Cardiovascular Physiology Part 2

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Cardiovascular Physiology Part 2

• Hemodynamics
• The Peripheral Circulation

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Some web sites for review

• http//www.cellsalive.com/myocyte.htm - look at
  some cardiac cells contracting in situ
• http//www.gwc.maricopa.edu/class/bio202/cyberhear
  t/cardio.htm - heart anatomy quizzes/tutorials

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Hemodynamics general patterns of blood flow

• Blood flows in a continuous circuit
• Same liters per minute flows through arteries
  capillaries veins unless there is a change in
  blood volume overall, a decrease in volume in one
  part must lead to an increase in another
• Velocity of flow sum of cross-sections of all
  vessels at that point highest velocities of
  flow occur in aorta pulmonary artery (mammals)
  (velocity drops in capillaries then picks up in
  veins

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Some Terms

• Laminar flow a gradient of relative velocity
  exists in which fluid layers closest to wall have
  lowest relative velocity flow is 0 at the wall
  maximal at center
• Pulsatile laminar flow characteristic of large
  arteries 1st blood is accelerated then slowed
  with each heartbeat since vessels walls are
  elastic expand relax as pressure oscillates
  with each HB velocity much flatter profile
  fig 12-24 p. 496

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Terms cont

• Turbulent flow fluid flows in directions not
  aligned with axis of flow increasing energy
  needed to move fluid through vessel uncommon in
  peripheral circulation
• Compliance ratio of change in volume to change
  in pressure (venous system is very compliant vs.
  arterial system where vessels have elastic fibers
  that enable them to be distended) venous system
  small changes in pressure produce large changes
  in volume volume reservoir vs. arterial system
  pressure reservoir

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Poiseuilles Law

• Flow Rate of Fluid (Q) is directly proportional
  to pressure difference (P1-P2) along the length
  of the tube fourth power of radius of tube, r
  inversely proportional to tube length (L) fluid
  viscosity, n
• Q (P1-P2) pir4 NB because Q
• 8Ln proportional to r4 very small changes
  in r have profound effect on Q e.g. a 2x increase
  in vessel diameter leads to 16x increase in flow
  if pressure difference along vessel remains
  unchanged

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Peripheral Circulation

• vessels in various parts of peripheral
  circulation are adapted for functions they serve
• Layer of endothelial cells (endothelium) lines
  lumen of all blood vessels
• In large vessels, endothelium is surrounded by
  layer of elastic collagenous fibers
• Walls of capillaries single layer of
  endothelial cells
• Circular longitudinal smooth muscle fibers may
  intermingle with/surround elastic collagenous
  fibers

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Walls of Large Vessels 3 layers

• Tunica adventitia limiting fibrous outer coat
• Tunica media middle layer circular
  longitudinal smooth muscle
• Tunica intima inner layer closest to lumen
  endothelial cells elastic fibers
• B/c arteries more muscular than veins they have
  thicker tunica media larger arteries closest to
  heart are more elastic with wide tunica intima
  vasa vasorum blood vessels own capillary
  circulation Fig. 12-26 p. 500 http//www.innerbody
  .com/image/card05.html

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Arterial System 4 Main Functions

• Conduit for blood between heart capillaries
• Act as a pressure reservoir for forcing blood
  into small-diameter arterioles
• Dampen the oscillations in pressure flow
  generated by HB produce more even flow of blood
  into capillaries
• Control distribution of blood to different
  capillary networks via selective constriction of
  terminal branches of arterial tree

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Arterial System cont

• Arterial BP determined by volume of blood in
  arterial system properties of walls if either
  altered, pressure will change
• Volume of blood in arteries is determined by rate
  of filling via cardiac contractions of empting
  via arterioles into capillaries
• If CO increase, arterial BP increases if
  capillary flow increase, arterial BP decreases
• Normally arterial BP varies little b/c rates of
  filling empting (CO capillary flow) are
  evenly matched

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Arterial System cont

• Blood flow through capillaries is proportional to
  pressure difference between arterial venous
  systems
• B/c venous pressure is low changes little,
  arterial pressure exerts primary control over
  rate of capillary blood flow is responsible for
  maintaining adequate perfusion of tissues
• Arterial pressure varies with species (50 150
  mm Hg

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Arterial System cont

• Oscillations in BP flow generated by
  contractions of heart are dampened in arterial
  system by elasticity of arterial walls
• As blood ejected into arterial system, pressure
  rises vessels expand as heart relaxes, blood
  flow to periphery is maintained by elastic recoil
  of vessel walls resulting in a reduction in
  arterial volume

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Laplaces Law

• Wall tension required to maintain a given
  transmural pressure within a hollow structure
  increases with increasing radius elastic
  vessels are unstable tend to balloon I.e. since
  cant develop high wall tension as pressure
  increase, they tend to bulge
• This instability is prevented by a collagen
  sheath that limits their expansion ballooning
  of blood vessel (aneurism) can occur, if collagen
  sheath breaks down

