REGULATION OF BLOOD FLOW.

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REGULATION OF BLOOD FLOW.
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Common characteristic of blood flow

• The direct effect of heart contraction is
  creation of certain level of blood pressure,
  which is allows blood circulation.
• Blood flow is continuous, although heart pumps
  the blood by separate portions. It caused by
  functioning of all components of cardio-vascular
  system heart, arteries, arterioles, capillaries,
  venuls and veins. Besides that continuous blood
  flow is caused by extracardial factors as
  skeletal muscle contraction and pressure gradient
  between abdominal and thoracic cavities. Cardiac
  output depends on high, mass and area of human
  body surface. Cardiac output is regulated by
  contractive activity of cardiac muscle valve
  function of full value blood volume, vascular
  tonus, blood flow in capillaries value of blood
  returning to the heart. In general distribution
  of cardiac output between different organs
  corresponds to its functional activity. Part of
  common blood supply, which every organ gets,
  depends on necessity in O2 and substrates of
  energy exchange. Average time of blood
  circulation measures 20-23 s.

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Powers, which causes blood flow

• Blood flows in vessels from high pressure to low.
  Heart pumping causes initial pressure. The
  highest pressure is large arteries ascending from
  the heart. Pressure in aorta at the end of
  systole is 110-125 mm Hg, at the end of diastole
  - 70-80 mm Hg. In pulmonary trunk during systole
  the blood pressure is 20-25 mm Hg, in diastole -
  10-15 mm Hg. In large arteries blood flow
  velocity is 0.1-0.2 m/s. In large veins returning
  blood flow to the heart is caused by lowest blood
  pressure - 0 mm Hg.

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Functional importance of blood circulation system.

• Both pulmonary and systemic circulation, compose
  entire system of blood circulation and function
  in correlation. The right ventricle is
  responsible for blood pumping into pulmonary
  circulation. Here blood is oxygenated and CO2 is
  taken out. The left ventricle pumps blood into
  systemic circulation. Blood flow in this part of
  vascular system provides performing of all other
  blood functions as regulatory, protective,
  excretory and others. Both right and left parts
  of heart pump equal portions of blood into
  corresponding vessels and function in
  interconnection to each other. The minute blood
  volume in pulmonary and systemic circulation is
  the same.

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Circulating blood volume

• Blood flowing in vessels is similar to stream of
  fluid in the pipe, but has a lot of
  specificities. Fluid stream in the pipe is
  described by formula
• Q (P1-P2)/R, where
• Q - fluid volume,
• P1 - pressure in the beginning of the pipe,
• P2 - pressure in the end of the pipe,
• R - peripheral resistance of the pipe.
• So fluid volume, which flows through the pipe is
  directly proportional to pressure difference from
  the end to beginning of pipe and inversely
  proportional to peripheral resistance of pipe. As
  vessels have elastic walls, the blood flow in it,
  is differ from the same in pipe. Vessel
  cross-section may change due to neural and
  endocrine influences according to necessity.
• Blood volume flowing through every part of
  vascular system per time unit is the same. It
  means that through aorta or cross-section of all
  arteries, capillaries or veins flows equal volume
  of blood. This volume per minute is called minute
  blood volume and measures in adults in rest 4.0 -
  6.5 l/min.

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Peripheral resistance of vessels

• Peripheral resistance in vessels according to
  Poiseuille's formula depends on length of vessels
  (l), viscosity of blood (?) and cross-section of
  vessel (r)
• R 8l?/pr.
• In accordance to this formula the highest
  peripheral resistance might be in the smallest
  vessels. In reality the highest resistance is
  observed in arterioles. Average blood flow
  resistance in adults is equal to 900-2500
  dins/sm5

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Paradoxes of blood flow.

• In capillaries blood flow resistance is a bit
  lower because of such mechanism. In capillaries
  blood cells move one after another, dividing only
  by plasma, which decreases friction between blood
  cells and capillary wall. On other side,
  capillaries are shorter, than arterioles, which
  caused lower blood flow resistance too.
• Viscosity of blood is also important for
  resistance of vessels. It depends on quantity of
  blood cells, protein rate in plasma, especially
  globulins and fibrinogen. Considerable increase
  of blood viscosity may cause lower blood
  returning to the heart and than disorders of
  blood circulation.
• In large arteries centralization of blood flow is
  observed. Blood cells moves in the central part
  of blood stream, and plasma is peripheral.
  Instead increase of blood viscosity in arterioles
  is caused by higher friction between cells and
  vessels wall.

