MICROCIRCULATION –
(CAPILLARY CIRCULATION)
Dr.Mohammed Sharique Ahmed Quadri
Asst Prof – Physiology
Al Maarefa College
Objectives
• Define capillary circulation.
• Recognize the importance of Mechanisms for
control of blood flow to the various organs
• Understand factors affecting capillary
circulation.
• Identify Starling forces acting on capillary.
• Appreciate the role of lymphatic.
• Apply the physiological knowledge of
microcirculation in understanding the causes
of edema.
Arterioles
• Major resistance vessels
• Radius of arterioles supplying
individual organs can be adjusted
independently to
–Distribute cardiac output among
systemic organs, depending on body’s
momentary needs
–Help regulate arterial blood pressure
Arterioles
• Mechanisms involved in adjusting arteriolar
resistance
– Vasoconstriction
• Refers to narrowing of a vessel
– Vasodilation
• Refers to enlargement in circumference and
radius of vessel
• Results from relaxation of smooth muscle layer
• Leads to decreased resistance and increased
flow through that vessel
Arteriolar Vasoconstriction and Vasodilation
Mechanisms for control of arteriolar
radius /blood flow to the various organs
• Blood flow to organ depends on the demands
of each organ.
a) Local(intrinsic) control of blood flow is the
primary mechanism that matches the blood
flow to the metabolic needs of the organ (imp
in determining distribution of cardiac out put).
b) Neural or Hormonal(extrinsic) control is
important in blood pressure regulation.
Local(intrinsic) control of
Arteriolar radius
• Only blood supply to brain remains constant
• Changes within other organs alter radius of
vessels and adjust blood flow to organ
• Local chemical influences on arteriolar radius
– Local metabolic changes
– Histamine release
• Local physical influences on arteriolar radius
– Local application of heat or cold
– Chemical response to shear stress
– Myogenic response to stretch
Local chemical influences on
arteriolar radius
• Following specific local metabolic factors are
produce in the tissue as results of metabolic
activity ,produces relaxation of arteriolar smooth
muscle
–
–
–
–
–
–
–
Decreased O2
Increased CO2
Increased acid
Increased K+
Increased osmolarity
Adenosine release
Prostaglandin release
These factors produces relaxation od arteriolar smooth
muscles by acting on vascular endothelium
Role of endothelial cells
• Release chemical mediators(vasoactive
substances) that play key role in locally
regulating arteriolar caliber
– Endothelial cells
• Release locally acting chemical messengers in
response to chemical changes as a result of
metabolic activity in their environment.
• Among best studied local vasoactive mediators
is nitric oxide (NO), a potent vasodialotor
Local physical response to stretch
Myogenic response to stretch
Increase in blood flow

Stretches the vessel

Contraction of vascular smooth muscle

Decrease blood flow back to normal
CONTROL OF REGIONAL
BLOOD FLOW
LOCAL CONTROL OF BLOOD FLOW
• Active Hyperemia - When any tissue becomes
highly active [eg. Skeletal muscle during
exercise]. the rate of blood flow through the
tissue increases.(as a results of vasoactive
metabolites)
• When cells are metabolically more active they
need more blood to bring in O2 and nutrients
and remove metabolic waste.
• Increase blood flow meets these increase local
needs
LOCAL CONTROL OF BLOOD FLOW
• Reactive hyperemia
When blood flow to a tissue is blocked for few
seconds and then is unblocked, the flow through
tissue increases almost 4-7 times normal.
• When blood flow to any region is blocked the
arteriole in the region dilate due to
– Myogenic relaxation in response to decrease stretch
– Changes in local chemical composition ( similar to
metabolically induced hyperemia)
• After the occlusion is removed , blood flow to
previously deprived tissue is transiently higher
than normal due to widely dilated arterioles
The excess blood flow lasts long enough to repay
the tissue oxygen deficit that has occurred during
occlusion.
LOCAL CONTROL OF BLOOD FLOW
• Autoregulation – maintenance of constant
blood flow to an organ in spite of
fluctuations in BP.
E.g. brain – auto regulation is best
kidney – auto regulation is good
skeletal muscle – auto regulation is poor
• Two basic mechanisms that explain
– Myogenic mechanism
– Metabolic mechanism
Two basic mechanisms that explain local
control of blood flow (auto regulation) :
When MAP falls as in hemorrhage , blood flow to
organs reduces
1. Myogenic mechanism
decrease in blood flow

Decrease Stretches on the vessel

Relaxation of vascular smooth muscle

Increase blood flow back to normal
Two basic mechanisms that explain local
control of blood flow (auto regulation) :
2. Metabolic mechanism
Decrease blood flow to organ

