AP 151

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AP 151

• Cardiovascular Physiology

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I. Principles of Physics Affecting Cardiovascular
Physiology

• Liquids and gases flow down a pressure gradient
  (?P)
• From a region of higher pressure to lower
  pressure.
• (?P) P1 -P2, where (?P) change in pressure
• Heart chambers contract (higher pressure) ?
  vessels (lower pressure)
• Higher pressure in Aorta (120 mm Hg) than in
  Right Atrium (0-1 mm Hg)

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Physical Laws Describing Blood Flow

• Blood flows through vascular system when there is
  pressure difference at its two ends (?P)
• Flow rate is directly proportional to pressure
  difference (?P P1 - P2)
• If ?P increases, blood flow speeds up if ?P
  decreases, blood flow declines

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Blood Flow

• Actual volume of blood flowing through a vessel,
  an organ, or the entire circulation in a given
  period
• Is measured in ml per min.
• Is equivalent to cardiac output (CO), considering
  the entire vascular system
• Is relatively constant when at rest
• Varies widely through individual organs,
  according to immediate needs (see next slide)

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Blood Flow Through Tissues

• Blood flow, or tissue perfusion, is involved in
• Delivery of oxygen and nutrients to, and removal
  of wastes from, tissue cells
• Gas exchange in the lungs
• Absorption of nutrients from the digestive tract
• Urine formation by the kidneys
• Blood flow is precisely the right amount to
  provide proper tissue function

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Velocity of Blood Flow

• Blood velocity
• Changes as it travels through the systemic
  circulation
• Is inversely proportional to the cross-sectional
  area the larger the cross-sectional area, the
  slower the speed of blood flow
• Slow capillary flow allows adequate time for
  exchange between blood and tissues

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Velocity of Blood Flow1. While the aorta is a
large vessel compared to a single capillary, its
cross sectional area is small compared to the
TOTAL CROSS SECTIONAL AREA of all the
capillaries.2. Note in the chart how the
velocity in the capillaries is slowest while that
in the aorta is fastest.3. What purpose is
served by the slow velocity of blood flow in the
capillaries?
Figure 19.13
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Resistance

• Resistance opposition to flow
• Measure of the amount of friction blood
  encounters as it passes through vessels
• Generally encountered in the systemic circulation
• Referred to as peripheral resistance (PR)
• The three important sources of resistance are
• blood viscosity
• total blood vessel length
• blood vessel diameter

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Resistance1. Effect of Viscosity on Resistance
to Blood Flow

• As viscosity increases, pressure required to flow
  increases
• Chocolate shake vs. soda passing through straws
• Viscosity influenced largely by hematocrit
  (percentage of the total blood volume composed of
  red blood cells).
• Dehydration and/or uncontrolled production of
  RBCs can lead to increased viscosity which
  increases the workload on the heart.

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Resistance2. Effect of Vessel Length on
Resistance to Blood Flow

• Greater resistance is encountered in longer tubes
  than in shorter tubes
• Long straw vs. short straw
• Blood vessel length does NOT change over short
  periods of time
• Obesity may add more b.v.s to adipose tissue and
  thus increase resistance to blood flow
• Therefore, not an important contributor to
  resistance to blood flow

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Resistance3. Effect of Vessel Diameter on
Resistance to Blood Flow

• Blood flow is inversely proportional to
  resistance (R)
• If R increases, blood flow decreases
• If R decreases, blood flow increases
• R is more important than ?P in influencing local
  blood pressure
• Flow ?P /R
• Resistance is directly proportional to length of
  vessel (L) viscosity of blood (?)
• Inversely proportional to 4th power of radius
• So diameter of vessel is very important for
  resistance

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Fig 14.14. Relationship between blood flow,
radius resistance
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Blood Pressure (BP)
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Systemic Blood Pressure

• Force per unit area exerted on the wall of a
  blood vessel by its contained blood
• Expressed in millimeters of mercury (mm Hg)
• Measured in reference to systemic arterial BP in
  large arteries near the heart
• Systemic pressure
• Is highest in the aorta
• Declines throughout the length of the pathway
• Is 0 mm Hg in the right atrium
• The steepest change in blood pressure occurs in
  the arterioles (see next slide)

