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LIU Chuan Yong
刘传勇
Institute of Physiology
Medical School of SDU
Tel 88381175 (lab)
88382098 (office)
Email: [email protected]
Website: www.physiology.sdu.edu.cn
Section 3
Physiology of the
Blood Vessels
I. Physiological Classification
of Blood Vessels
Windkessel Vessel --- Aorta and
big arteries.
 Contain a large amount of elastic tissue besides the
smooth muscle.
 Transiently store blood during systole, and then shrink
to produce onward blood flow during diastole.
 Convert the sharp pressure fluctuations in the left ventricle
(0 to 120 mmHg) into much smaller pressure fluctuations
in the arteries (80 to 120 mmHg).
 Convert the intermittent ventricular ejection into
continuous blood blood in the vessels
 This function of large arteries is known as Windkessel
effect.
2. Distribution Vessel – Middle arteries

Rich in smooth, systole or diastole under
some physical and chemical factors.
 Together with resistance vessels, they match
the blood flow to different organs with their
requirements.
Distribution of Cardiac Output
3. Precapillary Resistance Vessels –
Small arteries and arterioles
 Less elastic than the larger
arteries
 Hhave a thicker layer of
smooth muscle.
 Provide the greatest
resistance to blood flow
through the arterial system
 since they have narrow
lumina.
4. Precapillary Sphincter muscle Partially determines
the amount of blood
flowing through a
particular capillary
bed
 Allow only 5% 10% of the capillary
in bed skeletal
muscles, for
example, to be open
at rest.
5. Exchange Vessel – Capillary
the walls are composed
of only one cell layer
– a simple squamous
epithelium, or
endothelium.
permits a more rapid
transport of materials
between the blood and
the tissues.
Make Up of Blood Vessels: Capillaries
6. Capacitance Vessel – Systemic
veins
Have a large diameter but a thin wall,
which includes a thin muscle coat.
The number is about twice as much as
the number of arteries,
The large number and cross sectional
area gives them an enormous capacity to
hold blood.
Capacitance Vessel – Systemic
veins
 Most of the time, veins hold more than half
the blood volume .
 are known as capacitance vessels.
 the great distensibility of veins makes their
capacity adjustable.
 In times of need, a considerable amount of
blood can be squeezed from the veins to
areas where it may be needed.
II Basic Concept of
Hemodynamics:
Blood Flow,
Resistance of Blood Flow
and Blood Pressure
1. Blood Flow (Q)
 Concept: The quantity of blood that passes a
given point in the circulation in a given
period of time.
 The overall blood flow in the systemic
circulation is identical to the cardiac output
(2) Factors determining blood flow
(interrelationships among blood flow,
pressure and resistance.)
ΔP: the pressure difference between
the two ends of the vessels;
R: frictional force produced when
blood fIows through blood vessels.
Q = ΔP / R
(3) Laminar flow and turbulent flow
Laminar flow –
blood flows in
streamlines with
each layer of
blood remaining
the same distance
from the wall
Laminar flow
(3) Laminar flow and turbulent flow
Turbulent flow – blood
flow in all directions in
the vessel and
continually mixes within
the vessel.
because of
 the velocity of blood flow
is too great,
 is passing by an
obstruction,

making a sharp turn,
 passing over a rough
surface)
C, constriction;
A, anterograde;
R, retrograde
2. Resistance of Blood Flow
 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 a vessel 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 ).
 Normally, L and η have no change or almost no change.
 Therefore, the diameter of a blood vessel plays by far the
greatest role of all factors in determining the resistance ( R ) of
blood flow.
3. Blood pressure
 Blood pressure means the force exerted
by the blood against the vessel wall
 ( or the force exerted by the blood
against any unit area of the vessel wall)
 Blood Pressure is stored energy
(potential energy)
Formation of the blood pressure:
 (1) Mean circulatory filing 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.
 (2) Total peripheral resistance.
