OF LAMINAR & TURBULENT BLOOD FLOW
• Laminar flow is flow in
layers.
• Laminar flow occurs
throughout the normal
cardiovascular system,
excluding flow in the
heart.
• The layer with the
highest velocity is in the
center of the tube.
• Turbulent flow is non
layered flow.
• It creates murmurs.
These are heard as bruits
in vessels with severe
stenosis.
• It produces more
resistance than laminar
flow.
CRITICAL VELOCITY
• The maximal velocity at which the flow becomes
turbulent .
• Expressed in REYNOLDS NUMBER .
• R= PDV / 
• P= Density of blood (1), D = diameter of vessel ,
V= Velocity of blood flow (cm/sec) ,  = viscosity
in poises
• When number is 2000 – TURBULENCE occurs .
IN THE CLINIC – turbulent flow
• Usually accompanied by audible vibrations,
detected with a stethoscope .
• When the turbulence occurs in the heart,
the resultant sound is termed a murmur;
when it occurs in a vessel, the sound is
termed a bruit.
• E.g- In severe anemia, (1) the reduced
viscosity of blood and (2) the high flow
velocities associated with the high cardiac
output .
• Blood clots, or thrombi, are more likely to
develop in turbulent than in laminar flow.
Velocity effects
Axial streaming and flow velocity
Viscosity
• is directly proportional to haematocrit,plasma
prot, diameter of vessel(capillaries plasma
skimming).
• And inversely proportional to temp ,flow
rates.
Shear Stress on the Vessel Wall
Flowing blood creates a force on the
endothelium that is parallel to the long axis of
the vessel.
This shear stress (γ) is proportionate to viscosity
(ɳ) times the shear rate (dy/dr), which is the rate
at which the axial velocity increases from the
vessel wall toward the lumen.
IN THE CLINIC - Dissecting aneurysm
• In certain types of arterial disease, particularly
hypertension, the subendothelial layers of vessels
tend to degenerate locally, and small regions of the
endothelium may lose their normal support.
• The viscous drag on the arterial wall may cause a tear
between a normally supported and an unsupported
region of the endothelial lining.
• Blood may then flow from the vessel lumen through
the rift in the lining and dissect between the various
layers of the artery. Such a lesion is called a dissecting
aneurysm. It occurs most often in the proximal
portions of the aorta and is extremely serious.
WALL TENSION
• La Place law: States that tension in the wall of
a cylinder (T) is equal to the product of the
transmural pressure (P) and the radius (r)
divided by the wall thickness (w):
T= P r/w
• Because of their narrow lumens (i.e., small
radius), the thin-walled capillaries can
withstand high internal pressures without
bursting.
• This property can be explained in terms of the
law of Laplace
IN THE CLINIC- Dilated heart
• If the heart becomes greatly distended with
blood during diastole, as may occur with cardiac
failure, it functions less efficiently.
• More energy is required (greater wall tension) for
the distended heart to eject a given volume of
blood per beat than is required for a normal
undilated heart.
• The less efficient pumping of a distended heart is
an example of Laplace's law, which states that the
tension in the wall of a vessel or chamber (in this
case the ventricles) equals transmural pressure
(pressure across the wall, or distending pressure)
times the radius of the vessel or chamber.
Critical Closing Pressure
Estimation of blood flow
various parts of the body
through
• Use of flow meters – (Direct method) In animals
Electromagnetic flow meter
• Plethysmography
• Ficks principle – Also used to measure Cardiac
output ,renal/coronary / cerebral blood flow can be
estimated
• Indicator dilution technique
• PAH clearance
• By doppler study
Plethysmography
Poiseuille’s Law Describes the
Relationship
Between Pressure and Flow
• In electrical theory, Ohm's law states that the
resistance, R, equals the ratio of voltage drop,
E, to current flow, I.
Poiseuille Equation
EFFECT OF VESSEL DIAMETER ON BLOODFLOW
Viscosity
• Viscosity is a property of a fluid that is a
measure of the fluid’s internal resistance to
flow.
• Viscosity is the frictional resistance in between
the laminae of the flowing fluid .
• Frictional resistance is due to red cells and
plasma proteins.
• The greater the viscosity, the greater the
resistance.
• The prime determinant of blood viscosity is
the hematocrit.
Q
A 53-year-old woman is found, by
arteriography, to have 50% narrowing of her left
renal artery. What is the expected change in
blood flow through the stenotic artery?
(A)Decrease to ½
(B) Decrease to ¼
(C) Decrease to 1/8
(D) Decrease to 1/16
(E) No change
Hemodynamics - Summary
• A 56 yr old female is admitted to the hospital for
a hysterectomy. After surgery, she is transferred
to the intensive care unit. Her mean systemic
blood pressure is 100mmHg and her resting
cardiac output is 4 L/min. Which of the following
is total peripheral resistance in this patient?
a.
b.
c.
d.
e.
f.
0.025(ml/min)/mmHg
0.025 mmHg/(mL/min)
40( ml/min)/mmHg
40 (mmHg/(mL/min)
4000 (ml/min)/mmHg
4000 (mmHg/(mL/min)
Series Versus Parallel Circuits
• Series
Parallel
3.6ml/min
45ml/min
90ml/min
135ml/min
160ml/min
0.0625mm hg/l/min
0.05 mm hg/l/min
0.04 mm hg/l/min
0.03 mm hg/l/min
• The circuit below has an inflow pressure of 120
mmHg and an outflow pressure of 40 mmHg.
Resistance is each of the vessel shown is
2mmHg/ml/min(
R1=R2=R3=R4=2mmHg/ml/min).What is the total
peripheral resistance of the circuit shown in the
picture below?
a.
b.
c.
d.
e.
8 mmHg/ml/min
4 mmHg/ml/min
2 mmHg/ml/min
1 mmHg/ml/min
0.5 mmHg/ml/min