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Systemic Blood Pressure
Figure 19.5
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Arterial Blood Pressure

• Arterial BP reflects two factors of the arteries
  close to the heart
• Their elasticity (compliance or distensibility)
• The amount of blood forced into them at any given
  time
• Blood pressure in elastic arteries near the heart
  is pulsatile (BP rises and falls)
• Due to the greater amount of elastic tissue in
  the walls of elastic arteries

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Capillary Blood Pressure

• Capillary BP ranges from 20 to 40 mm Hg
• Low capillary pressure is desirable because high
  BP would rupture fragile, thin-walled capillaries
• Low BP is sufficient to force filtrate out into
  interstitial space and distribute nutrients,
  gases, and hormones between blood and tissues

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Arterial Blood Pressure

• Systolic pressure pressure exerted on arterial
  walls during ventricular contraction
• Diastolic pressure lowest level of arterial
  pressure during a ventricular cycle
• Pulse pressure the difference between systolic
  and diastolic pressure
• Mean arterial pressure (MAP) pressure that
  propels the blood to the tissues
• MAP diastolic pressure 1/3 pulse pressure

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Venous Blood Pressure

• Venous BP is steady and changes little during the
  cardiac cycle
• The pressure gradient (from beginning to end) in
  the venous system is only about 20 mm Hg
• A cut vein has even blood flow a lacerated
  artery flows in spurts

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Factors Aiding Venous Return

• Venous BP alone is too low to promote adequate
  blood return and is aided by the
• Respiratory pump pressure changes created
  during breathing suck blood toward the heart by
  squeezing local veins
• Muscular pump contraction of skeletal muscles
  milk blood toward the heart
• Valves prevent backflow during venous return

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Factors Aiding Venous Return
Figure 19.6
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Blood Volume

• Constitutes small fraction of total body fluid
• 2/3 of body H20 is inside cells (intracellular
  compartment)
• 1/3 total body H20 is in extracellular
  compartment
• 80 of this is interstitial fluid 20 is blood
  plasma

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Capillary Exchange Fluid Movements

• Direction and amount of fluid flow depends upon
  the difference between
• Capillary hydrostatic pressure (HPc) - a function
  of blood pressure
• Capillary colloid osmotic pressure (OPc) - due to
  plasma proteins
• HPc pressure of blood against the capillary
  walls
• Tends to force fluids through the capillary walls
  
• Is greater at the arterial end (35 mm Hg) than at
  the venule end (17)
• OPc created by nondiffusible plasma proteins,
  which draw water toward themselves
• Does NOT change over the length of the vessel as
  these are nondiffusible proteins. Why
  nondiffusible?

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Net Filtration Pressure (NFP)

• Net filtration pressure (NFP) 10 mm Hg at
  arterial end
• NFP -8 mm Hg at venule end
• Thus, more fluid leaves than enters capillary

Figure 19.15
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Edema and Capillary Exchange

• If capillaries become more permeable, proteins
  can leak into the interstitial fluid increasing
  OPif. More fluid moves from the capillaries into
  the interstitial fluid edema.
• Chemicals of inflammation increase permeability
• Decreases in plasma concentration of protein
  reduces OPc more fluid moves into interstitial
  fluid
• Liver disease resulting in fewer plasma proteins
• Loss of plasma proteins through the kidneys
• Protein starvation
• Blockage of veins increases capillary BP reduced
  venous return due to gravity
• Blockage or removal of lymphatic vessels
  (blockage elephantiasis removal cancer)

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Effect of Cross-Sectional Area of Blood Vessels
on Rate of Blood Flow

• As diameter of vessels decreases, the total
  cross-sectional area increases and velocity of
  blood flow decreases. Only one aorta with a
  cross-sectional area of 5 cm2. Total
  cross-sectional area of the millions of
  capillaries is 2500 cm2.
• Much like a stream that flows rapidly through a
  narrow gorge but flows slowly through a broad
  plane