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Linear velocity of blood flow

• Blood flow also is characterized by linear
  velocity of blood circulation
• VQ/pr2, where
• V - linear velocity,
• Q - blood volume,
• r - radius of vessel.
• So it is clear the wider cross-section of vessel
  the slower linear velocity of blood stream. In
  large arteries linear velocity is highest
  (0.1-0.2 m/s). In arterioles it measures 0.002 -
  0.003 m/s, in capillaries - near 0.0003 m/s. In
  veins cross-section decreases and linear velocity
  increases to 0.001 - 0.05 m/s in large veins and
  to 0.1 - 0.15 in vena cava.

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Blood pressure

• Transversal pressure - is difference between
  pressure inside the vessel and squeeze of it from
  the tissues. When increasing the tissue pressure
  to vessel wall, it closes. Hydrostatic pressure
  is corresponding to weight of all blood in vessel
  when it has vertical position. For vessels of
  head and neck this pressure decreases towards the
  heart. For vessels of limbs it has outward
  direction. That is why hydrodynamic pressure in
  vessels over heart is decreased due to
  hydrostatical pressure. Below heart hydrodynamic
  pressure is increased, because it is summarized
  with hydrodynamic pressure.

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Role of changing body position

• In horizontal position of the body hydrostatical
  pressure is equal in every part of the body and
  hydrodynamic pressure doesn't depend on it. In
  vertical position transversal pressure in vessels
  of limbs creates tension of vessels walls (Laplas
  low)
• PtF/r, where
• Pt transversal pressure,
• F - vessel tension,
• r - radius of vessel.
• So it is shown the smaller radius of vessel, the
  lower tension in vessels walls. Due to this
  capillaries with thinnest wall don't crush
  because of its smallest diameter. Existence of
  precapillary sphincters permits proper direction
  of blood pressure so that capillaries may close
  (plasmatic capillaries).

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Local regulatory mechanisms

• Collagen fibers of vessels walls form net, which
  prevent its tension or decrease tone. Smooth
  muscle cells combine with elastic and collagen
  fibers in vessels walls. Contracting and
  stretching these fibers smooth muscle cells
  produce active tension of vessel wall - tonus of
  vessels.
• There are some mechanisms in regulation of vessel
  tonus by smooth muscles. When rapid increasing of
  blood pressure, smooth muscles contract and
  decrease tension by decreasing vessel diameter.
  In slow rising of pressure tension decrease by
  dilation of smooth muscles and increase vessel
  diameter. These mechanisms occur more often in
  veins than in arteries. Veins have equal
  elasticity in systemic and pulmonary circulation,
  but arteries are more extensible in pulmonary
  circulation. Arteries in pulmonary circulation
  contain a lot of elastic and smooth muscle fibers.

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Functional types of vessels

• - Elastic (damping) vessels. Large arteries
  belong to this group. The main function of these
  vessels is to turn ejection of blood into
  continuous blood flow. It is possible due to
  elastic properties of its wall
• - Resistive vessels are arterioles, precapillary
  sphincters and venuls. These vessels may regulate
  the blood flow in capillaries by changing their
  tonus
• - Exchange vessels are capillaries. Their walls
  due to the special structure permit exchange of
  materials between blood and tissues
• -Capacitive vessels are veins. To sure one-way
  direction of blood flow veins have valves if
  lying below the heart. Veins contain 75-80 of
  circulating blood. Veins of skin and abdominal
  cavity may function as depot of blood.

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Local regulation of blood flow

• Role of metabolic factors Greater rate of
  metabolism or less blood flow causes decreasing
  O2 supply and other nutrients. Therefore rate of
  formation vasodilator substances (CO2, lactic
  acid, adenosine, histamine, K and H) rises.
  When decreasing both blood flow and oxygen supply
  smooth muscle in precapillary sphincter dilate,
  and blood flow increases. In is a vasodilator
  substance as nitric oxide released from
  endothelial cells released from endothelial
  cells. It causes secondary dilation of large
  arteries when micro vascular blood flow
  increases. Cardiac muscle utilizes fatty acids
  for energy. Cardiac muscle utilizes glucose
  through glycolisis that results in formation of
  lactic acid.

