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Section Three
Introduction
Arterial Pressure
Venous Circulation
Microcirculation
Blood Vessels
ANATOMY CONSIDERATIONS
ANATOMY CONSIDERATIONS
VESSEL LUMEN DIAMETER WALL THICKNESS
Aorta
2.5cm
2mm
Artery
0.4cm
1mm
Arteriole
30um
20um
Capillary
5um
1um
Venule
20um
2um
Vein
0.5cm
0.5mm
Vene Cava
3cm
1.5mm
I. Physiological Classification of
Blood Vessels
1. Windkessel Vessel --- Aorta and big arteries.
In addition to smooth muscle, aorta and big arteries
contain a large amount of elastic tissue.
They are stretched during
systole and recoil on the blood
during diastole, which dampen
blood pressure pulsation.
Convert the intermittent
ventricular ejection into
continuous blood flow in the
vessels.
This function of large arteries
is known as Windkessel effect.
2. Distribution Vessel – Middle arteries
These arteries are rich in smooth, which
will systole or diastole under some physical
and chemical factors.
Together with resistance vessels, they
match the blood flow to different organs
with their requirements.
3. Precapillary Resistance Vessels –
Small arteries and arterioles
⑴ narrow lumina
the greatest resistance
to blood flow
⑵ a thicker layer of
smooth muscle.
control vascular
caliber and local
blood flow
4. Precapillary Sphincter muscle （control switch）
The amount of blood
flowing through a
particular capillary bed
is determined in part by
the action of the
precapillary sphincter
muscle.
These muscles allow
only 20% - 35% of the
capillary bed in skeletal
muscles, for example, to
be open at rest.
5. Exchange Vessel – True Capillary
The walls of true
capillaries are made up of
a single layer of
endothelial cell and a thin
basement membrane.
The absence of smooth
muscle and connective tissue
layers permit a more rapid
transport of materials
between the blood and the
interstitial fluid.
6. Postcapillary Resistance Vessels–Veinule
⑴ narrow lumina
the great resistance
to blood flow
⑵ Action of veinule
change capillary
blood pressure, and
affect formation of
interstitial fluid.
7. Capacitance Vessel : Veins
The walls of veins are thin and easily distended. Their
lumina are larger.
The number veins is about
twice as much as the number of
arteries.
The large number and cross
sectional area give them an
enormous capacity to hold
blood.
Veins hold 60%-70% of the
circulating blood volume . That
is why veins are known as
capacitance vessels.
8. Arteriovenous
Shunt vessels
Direct connect an
arteriole and a venule.
Function:
Regulates the body
temperature
II Basic Concept of Hemodynamics:
Blood Flow, Resistance of Blood Flow
and Blood Pressure
1. Blood Flow (Q)
(1) Concept: The quantity of blood that passes
through a cross section of blood vessel in
unit time is called blood flow(also called
volume velocity).
Velocity of blood flow(V) means line speed
of a particle in the blood circulation.
Q ↑, V ↑
Area of a cross section ↑, V ↓
(2) Factors determining blood flow
(interrelationships among blood flow, pressure
and resistance.)
1) ΔP: the pressure difference between the two
ends of the vessels;
2) R: frictional force produced when blood
flows through blood vessels.
Q = ΔP / R
Poiseuille’s law, Q =πΔP r4/8ηl
(3) Laminar flow and turbulent flow
Laminar flow – flow direction of each particle in
blood is consistent with vascular long axis ;
the velocity of flow
of a particle in the
center of the vessel is
the greatest.