Formation of the blood pressure:
 (3) Cardiac pumping
 Energy released from heart contraction is
transferred into 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
Blood Pressure:
Generated by Ventricular Contraction
Formation of the blood pressure:
 （4）Elasticity of Windkessel vessel
 ① diastolic blood pressure
 ② continuous blood flow in diastole
 ③ buffering blood pressure
4. Physical Characteristics of the
Systemic Circulation
 (1)The velocity of
blood flow in each
segment of the
circulation is
inversely
proportional to its
cross-sectional
area.
4. Physical Characteristics of the
Systemic Circulation
 (2) Pressure and resistance in the various portion
of the systemic circulations.
 The decrease in pressure in each part of the systemic
circulation is directly proportional to the vascular
resistance.
III. Arterial Pressure
1. Concept of Arterial
Pressure
 Blood pressure in the aorta and
other big arterials.
2. Normal Range of Arterial Pressure
 Systolic pressure (Ps) – the maximum of the
pressure during systole
 Diastole pressure (Pd) – the minimum pressure
during diastole
 Pulse pressure – the difference between Ps and
Pd
 Mean arterial pressure – the average pressure
throughout each cardiac cycle.
 Mean arterial pressure (Pm) = Pd + Pulse
pressure / 3
Mean arterial pressure
Normal range of arterial pressure
 At rest, the arterial pressure of Chinese adult young people
should be
 Ps 100 – 120 mmHg
 Pd 60 – 80 mmHg
 Pulse pressure 30 – 40 mmHg
Measurement of the arterial pressure
 Direct (inserting a cannula into the artery)
Measurement of the arterial pressure
Indirect
(auscultatory)
method
Stethoscope
Blood Pressure (BP):
3. Factors Determining Arterial Pressure
 Stroke volume ---- Ps
 Heart rate ---- Pd
 Total peripheral
resistance (Ps)
 Action of Windkessel
vessel (aorta and other
large arteries) – Pulse
pressure
 Mean circulatory
filling pressure
IV. Venous Pressure and Venous
Return
Venous Pressure
Central venous pressure
Peripheral venous pressure
Central venous pressure
 The pressure in the right atrium.
 Normally about 0 mmHg.
 Regulated by a balance between
 the ability of the right ventricle to pump blood
out
 the tendency of blood to flow from the
peripheral back into the right atrium.
 Clinical importance:
 the hemorrhage
 right heart failure
Peripheral venous pressure
 Venous pressure in
the organs
 Properties:
 Low pressure
 Affected by the
hydrostatic
pressure
 Usually veins are
collapsed. (Why?)
Transmural pressure
= Blood pressure - The pressure adjacent
tissues exerted on the blood vessel.
If the transmural pressure is negative
(smaller than 0), the vein is collapsed
Venous Return
 Concept: The quality of blood flowing from
veins into the right atrium per minute
 Factors affecting venous return





Mean system filling pressure
Cardiac contractility
Position of the body
Action of “muscular pump”
Respiration movement
Factors affecting venous return
 1) Mean systemic filling pressure
 2) Cardiac contractility
Cardiac contractility – stroke volume –
ventricular pressure in diastole period – blood
from atria and large veins to ventricle – venous
return
 3) Position of the body
From lying to standing – increase of the blood
in veins – dilation of veins in the lower part of
the body – decrease of venous return
Factors affecting venous return
4) Action of
“muscular pump”
(or venous pump)
Factors affecting venous return
 (5) respiration movement.
Negative pressure in the thoracic cavity that changes with
respiratory movement – dilation of venae cave – increase of
venous return
V Microcirculation
1.Functional anatomy of the
microcirculation
2. “pores” in the capillary membrane
A
A, Continuous Capillaries
B
B, Fenestrated Capillary
2. “pores” in the capillary
membrane
 Structurally, capillaries have no
smooth muscle in their walls.
 They are lined by only a single layer
of endothelial cells.
 There are gaps between endothelial
cells to allow for exchange of
nutrients and metabolites.