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Basal tone of vessels.

• When arterial pressure suddenly increases local
  blood flow tends to increase. It leads to sudden
  stretch of arterioles cause smooth muscles in
  their wall to contract. Than local blood flow
  decreases to normal level. Vessel walls are
  capable to prolonged tonic contraction without
  tiredness even at rest. Such a condition is
  supported by spontaneous myogenic activity of
  smooth muscles and efferent impulsation from
  autonomic nerve centers, which control arterial
  pressure. Partial state of contraction in blood
  vessels caused by continual slow firing of
  vasoconstrictor area is called vasculomotor tone.
  Due to regulatory nerve and humoral influences
  this basal ton changes according to functional
  needs of curtain organ.

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Neuro-humoral regulation of systemic circulation

• a) Afferent link. Nerve receptors, which are
  capable react to changing blood pressure, lays in
  heart cameras, aorta arc, bifurcation of large
  vessels as carotid sinus and other parts of
  vascular system. Irritation of these mechanical
  receptors produce nerve impulses, which pass to
  higher nerve centers for processing sensory
  information from visceral organs.

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• b) Central link. Vasoconstrictor area of
  vasculomotor center is located bilaterally in
  dorsolateral portion of reticular substance in
  upper medulla oblongata and lower pons. Its
  neurons secrete norepinephrine, excite
  vasoconstrictor nerves and increase blood
  pressure. It transmits also excitatory signals
  through sympathetic fibers to heart to increase
  its rate and contractility.
• Vasodilator area is located bilaterally in
  ventromedial of reticular substance in upper
  medulla oblongata and lower pons. Its neurons
  inhibit dorsolateral portion and decrease blood
  pressure. It transmits also inhibitory signals
  through parasympathetic vagal fibers to heart to
  decrease its rate and contractility.
• Posterolateral portions of hypothalamus cause
  excitation of vasomotor center. Anterior part of
  hypothalamus can cause mild inhibition of one.
  Motor cortex excites vasomotor center. Anterior
  temporal lobe, orbital areas of frontal cortex,
  cingulated gyrus, amygdale, septum and
  hippocampus can also control vasomotor center.

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• c) Efferent link. Stimulation of sympathetic
  vasoconstrictor fibers through alfa-adrenoreceptor
  causes constriction of blood vessels.
  Stimulation of sympathetic vasodilator fibers
  through beta-adrenoreceptors as in skeletal
  muscles causes dilation of vessels.
• Parasympathetic nervous system has minor role and
  gives peripheral innervations for vessels of
  tong, salivatory glands and sexual organs.

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Mechanical receptors reflexes.

• These are spray-type nerve endings, which are
  stimulated by stretch. In increasing blood
  pressure, from the wall of carotid sinus impulses
  pass through Hering's nerve to glossopharyngeal
  nerve to solitary tract in medulla. Secondary
  signals inhibit vasoconstrictor center and excite
  vagal center. It results in peripheral
  vasodilatation and decreasing heartbeat. When
  arterial pressure decreases whole processes lead
  to exciting dorsolateral portion of vasomotor
  center and increasing blood pressure and
  heartbeat. Similar reflex mechanism starts from
  receptors of aortic arc.
• Bainbridge reflex is observed when arterial
  pressure increases due to increasing blood volume
  and blood return. Atria and SA node are stretched
  and send nerve signals to vasomotor center.
  Increasing heart rate and heart contractility
  prevent damming up of blood in pulmonary
  circulation.

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Reflexes from proprio-, termo- and
interoreceptors

• Contraction of skeletal muscle during exercise
  compress blood vessels, translocate blood from
  peripheral vessels into heart, increase cardiac
  output and increase arterial pressure.
  Stimulation of termoreceptors cause spreading
  impulses from somatic sensory neurons to
  autonomic nerve centers and so leads to changing
  tissue blood supply. Irritation of
  visceroreceptors results in stimulation of vagal
  nuclei, which cause decreasing blood pressure and
  heartbeat.