The more near the
vessel wall, the
slower the velocity

Turbulent flow – Flow direction of each particle
in blood is not consistent,
- Each particle in blood flows in all directions in
the vessel to form whirlpool.
Reynolds equation
V Dρ
Re= ———
η
V-mean velocity of
blood flow.
D-diameter of a blood vessel.
ρ-blood density.
η-blood viscosity
Re>2000, easy to
generate turbulence.
2. Resistance of Blood Flow(R)
-Blood flowing within the vessel encountered
resistance.
-Friction between blood and blood vessel, and
friction between blood components.
- R of turbulent flow is greater than that in
laminar flow.
Resistance of Blood Flow(R)
From Q = ΔP / R (1)
we get R = ΔP / Q (2)
According to Poiseuille’s law, Q =πΔP r4/8ηl (3)
From (3) and (2), we get R = 8 ηl/ π r4
π is constant.
Note that the resistance (R) of blood flow is
directly proportional to the blood viscosity (η) and
length (l) of the vessel, but inversely proportional to
the fourth power of the radius ( r ) of the vessel.
Normally, L and η have no change or almost
no change.
Therefore, the radius of a blood vessel plays
the greatest role in determining the resistance
( R ) of blood flow.
★ Distribution of resistance of blood flow
Aorta and large arteries
9%
Small arteries and branches 16%
Arteriole
41%
Capillary
27%
Vein
the radius ( r )
of the vessel
7%
the organ blood flow
distribution
Blood viscosity (η)
1) erythrocrit ↑, η↑.
2) At laminar flow:
shearing rate of blood flow =ΔV / d,
d-thickness of adjacent two layers of blood layers .
ΔV- V difference of adjacent two layers of blood layers.
Newtonian fluid: homogeneous liquid, such as
plasma. Its shearing rate changes, but itsηdo not
change.
Non-Newtonian fluid: Non homogeneous liquid,
such as blood.
Its shearing rate changes ↓, its η↑.
Axial flow- when blood flows in the form of
laminar flow, red cells have a trend of moving to
central axis.
When shearing rate is higher, axial flow
phenomenon is more obvious, its η is lower.
3) Calibre of blood vessel:
Fahraeus-Lindqvist effect. If calibre of blood
vessel is smaller than 0.2~0.3 mm, shearing
rate of blood is enough high, within a certain
range, decrease inηaccompanys with calibre of
blood vessel becoming smaller.
4)Temperature ↓,η↑.
3. Blood Pressure
The lateral pressure that
the blood effects on unit
area of the vessels wall is
called the blood pressure.
Unit ： kPa/mmHg
1mmHg=1.36cmH2O
1mmHg=0.133KPa =133Pa
1cmH2O=0.098KPa
III. Arterial Blood Pressure
Arterial blood pressure
means the force exerted
by the arterial blood
against unit area of the
arterial vessel wall.
Formation Of Arterial Pressure
Conditions：
+ There is a enough blood in the cardiovascular
system.
+ cardiac pumping and peripheral resistance
+ windkessel of aorta and big arteries
+ There is a enough blood in the cardiovascular
system.
Mean circulatory filling pressure (MCFP):
when heart beat is stopped, the pressure in any
point of cardiovascular system is equal. This
pressure is called MCFP.
systemic circulation, 7 mmHg;
pulmonary circulation, 10 mmHg.
+ cardiac pumping and peripheral resistance