3. Capillary pressure.
Arterial end 30 – 40 mmHg;
Venous end, 10 – 15 mmHg;
Middle part 25 mmHg
4. Exchange of nutrients and other substances
between the blood and interstitial fluid
(1) Diffusion through the capillary membrane.
Lipid-solute substance can diffuse directly through the
cell membranes of the capillary
Water-soluble, liquid-insoluble substance, such as Na ,
Cl, glucose and so forth, diffuse only through the
capillary pores.
4. Exchange of nutrients and other substances
between the blood and interstitial fluid
 (2) Transport through the capillary membrane by
pinocytosis.
Proteins and many much large substance in the plasma
(such as lipoprotein) are transported through the capillary
membrane by means of pinocytosis.
 (3) Filtration.
When the hydrostatic pressure is different on the two sides
of membrane, the greater pressure on one side causes
slightly increased diffusion of water and dissolved
substances toward the opposite side.
VI The Interstitial Fluid
 Water within the body accounts
for 60% of the total body weight
(body fluid)
 2/3 intracellular compartment
 1/3 extracellular compartment
 80%, interstitial fluid;
 20%, blood plasma
The distribution of
extracellular fluid
between the plasma
and interstitial
compartments is in a
state of dynamic
equilibrium.
Tissue fluid is not
normally a “stagnant
pond” but is rather a
continuously
circulating medium,
formed from and
returning to the
vascular system.
In this way, the
tissue cells receive
a continuously
fresh supply of
glucose and other
plasma solutes
that are filtered
through tiny
endothelial
channels in the
capillary walls
The daily intake and excretion of body water and its distribution between
different intracellular and extracellular compartment
1. Formation of the interstitial fluid
• Effective Filtration Pressure = (Capillary Pressure
+ Interstitial Colloid Osmotic Pressure) – (Plasma
Colloid Osmotic Pressure + Interstitial Hydrostatic
Pressure)
（crystal pressure？）
2. Factors Determining Formation of the
Interstitial Fluid (Mechanism of Edema)
 Edema is an abnormally large
collection of fluid in the interstitial
space.
 From the physiology of capillaries
and lymphatics,
 edema may be due to one or more of the
following causes:
Mechanism of Edema
 (1) Capillary pressure
 Right heart failure –systemic edema
 Left heart failure – pulmonary edema
 Late pregnancy – edema in legs and foot
(pressure of uterus on inferior vena cava)
Mechanism of Edema
 (2) Plasma colloid osmotic pressure
 Protein malnutrition, liver disease (inadequate
albumin synthesis ) or renal disease (protein
loss in urine) – hypoproteinemia – low plasma
colloid osmotic pressure
 (3) Permeability of capillary wall
 Inflammation or allergy – leakage of
abnormally large quantities of proteins from
capillaries
Mechanism of Edema
• (4) Lymphatic drainage
• Lymphatics are the second circulatory system.
• Structurally, lymphatics are a network of blindended thin endothelial tubes.
• Although the endothelial lining is not
fenestrated,
• the intercellular junctions are permeable to
large molecules.
Mechanism of Edema
 (4) Lymphatic drainage (continued)
 Lymphatics collect proteins, lipids and other
large molecules which leak out of capillaries
into the interstitial space,
 to prevent the osmotic pressure of interstitial
space from rising,
 and thereby prevent abnormal accumulation
of fluid in the interstitial space.
 Reduced lymphatic drainage, e.g. in filariasis,
or involvement of lymph nodes in malignancy
 – local or systemic edema.
3. The function of lymph
 Removing protein from interstitial fluid.
 Regulating balance between plasma and
interstitial fluid (reabsorption l0% of filtration
fluid).
 Absorption nutrients (80%~90% of fat) from
gastrointestinal tract.
 Removing the particles such as RBC, bacteria,
lymphatic cell, tissue cell in the interstitium.
 Defense function (to ingest bacteria and to
produce antibodies)