MICROCIRCULATION

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MICROCIRCULATION (CAPILLARY CIRCULATION)

• Dr.Mohammed Sharique Ahmed Quadri
• Asst Prof Physiology
• Al Maarefa College

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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.

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Arterioles

• Major resistance vessels
• Radius of arterioles supplying individual organs
  can be adjusted independently to
• Distribute cardiac output among systemic organs,
  depending on bodys momentary needs
• Help regulate arterial blood pressure

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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

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Arteriolar Vasoconstriction and Vasodilation
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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.

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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

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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
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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

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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

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• CONTROL OF REGIONAL
• BLOOD FLOW

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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

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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.

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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

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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

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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.

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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.

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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.

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The Microcirculation

• Over 10 billion capillaries with surface area of
  500-700 square meters
• Small volume of blood is exposed to larger
  surface area

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• 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.

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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.

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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

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• 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

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Fluid movement Kf (Pc Pi) ( ?c - ?i )
Kf filtration coefficient
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• 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.

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Starlings forces across capillary wall
Absorption
Filtration
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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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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 .

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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

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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

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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)

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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