Energy released from heart contraction is
transferred into two parts,
1) kinetic energy (1% of the total),
2) potential energy (pressure) (99% of the total).
That means most part of energy used to create the blood
pressure.
There is a resistance of blood flow in the blood
vessels, especially in small arteries and arterioles.
+ windkessel of aorta and big arteries
①buffering arterial blood pressure fluctuation.
②convert the intermittent pumping blood of heart
to continuous blood flow within arteries.
2/3
1/3
Measurement
of the arterial
pressure
Direct :
(inserting a
cannula into
the artery)
Indirect
(auscultatory)
method:
Stethoscope
Blood Pressure (BP):
Normal value of arterial blood pressure
Systolic pressure The pressure in the aorta and other
arteries rises to a peak value during each heart cycle.
Diastolic Pressure The pressure in the aorta and other
arteries falls to a minimum value during each heart cycle.
arterial pressure
Pulse Pressure = Systolic pressure - Diastolic Pressure
MAP
= Diastolic pressure ＋
Pulse Pressure
3
The mean arterial pressure (MAP) is the average
pressure throughout the cardiac cycle.
systolic pressure ：
diastolic pressure ：
pulse pressure :
mean arterial pressure :
100-120mmHg
60-80mmHg
30-40mmHg
100mmHg
hypertension
diastolic pressure >90mmHg
arterial pressure >140/90mmHg
hypotension
arterial pressure<90/50mmHg
, pressure drop
resistance of blood flow
*
Factors Controlling Arterial Pressure
Stroke Volume
Heart Rate
Resistance
Windkessel effect of aorta and big arteries
Relationship Between Blood Volume and
Vascular Volume
(1) Stroke volume :
stroke volume↑
the systolic pressure ↑
pulse pressure↑
• An increase in stroke volume results in
a rise in blood pressure, and vice versa.
However, change in stroke volume mainly
affects the systolic pressure provided the
peripheral resistance remains unchanged.→
pulse pressure↑
(2) Heart rate :
heart rate↑
the diastolic pressure ↑
pulse pressure ↓
• When heart rate is increased , arterial
pressure is also elevated . But change in
heart rate mainly influences the diastolic
pressure, if peripheral resistance does not
change .→ pulse pressure↓
(3) Peripheral resistance :
peripheral resistance↑
the diastolic pressure ↑ , pulse pressure ↓
• An increase in the peripheral resistance causes
mainly an elevation of the diastolic pressure .
Systolic pressure also rise but the alteration is less
prominent .→ pulse pressure↓
(4) Windkessel effect of aorta and big
arteries(Elasticity of the vessel wall) :
• Windkessel effect of aorta and big arteries
helps to reduce the pressure variation
produced by the ventricular contraction and
relaxation.
windkessel effect ↓
the systolic pressure ↑
the diastolic pressure ↓
pulse pressure↑
(5) Relationship Between Blood Volume
and Vascular Volume
• The greater the extent of overfilling , the
greater is the arterial blood pressure . A
precondition of formation of arterial pressure
is that there is adequate blood volume in the
arterial system.
Blood volume↑
the blood pressure ↑
vascular volume↑ the blood pressure ↓
Systolic P
Factors
Diastolic P
Pulse P