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Haemodinamic in special body conditions

• Changing body position.
• Change body position from vertical to horizontal
  and vice versa is followed by redistribution of
  blood. Under the influence of gravity veins in
  lower half of the body are dilated and may
  contain additional near 0,5 l of blood. After
  this impulsations from baroreceptors is activated
  and resistive vessels are contracted, mainly in
  skin and muscles. At the same time rate of
  heartbeat increases, which permit make up for
  cardiac output. In insufficient reflex regulation
  orthostatic unconsciousness may occur.

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Regulation of blood flow in physical exercises

• In physical exercises impulses from pyramidal
  neurons of motor zone in cerebral cortex passes
  both to skeletal muscles and vasomotor center.
  Than through sympathetic influences heart
  activity and vasoconstriction are promoted.
  Adrenal glands also produce adrenalin and release
  it to the blood flow. Proprioreceptor activation
  spread impulses through interneurons to
  sympathetic nerve centers. So, contraction of
  skeletal muscle during exercise compress blood
  vessels, translocate blood from peripheral
  vessels into heart, increase cardiac output and
  increase arterial pressure.

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Changing blood volume after bleeding.

• In changing blood volume volumic receptors in
  vena cava or atria are activated. These impulses
  spread to both medulla oblongata and osmolarity
  regulating neurons in hypothalamus. In
  consequence decreasing blood volume heart
  activity rises through sympathetic activation and
  vasopressin in released from hypophisis.

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Cerebral circulation

• Intensity of blood flow is 750 mL/min 54 mL/100
  g/min.
• Except for a small contribution from the anterior
  spinal artery to the medulla, the entire arterial
  inflow to the brain in humans is via 4 arteries
  2 internal carotids and 2 vertebrals. The
  vertebral arteries unite to form the basilar
  artery and the circle of Willis, formed by the
  carotids and the basilar artery, is the origin of
  the 6 large vessels supplying the cerebral
  cortex. Venous drainage from the brain by way of
  the deep veins and dural sinuses empties
  principally into the internal jugular veins in
  humans, although a small amount of venous blood
  drains through the ophthalmic and pterygoid
  venous plexuses, through emissary veins to the
  scalp, and down the system of paravertebral vein
  in the spinal canal.

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Regulatory influences

• The sympathetic fibers on the pial arteries and
  arterioles come from the cervical ganglia of the
  sympathetic ganglion chain. The parasympathetic
  fibers pass to the cerebral vessels from the
  facial nerve via the greater superficial petrosal
  nerve. Cerebral has autonomic circulation.
  Adenosine, histamine, serotonine, prostaglandines
  caused vasodilatation.

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Coronary circulation

• 2 coronary arteries that supply the myocardium
  arise from the sinuses behind the cusps of the
  aortic wave at the root of the aorta. The right
  coronary artery has a greater flow in 50 of
  individuals, the left has a greater flow in 20 ,
  and the flow is equal in 30 . There are 2 venous
  drainage systems a superficial system, ending in
  the coronary sinus and anterior cardiac veins,
  that drains the left ventricle and a deep system
  that drains the rest of the heart.

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Compressive influences role

• The heart is a muscle that compresses its blood
  vessels when it contracts. The pressure inside
  the left ventricle is slightly greater than in
  the aorta during systole. Consequently, flow
  occurs in the arteries supplying the
  subendocardial portion of the left ventricle only
  during diastole, although the force is
  sufficiently dissipated in the more superficial
  portions of the left ventricular myocardium to
  permit some flow in this region throughout the
  cardiac cycle.

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Metabolic and chemical factors significance

• Factors that cause coronary vasodilatation O2,
  CO2, H, K, lactic acid, prostaglandines,
  adenine nucleotides, adenosine. Asphyxia,
  hypoxia, intracoronary injections of cyanide all
  increase coronary blood flow 200-300 in
  denervated as well as intact hearts.

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Neural regulation

• The coronary arterioles contain alpha-adrenergic
  receptors, which mediate vasoconstriction, and
  beta-adrenergic receptors, which mediate
  vasodilatation. Activity in the noradrenergic
  nerves cause coronary vasodilatation.

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