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Functional Characteristics of Veins

• Cardiac output depends on preload
• Latter determined by volume of blood that enters
  the heart from the veins
• Venous return to heart increases due to increase
  in blood volume (transfusion), venous tone, and
  arteriole dilation
• Venous tone continual state of partial
  contraction of the veins as a result of
  sympathetic stimulation
• Forces large venous volume to flow toward the ?
  )
• Results in an ? in VR and preload (EDV), causing
  an ? in cardiac output
• Conversely, ? sympathetic stimulation ? venous
  tone, allowing veins to relax, dilate, fill w/
  blood, reducing VR to heart, preload, and CO

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Control of Blood Flow in Tissues

• Blood flow to the tissues is controlled by
  several different mechanisms
• Local control in most tissues, blood flow is
  proportional to metabolic needs of tissues or
  myogenic responses
• Nervous System responsible for routing blood
  flow and maintaining blood pressure
• Hormonal Control sympathetic action potentials
  stimulate epinephrine and norepinephrine
  secretion from adrenal medulla

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Local Control of Blood Flow (Autoregulation)
Metabolic Control

• Metabolic control mechanism matches blood flow to
  local tissue needs
• Metabolic conditions that cause vasodilation
• Low O2 levels of tissues
• High CO2 levels
• Low pH (acid)
• ? Adenosine or K
• Known as active hyperemia

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Local Control2. Myogenic Controls

• Maintain fairly constant blood flow despite BP
  variation
• Inadequate blood perfusion (low BP) or
  excessively high arterial pressure
• Are autoregulatory
• Provoke myogenic responses stimulation of
  vascular smooth muscle
• Vascular muscle responds directly to
• Increased vascular pressure (high BP) with
  increased tone, which causes vasoconstriction
• Reduced stretch (low BP) with vasodilation, which
  promotes increased blood flow to the tissue

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Paracrine Regulation of Blood Flow

• Endothelium produces several paracrine regulators
  that promote relaxation
• Nitric oxide (NO), bradykinin, prostacyclin
• NO is involved in setting resting tone of
  vessels
• Levels are increased by parasympathetic activity
• Vasodilator drugs such as nitroglycerin or Viagra
  act thru NO
• Endothelin 1 is vasoconstrictor produced by
  endothelium
• Remember
• Vasoconstriction leads to an increase in
  resistance and a decrease in blood flow.
• Vasodilation leads to a decrease in resistance
  and an increase in blood flow.

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Controls of Blood Pressure

• Short-term controls
• Are mediated by the nervous system and bloodborne
  chemicals
• Counteract moment-to-moment fluctuations in blood
  pressure by altering peripheral resistance (blood
  vessel diameter)
• Long-term controls regulate blood pressure by
  regulating blood volume

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Short-Term Regulation ofBlood Pressure

• Baroreceptor reflexes change peripheral
  resistance, heart rate, and stroke volume in
  response to changes in blood pressure
• Chemoreceptor reflexes sensory receptors
  sensitive to oxygen, carbon dioxide, and pH
  levels of blood
• Central nervous system ischemic response results
  from high carbon dioxide or low pH levels in
  medulla and increases peripheral resistance

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Nervous Regulation of Blood Vessels

• Important in minute-to-minute regulation of local
  circulation
• Provides a means by which blood can be shunted
  from one large area of the peripheral circulatory
  system to another area by increasing resistance
• Sympathetic division most important. Innervates
  all vessels except capillaries, precapillary
  sphincters, and most metarterioles.
• Vasomotor center in medulla oblongata.
• Excitatory part is tonically active. Causes
  vasomotor tone. Norepinephrine
• Inhibitory part can cause vasodilation by
  decreasing sympathetic output
• Sympathetic stimulation of adrenal medulla causes
  output of norepinephrine and epinephrine into
  circulation. Cause vasoconstriction in vessels
  (a-adrenergic receptors) except in skeletal
  muscle where vasodilation takes place
  (ß-adrenergic receptors)

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Short-Term Mechanisms Vasomotor Center