Heart rate



Resistance



Windkessel effect





Stroke volume
Blood volume

vascular volume↑
Venous Circulation
1. Venous Pressure and Flow
(1)Peripheral Venous Pressure : The pressures in the
veins of each organ outside the thorax.
(2)Central Venous Pressure :The pressures in the right
atrium and great veins inside the thorax.
Normally about 0 mmHg. 4~12 cmH2O
It is regulated by a balance between the heart ejection
ability and the venous return.
Central Venous Pressure
Peripheral Venous Pressure
Blood
volume↑
Arterioles veins
diastole
systole
Ejection ability
Venous
return
Central
Venous
Pressure
Significances of measuring the CVP
① judge the cardiac functions and blood volume.
② determine the transfusion volume.
Effect of Gravity on Venous Pressure
Position:
lying position：
静水压
hydrostatic pressure
Pressure in everywhere is similar.
upright position：
The venous pressure in lower limbs is higher.
颅顶矢状窦calotte sagittal sinus -10mmHg
Transmural pressure is the difference in
pressure between two sides of a vein wall.
Filling degree of vessels is affected by transmural pressure.
Transmural pressure reduces
Vein collapses
Vein volume reduces
Transmural pressure increases
Vein fills
Vein volume increases
Factors Influencing Venous Return
⑴Mean Circulatory Filling Pressure ↑---- V return ↑
⑵Myocardial Contractibility ↑ ----- V return ↑
Cardiac contractility ↑– stroke volume ↑ –
ventricular pressure in diastole period ↓– blood
from atria and large veins to ventricle ↑– venous
return ↑
⑵Myocardial Contractibility ↑ ----- V return ↑
Cardiac contractility ↓– stroke volume ↓ –
ventricular pressure in diastole period↑– blood
from atria and large veins to ventricle ↓– venous
return ↓
Right heart failure–The external jugular vein
engorgement， Liver congestion enlargement，
Edema of lower extremity
Left heart failure–Pulmonary congestion，
Pulmonary edema
⑶ Muscle Pump
{
Muscle rhythmic contraction
and relaxation
venous valve
Promote blood flow
significance： At upright position, muscle pump
decreases venous pressure of lower limb, and
reduces venous blood retention of lower limb.
⑷ Respiratory movement
inspiration
↓
thoracic cage↑
↓
intrapleural negative pressure↑
↓
↓
Pulmonary vessels Atrium,big veins inside
expand
the thorax expand
↓
Pulmonary vein
return ↓
↓
Cardiac output of
LV↓
↓
Bp↓
↓
Central venous pressure ↓
↓
Venous return↑
↓
Cardiac output of RV↑
⑸ Stand from lying position ----- V return ↓
From lying to standing
increase of transmural pressure
dilation of veins in the lower
part of the body ↑
decrease of venous return
400 — 600ml
megatemperature environment
Microcirculation
Microcirculation is the circulation between
arterioles and veinules. In the microcirculation,
the most purposeful function of the circulation
occurs: transport of nutrients to the tissues and
removal of cellular excreta.
Composition of microcirculation
main switch
arteriole
metarteriole
precapillary sphincter
true capillary
}
branch switch
trophism vessel
thoroughfare capillaries thoroughfare channel
arteriovenous anastomosis regulate body temperature
veinule
post switch
Ⅰ.Structure of the microcirculation
PASSAGES:
⑴ Thoroughfare channel: often open
Arterioles → metarteriole→ thoroughfare
capillaries →veinules
Function:
the blood pass the microcirculation rapidly
→ return the heart rapidly.
Distributing: skeletal muscle tissues
⑵ A-V Shunt
Arterioles → Arteriovenous vessel
(Anastomoses) →veinules
Function: Regulating
the body temperature.
Distributing: skin and
hypodermic tissue
e.g. finger, toe
(3) Exchange Channel
Arterioles → metarteriole → precapillary
sphincter → true capillary → veinules
Function:
There is the
exchange of
materials across
the vessel wall
between the
blood and tissues.
Ⅱ.Structure of the Capillary wall
Total thickness:0.5µm, areas:1000m2
+ a single layer of
endothelial cell.
+ a thin basement
membrane.
+ no smooth muscle
+ gaps
Regulation of blood flow of microcirculation：
Vasomotion is controlled by local tissue metabolism
Local metabolites
组织胺 ，PO2
True cap.close
Q
,
V
Metarteriole and
precapillary
sphincter relax
True cap.open
Q
，
V
Metarteriole and
precapillary sphincter
contract
Local metabolites
组织胺 ，PO2
Ⅲ . Exchange of nutrients and other
substances between the blood and
interstitial fluid
1. diffusion :
⑴Lipid-soluble
substances can
diffuse directly
through the cell
membranes of
the capillary
endothelium.
(2) Water-soluble, non-lipid-soluble substances
diffuse only through intercellular “pores” in
the capillary membrane.
(3) Effecting
factors :
• the
concentration
difference
between the two
sides of the
membrane.
2. Pinocytosis
• endocytosis and exocytosis
3. Filtration and reabsorption.
(1).Filtration :substances move from blood
toward the interstitial fluid.
(2).Reabsorption : substances move from
interstitial fluid to blood .
Ⅳ. Interstitial fluid
Water within the body accounts for 60%