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Arterial System cont

• Generally, elasticity of arterial wall
  thickness of muscular layer decreases with
  increasing distance from heart
• BP transmural pressure I.e. difference in
  pressure between inside outside across wall of
  blood vessel
• Maximum arterial pressure during a cardiac cycle
  systolic pressure minimum as diastolic
  pressure the difference is pressure pulse

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Venous System

• venous system conduit for return of blood from
  capillaries to heart
• large-volume, low-pressure system consisting of
  vessels with a larger inside diameter than
  corresponding arteries
• in mammals, 50 of total BV is contained in
  venous system

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Venous System cont

• venous pressure seldom gt 11mm Hg (10 of
  arterial pressure) venous system functions also
  as a storage reservoir for blood
• walls are much thinner, contain less smooth
  muscle, less elastic than artery wall (veins
  contain more collagenous than elastic fibers)
  exhibit much less recoil than arteries

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Venous System cont

• -   NB in event of blood loss, venous BV (not
  arterial volume) is decreased to maintain
  arterial pressure capillary blood flow this
  decrease is compensated by decrease in internal
  volume of venous system itself
• Walls of many veins are covered by smooth muscle
  innervated by sympathetic adrenergic fibers
  stimulation of these fibers causes
  vasoconstriction reduction in size of venous
  reservoir (allows some bleeding to occur without
  drop in venous BP e.g. blood donors venous system
  expands as blood is replaced by fluid retention
  volume can be quickly replaced by drinking
  other blood constituents e.g. RBCs can take up to
  one week)

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Factors Affecting Venous Blood Flow (other than
contraction of the heart)

• 1. contraction of limb muscles
• veins contain pocket valves allow flow only
  towards heart thus squeezing augments return
  increases CO - squeezing augments return
  increases CO activation of skeletal muscle venous
  pump is associated with increased, localized
  activity in sympathetic fibers innervating venous
  smooth muscle increases smooth muscle tone NB
  sympathetic response ensure that skeletal
  muscle pump specifically increases return to
  heart rather than simply distending another part
  of venous system without this pump there may
  be considerable pooling of blood in venous system
  of limbs

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Factors affecting Venous Blood Flow (other than
contraction of the heart)

• 2. pressure created by diaphragm on gut
• breathing (mammals) expansion of thoracic cage
  reduces pressure within chest draws air into
  lungs pressure reduction sucks blood from veins
  of head abdominal cavity into heart large veins
  situated within thoracic cavity

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Blood Distribution (veins)

• lying down vs. upright head, heart limbs etc
  same plane vs. stand-up relationships all
  change relative to gravity average person
  heart now meter above lower limbs lower than
  head
• gravity has little to do with capillary flow
  (which is determined by arterial-venous
  difference i.e. gravity raises arterial venous
  pressure by same amount thus doesnt affect
  pressure across cap bed very much) consider
  animals with long necks e.g. giraffe how
  pressures vary when erect vs. lowers head very
  interesting reading including effects at the
  level of the kidney

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Blood Distribution (veins) cont

• baroreceptors pressure receptors
• evolutionary changes in venous system as animals
  moved from water to land (especially as they lost
  the support of water) - pooling of blood not a
  problem for animals in water as density of water
  is only slightly less than that of blood in
  water, hydrostatic pressure increases with depth
  effectively matches increase in blood pressure
  due to gravity thus transmural pressure does not
  change that much blood doesnt pool

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Blood Distribution (veins) cont

• pooling became immediate problem for terrestrial
  animals because air is much less dense than blood
  ( changes in venous system) are paralleled also
  by changes in arterial system particularly those
  required to maintain separation of oxygenated
  deoxygenated blood moving through the heart
  challenges with modern day fish pooling at tail
  due to inertia to compression waves associated
  with swimming movements passing down body (to
  counteract most veins returning to heart pass
  down center of fishs body some with caudal
  heart in tail propelling blood forward

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Countercurrent exchangers (review spotlight 14-2
p. 611)

• most animals arteries veins run next to each
  other with blood flowing in opposite directions
  (i.e. countercurrent blood flow)
• in many instances, if vessels are small, there is
  an exchange of heat between the countercurrent
  blood flow
• countercurrent arrangement of small arterioles
  venules rete mirabile

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Pressure Flow

• Energy is expended in setting blood in motion,
  but once in motion, flowing blood has inertia
  fluid motion possess kinetic energy
• Maximum velocity of blood flow occurs at base of
  aorta (mammals)
• Flow of blood is inversely related to its
  viscosity (plasma has viscosity relative to water
  of about 1.8) the addition of RBCs further
  increases relative viscosity of blood blood
  behaves as though it were 3-4 Xs more viscous
  than water larger pressure gradients are
  required to maintain flow of blood through a
  vascular bed than would be needed if the vascular
  bed were perfused by plasma alone

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Pressure cont

• http//www.interactivephysiology.com/demo/systems/
  buildframes.html?cardio/mainbp/01