• Vasomotor center a cluster of sympathetic
  neurons in the medulla that oversees changes in
  blood vessel diameter
• Maintains blood vessel tone by innervating smooth
  muscles of blood vessels, especially arterioles
• Cardiovascular center vasomotor center plus the
  cardiac centers that integrate blood pressure
  control by altering cardiac output and blood
  vessel diameter

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Short-Term Mechanisms Vasomotor Activity

• Sympathetic activity causes
• Vasoconstriction and a rise in blood pressure if
  increased
• Blood pressure to decline to basal levels if
  decreased
• Vasomotor activity is modified by
• Baroreceptors (pressure-sensitive),
  chemoreceptors (O2, CO2, and H sensitive),
  higher brain centers, bloodborne chemicals, and
  hormones

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Baroreceptor Reflex

• Is activated by changes in BP
• BP is detected by baroreceptors (stretch
  receptors) located in aortic arch carotid
  sinuses
• Increase in BP causes walls of these regions to
  stretch, stimulating baroreceptors.
• Baroreceptors send action potentials to vasomotor
  cardiac control centers in medulla
• Is most sensitive to decrease sudden changes in
  BP

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Baroreceptor Reflexes1. Increased blood
pressure - - Decreases heart rate -
Decreases cardiac output, - Decreases
peripheral resistance through vasodila-
tion which - Decreases blood pressure
2. Declining blood pressure - - Increase
cardiac output - Increased peripheral
resistance through vasoconstriction which
- Increases blood pressure
Figure 19.8
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Short-Term Mechanisms Chemical Controls

• Blood pressure is regulated by chemoreceptor
  reflexes sensitive to oxygen and carbon dioxide
• Prominent chemoreceptors are the carotid and
  aortic bodies
• Reflexes that regulate blood pressure are
  integrated in the medulla
• Higher brain centers (cortex and hypothalamus)
  can modify BP via relays to medullary centers
• Will be covered in greater detail in resp. system

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Chemoreceptor Reflex Control
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Long Term Mechanisms of Blood Pressure
ControlKidney Action

• Kidneys act directly and indirectly to maintain
  long-term blood pressure
• Direct renal mechanism alters blood volume
• Indirect renal mechanism involves the
  renin-angiotensin mechanism

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Kidney Action and Blood Pressure

• Declining BP causes the release of renin, which
  stimulates the conversion of Angiotensinogen into
  Angiotensin I
• Angiotensin I converted to angiotensin II by ACE
• Angiotensin converting enzyme
• Angiotensin II is a potent vasoconstrictor that
  stimulates aldosterone secretion
• Aldosterone enhances renal reabsorption of NaCl
  and stimulates ADH release
• ADH (antidiuretic hormone) stimulates increased
  water reabsorption by the kidney collecting ducts

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Kidney Action and Blood Pressure
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Renin-Angiotensin-Aldosterone Mechanism
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Chemicals that Increase Blood Pressure

• Adrenal medulla hormones norepinephrine and
  epinephrine increase blood pressure
• Antidiuretic hormone (ADH) causes intense
  vasoconstriction in cases of extremely low BP
• Angiotensin II kidney release of renin
  generates angiotensin II, which causes intense
  vasoconstriction
• Endothelium-derived factors endothelin and
  prostaglandin-derived growth factor (PDGF) are
  both vasoconstrictors

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Vasopressin (ADH) Mechanism
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Monitoring Circulatory Efficiency

• Efficiency of the circulation can be assessed by
  taking pulse and blood pressure measurements
• Vital signs pulse and blood pressure, along
  with respiratory rate and body temperature
• Pulse pressure wave caused by the expansion and
  recoil of elastic arteries
• Radial pulse (taken on the radial artery at the
  wrist) is routinely used
• Varies with health, body position, and activity

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Measuring Blood Pressure

• Systemic arterial BP is measured indirectly with
  the auscultatory method
• A sphygmomanometer is placed on the arm superior
  to the elbow
• Pressure is increased in the cuff until it is
  greater than systolic pressure in the brachial
  artery
• Pressure is released slowly and the examiner
  listens with a stethoscope