Accumulation of vasodilator substances in active
tissues

Blood vessels dilate

Increase blood flow
Vasodilator metabolites
 O2 tension, H,  CO2 tension,  Temperature, K+,
lactate, Adenosine, Histamine.
control of arteriolar radius
/extrinsic control
• Extrinsic control
–Accomplished primarily by
sympathetic nerve influence
–Accomplished to lesser extent by
hormonal influence over arteriolar
smooth muscle
• Epinephrin,norepinephrine, angiotensin II, vasopressin,
etc.
SYMPATHETIC CONTROL OF
ARTERIOLE
• Sympathetic Control of Arterioles is important in
regulating blood pressure.
• Sympathetic ANS supplies arteriolar smooth
muscle all over body except brain.
• Increase sympathetic stimulation causes
arteriolar vasoconstriction.
• Decrease sympathetic stimulation causes
arteriolar vasodilation.
• There is no parasympathetic innervations to
arterioles.
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The Microcirculation
• The term Microcirculation refers to the
functions of the capillaries and the
neighboring lymphatic vessels.
• 5 % of circulating blood volume( 250 ml) is
present in the capillaries at any given time.
• This takes part into the exchange of nutrients,
gases and waste products between the blood
& tissues.
The Microcirculation
● Over 10 billion capillaries with surface area of 500700 square meters
● Small volume of blood is exposed to larger surface
area
• Arteriole  Meta arteriole  Capillaries 
Venules.
• Pre capillary sphincter is present at the junction
where the capillary arises from the Meta arteriole.
This opens and closes the entrance of capillary and
hence regulates the blood flow through the capillary.
• The capillary wall is thin & consists of a single layer
of endothelial cells on basement membrane. Pores
are present between the endothelial cells that allow
transport of substances including water.
Types of capillaries
Classified according to the size of the pores
• Brain – the pores are very tight and allow only very
small molecules to pass thru.
• Kidney & Intestine - the pores are wider –
fenestrations
• Liver - the endothelium is discontinuous with wide
gaps between the cells.
EXCHANGE OF SUBSTANCES ACROSS THE
CAPILLARY WALL
This occurs by.
• Simple diffusion Lipid soluble gases such as
O2 & CO2 readily diffuse thru the endothelial
cells.
• Bulk Flow (various constituents of fluid
moves in bulk/ unit ) - the most important
mechanism for fluid transfer driven by
Starlings forces.
– A volume of protein free plasma filters out and
reabsorbs
The rate of filtration at any point along the capillary
depends on a balance of forces – STARLINGS FORCES.
• 1. Capillary hydrostatic pressure
Arterial end = 37 mmHg
Venous end = 17 mmHg
• 2. Plasma colloid osmotic pressure = 25 mmHg
• 3. Interstitial hydrostatic pressure = 0 -1
• 4. Interstitial colloid osmotic pressure = 0 mmHg
Fluid movement = Kf [ (Pc – Pi) – ( Πc - Πi )
Kf = filtration coefficient
• Arteriolar end  fluid moves out into tissue spaces
• Venous end fluid enters into capillaries.
• Any decrease in plasma proteins (hypoproteinemia)
or increase in capillary hydrostatic pressure (cardiac
failure) causes edema. (abnormal increase in
interstitial fluid volume )
• Histamines, bradykinin increases capillary
permeability  edema.
Starling’s forces across capillary wall
Filtration
4/13/2015
Absorption
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Lymphatic circulation
• Lymphatic system is responsible for bringing
the interstitial fluid to vascular compartment.
• Normal 24 hrs lymph flow is 2-4 L
• Lymphatic capillaries lie in interstitial fluid
close to vascular capillaries ,these capillaries
merge into large lymphatic vessels &
eventually into largest vessel, thoracic duct
which empties into large veins .
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4/13/2015
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Lymphatic circulation
• The interstitial fluid enter lymphatic capillaries
through loose junctions between endothelial
cells .
• Lymph flow back to the thoracic duct is
promoted by contraction of smooth muscle in
wall of lymphatic vessels & contraction of
surrounding skeletal muscle .
• Failure of lymphatic drainage can lead to
Edema .
4/13/2015
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What is Edema?
• Accumulation of fluid
beneath the skin or in a
body cavity
• Palpable swelling produced
by expansion of the
interstitial fluid volume
Causes of Edema
• Increase capillary
pressure
– Increase vascular
volume
• Heart failure
• Kidney disease
• Pregnancy
– Venous obstruction
• Thrombosis
• Liver disease
• Decrease colloidal
osmotic pressure
– Increase loss of
proteins
• Nephrotic syndrome
• Burns
– Decrease production
• Starvation,
Malnutrition
• Liver disease
Causes of Edema
• Increase capillary
permeability
– Inflammation
– Allergic reaction
– Tissue injury
– Malignancy
• Obstruction of
lymphatic flow
– Surgical removal of
lymph nodes
– Malignant
obstruction
– Infection ( filariasis)
References
• Human physiology by Lauralee Sherwood,
seventh edition
• Text book physiology by Guyton &Hall,11th
edition
• Text book of physiology by Linda .s
contanzo,third edition