of the total body weight (body fluid).
2/3 intracellular compartment
1/3 extracellular compartment
(75%, interstitial fluid; 25%, blood plasma)
• The interstitial fluid is derived by
filtration and diffusion from the capillaries,
it contains almost the same constituents as
plasma except for proteins because proteins
do not pass outward through the pores of
the capillaries with ease.
Interstitial Fluid
Formation of the interstitial fluid
Place：true capillary
(Ⅰ) Four primary forces that determine fluid
movement through the capillary membrane.
• 1.The capillary pressure (Pc), which tend
to force fluid outward through the capillary
membrane.
2.The interstitial fluid pressure (Pi):
Interstitial fluid pressure is very low.
And it is not same in different tissues. In
loose tissues, interstitial fluid pressure is
lower than atmospheric pressure. It is
about -2mmHg. But in capsula organ , e.g.
kidney, muscle, brain, interstitial fluid
pressure is positive. For example,
interstitial fluid pressure of kidney is about
6mmHg.
• When Pi is positive, it tends to force fluid
inward through the capillary membrane.
But when Pi is negative. it tends to force
fluid outward.
3.The plasma colloid osmotic pressure ( πc):
The plasma colloid osmotic pressure is
formed by plasma proteins. Its normal value
is about 25mmHg. Ions diffuse rapidly across
the capillary wall but proteins do not, so colloid
osmotic pressure of plasma plays an important
role in determining the balance of distribution
of fluid between extravascular and intravascular
spaces and maintaining normal blood volume.
• Fluid always flows from a region of low
osmotic pressure to a region of high osmotic
pressure. So the plasma colloid osmotic
pressure tends to cause osmosis of fluid inward
through the capillary membrane.
4. The interstitial fluid colloid osmotic
pressure (πi ) , which tends to cause
osmosis of fluid outward through the
capillary membrane. It is about 10
mmHg.
(Ⅱ) Exchange of fluid volume
through the capillary membrane
Effective Filtration Pressure(EFP, ΔPf)
ΔPf = (Pc +πi)-(Pi + πc)
A end
ΔPf=（32+10）-（10+22）= +10mmHg
V end
ΔPf=（15+10）-（10+22）= - 7mmHg
At the
arteriolar end
of the capillary,
the effective
filtration
pressure is
positive, and
the fluid moves
into the
interstitial
space.
At the venous
end of the
capillary, the
effective
filtration
pressure is
negative, and
the fluid moves
into the
capillaries.
Filtration Volume=Kf× ΔPf
Kf : capillary filtration coefficient.
Permeability of capillary and filtration area
Arteriole
Venule
0.5-2%
Interstitial fluid
10%
Lymph
90%
• Most of the fluid filtering from the blood
capillaries flows among the cells and
finally is reabsorbed back into the venous
ends of the blood capillaries;
• but on the average, about 1/10 of the fluid
instead enters the lymphatic capillaries
and returns to the blood system through
the lymphatic system.
• The total quantity of this lymph is
normally only 2 to 4 liters each day.
Factors influencing the Formation of the Interstitial Fluid
Capillary pressure
edema
Arteriole relax
interstitial fluid ↑
ΔPf ↑
venule contract
right heart failure
ΔPf = (Pc +πi)-(Pi + πc)
Capillary Colloid
Osmotic pressure
albuminuria or liver function
ΔPf = (Pc +πi)-(Pi + πc)
Capillary Colloid
Concentration of
Osmotic pressure ↓
plasma protein↓
ΔPf ↑
interstitial fluid ↑
edema
Capillary
Permeability
empyrosis、hypersensitiveness
ΔPf = (Pc +πi)-(Pi + πc)
Capillary
Permeability ↑
πi ↑, ΔPf ↑ , interstitial fluid ↑
edema
Lymphatic
Circulation
Filariasis
block lymphatic
interstitial fluid
return
volume↑
edema
Ⅴ. Lymphatic system
The lymphatic system represents an
accessory route by which interstitial fluid
can flow from the interstitial spaces into
the blood vessels.
Its function is very important.
The lymphatic ducts begin as dead end-endothelial bulbs in the interstitium known as
lymphatic capillaries or terminal lymphatic.
Lymphatic capillaries are much more
permeable than blood capillaries, allowing
proteins and even whole cells to enter.
• Tissue fluid can flow into the lymphatics. Then the
tissue fluid in the lymphatics is called lymph.
• Lymph passes into capillary lymphatic ducts and
collecting lymphatic ducts.
• Eventually the lymph empties into the central veins by
the right lymphatic duct and the thoracic duct.
• The total quantity of the lymph is normally 2 to 4
liters each day at rest.
• Lymph flow is unidirectional from lymphatic ducts to
the blood vessels because of valves in lymphatic ducts.
Function of Lymphatic Circulation
Returns filtrate to
blood.
Regulates the fluid
equilibrium between
the blood and the
interstitial fluid.
Carries large proteins
and particulates from
interstitium to blood
vessels. Absorption of fat.
Protects the body.
Factors Affecting Lymph Flow

Capillary pressure (Pc)

Plasma colloid pressure (c)

Interstitial fluid protein

Capillary permeability