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Measuring Blood Pressure

• Systemic arterial BP is measured indirectly with
  the auscultatory method
• A sphygmomanometer is placed on the arm superior
  to the elbow
• Pressure is increased in the cuff until it is
  greater than systolic pressure in the brachial
  artery
• Pressure is released slowly and the examiner
  listens with a stethoscope

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Measuring Blood Pressure

• The first sound heard is recorded as the systolic
  pressure
• The pressure when sound disappears is recorded as
  the diastolic pressure

InterActive Physiology Cardiovascular System
Measuring Blood Pressure
PLAY
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Variations in Blood Pressure

• Blood pressure cycles over a 24-hour period
• BP peaks in the morning due to waxing and waning
  levels of retinoic acid
• Extrinsic factors such as age, sex, weight, race,
  mood, posture, socioeconomic status, and physical
  activity may also cause BP to vary

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Alterations in Blood Pressure

• Hypotension low BP in which systolic pressure
  is below 100 mm Hg
• Hypertension condition of sustained elevated
  arterial pressure of 140/90 or higher
• Transient elevations are normal and can be caused
  by fever, physical exertion, and emotional upset
• Chronic elevation is a major cause of heart
  failure, vascular disease, renal failure, and
  stroke

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Hypertension

• Hypertension maybe transient or persistent
• Primary or essential hypertension risk factors
  in primary hypertension include diet, obesity,
  age, race, heredity, stress, and smoking
• Secondary hypertension due to identifiable
  disorders, including excessive renin secretion,
  arteriosclerosis, and endocrine disorders

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Hypotension

• Orthostatic hypotension temporary low BP and
  dizziness when suddenly rising from a sitting or
  reclining position
• Chronic hypotension hint of poor nutrition and
  warning sign for Addisons disease
• Acute hypotension important sign of circulatory
  shock
• Threat to patients undergoing surgery and those
  in intensive care units

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Circulatory Shock

• Circulatory shock any condition in which blood
  vessels are inadequately filled and blood cannot
  circulate normally
• Results in inadequate blood flow to meet tissue
  needs

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Circulatory Shock

• Three types include
• Hypovolemic shock results from large-scale
  blood loss
• Vascular shock poor circulation resulting from
  extreme vasodilation
• Cardiogenic shock the heart cannot sustain
  adequate circulation

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Events of Hypovolemic Shock
Figure 19.16
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Blood Flow To The Brain

• Blood flow to the brain is constant, as neurons
  are intolerant of ischemia
• Metabolic controls brain tissue is extremely
  sensitive to declines in pH, and increased carbon
  dioxide causes marked vasodilation
• Myogenic controls protect the brain from damaging
  changes in blood pressure
• Decreases in MAP cause cerebral vessels to dilate
  to ensure adequate perfusion
• Increases in MAP cause cerebral vessels to
  constrict

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Blood Flow Lungs

• Blood flow in the pulmonary circulation is
  unusual in that
• The pathway is short
• Arteries/arterioles are more like veins/venules
  (thin-walled, with large lumens a low resistance
  circuit)
• They have a much lower arterial pressure (24/8 mm
  Hg versus 120/80 mm Hg)

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Blood Flow Lungs

• The autoregulatory mechanism is exactly opposite
  of that in most tissues
• Low oxygen levels cause vasoconstriction high
  levels promote vasodilation
• This allows for proper oxygen loading in the lungs

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Blood Flow Heart

• Small vessel coronary circulation is influenced
  by
• Aortic pressure
• The pumping activity of the ventricles
• During ventricular systole
• Coronary vessels are compressed and flow
  decreases
• Myocardial blood flow ceases
• Stored myoglobin supplies sufficient oxygen
• During ventricular diastole, oxygen and nutrients
  are carried to the heart

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Blood Flow Heart

• Under resting conditions, blood flow through the
  heart may be controlled by a myogenic mechanism
• During strenuous exercise
• Coronary vessels dilate in response to local
  accumulation of carbon dioxide
• Blood flow may increase three to four times
• Blood flow remains constant despite wide
  variation in coronary perfusion pressure