Cardiovascular Physiology Part 1

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Cardio-vascular PhysiologyPart 1

• Circulation
• The Heart

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Circulation Introduction

• In small animals lt 1 mm transport of materials
  totally by diffusion
• In larger animals, transport orchestrated by
  varying levels of organization circulatory
  system nutrients, gases, wastes, hormones,
  antibodies, salts, etc. moved in blood (a complex
  tissue containing specialized cells)

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General Plan

• Main propulsive organ (e.g. heart) forces blood
  through body
• An arterial system that distributes blood acts
  as a pressure reservoir
• Capillaries transfer material between blood and
  other tissues
• A venous system that acts as blood storage
  reservoir as a system for returning blood to
  heart
• 1 major organs leaving central circulation 2
  through 4 peripheral circulation

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Mechanisms Movement of Blood

• Forces imparted by rhythmic contractions of the
  heart
• Elastic recoil of arteries following filling by
  the action of the heart
• Squeezing of blood vessels during body movements
• Peristaltic contractions of smooth muscle
  surrounding blood vessels
• Importance of each varies with the species

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Movement of Blood cont

• Valves or septa determine direction of blood flow
• smooth muscle surrounding blood vessels alters
  vessel diameter
• regulating amount of blood that flows through a
  particular pathway controlling the distribution
  of blood within the body

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Open Circulation

• E.g. invertebrates blood pumped by heart
  empties via an artery into an open, fluid-filled
  space hemocoel lying between ectoderm
  endoderm
• Fluid contained within hemocoel hemolymph not
  circulated through capillaries but bathes tissues
  directly
• Pressures open circulation low blood volume
  20-40 of body volume

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Closed Circulation

• Blood flows in a continuous circuit from arteries
  to veins through capillaries I.e. all vertebrates
  and some invertebrates such as cephalopods
  (octopus/squid) fig 12-3 p. 476
• More complete separation of function
• Blood volume generally about 5-10 body volume
• Heart propulsive organ maintains BP

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Closed Circulation cont

• Arterial system acts as a pressure reservoir
  forcing blood through capillaries
  (microcirculation)
• Capillaries thin walls allowing high rates of
  transfer of material between blood tissues by
  diffusion, transport or filtration
• Each cell is no more than 2-3 cells away from
  capillary cap networks have many branches in
  parallel allows fine control of blood
  distribution O2 delivery to tissues

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Closed Circulation cont

• Ultrafiltration separation of an ultrafiltrate
  (fluid devoid of colloidal protein particles )
  from blood plasma by filtration through a
  semi-permeable membrane (I.e. cap. wall) using
  pressure (BP) to force fluid through the membrane
• Lymphatic system evolved to recover fluid lost
  to tissues from blood via ultrafiltration
  returns it to the venous system

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Closed Circulation cont

• Permeability of caps varies among tissues as does
  BP with circulatory conditions e.g. liver high
  permeability permits rapid transfer of substrates
  products of metabolism pressures are lower
  than rest of body vs. low pressure in lung
  capillaries would reduce filtration into the gas
  space of lungs which would impair gas transfer

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Closed Circulation cont

• Systemic circuit circulates blood thru body
• Respiratory (or pulmonary) circuit circulates
  blood to organs of gas exchange
• Mammals maintain different pressures in these two
  circuits because they are equipped with a
  completely divided heart i.e. right side pumps
  blood through pulmonary circuit left side
  through systemic circuit

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Closed Circulation cont

• Venous system collects blood from capillaries
  delivers it to heart via veins typically
  low-pressure, flexible structures (large changes
  in blood volume have little effect on venous
  pressure) contains most blood reservoir

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The Heart

• Valved, muscular pumps propel blood around body
  fig. 12-4 p. 477
• 1 or more muscular chambers connect in series
  guarded by valves (some animals sphincters e.g.
  mollusks) allowing blood to flow in only one
  direction
• Mammalian 4 chambered 2 atria 2 ventricles
  contraction of heart ejection of blood

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Cardiac Muscle more info

• Presence of gap junctions electrically coupled
  to each other except for the uptake release
  of Ca2, similar to skeletal muscles
• Myocarium - heart muscle 3 types of fibers
• Sinus node (aka sinoatrial node)
  atrioventricular node smaller, autorhythmic,
  weakly contractile exhibit very slow electrical
  conduction between cells

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Cardiac Muscle more info

• Largest myocardial cells inner surface of
  ventricular wall, also weakly contractile but
  specialized for fast electrical conduction
  constitute system for spreading excitation over
  heart
• Intermediate-sized myocardial cells strongly
  contractile constitute the bulk of heart

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Electrical Properties of the Heart

• Heartbeat rhythmic contraction (systole)
  relaxation (diastole) of whole cardiac muscle
• Electrical activity initiated in pacemaker region
  spreads over heart from 1one cell to another
  since cells electrically coupled by gap junctions
• Sinoatrial node location of pacemaker a group
  of small, weakly contractile, specialized muscle
  cells capable of spontaneous activity either
  neurogenic (most invertebrates) or myogenic
  (vertebrates some invertebrates)

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Electrical Properties of the Heart

• Myogenic pacemakers may be several cells with
  capacity to stimulate heart beat but those with
  fastest intrinsic activity generally determine HR
  if main pacemaker stops, other cells can take
  over
• Cardiac pacemaker potentials NB pacemaker
  cells lack stable resting potential after AP,
  pacemaker membrance undergoes a steady
  depolarization pacemaker potential

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Cardiac Potentials cont

• Pacemaker potential brings membrane to threshold
  potential usually inless than a second, giving
  rise to another all-or-none cardiac AP
• Interval between APs determines HR depends on
  rate of pacemaker potential extent of
  repolarization threshold potential for cardiac
  AP fig 12-5 p. 478

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Cardiac Potentials cont

• Only small currents needed to change pacemaker Vm
• Origin of activity interaction between several
  time-dependent voltage dependent membrane
  currents in combination with time-independent
  background currents I.e. at least 6-time
  voltage-dependent ligand-operated K channels,
  several different Ca2 Na channels found in
  pacemaker cells

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Cardiac Potentials cont

• Result pacemaker can funciton for many years
  without interruption
• Ach (from ParaSym terminals of vagus nerve Xth
  cranial nerve) innervates the heart I.e. slows
  HR by increasing K conductance reducing Ca2
  conductance of pacemaker cells vs. norepinephrine
  (Sym NS) accelerates pacemake potential
  increasing HR

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Cardiac APs

• Skeletal muscle AP is completed membrane in
  nonrefractory state before onset of contraction
  repetitive stimulation tetanic contraction
  possible
• Cardiac muscle AP reaches a plateau which
  remains for 100s milliseconds membrane remains
  in refractory state until heart has returned to a
  relaxed state summation of contractions does
  not occur in cardiac muscle fig. 12-7 p. 479

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Cardiac APs

• Long cardiac AP produces prolonged contraction
  entire heart chamber can fully contact before
  any portion begins to relax essential for
  efficient pumping of blood
• Duration of plateau phace rates of
  depolarization repolariza6ion varies among
  different cells summation of these changes
  recorded by an ECG characteristic pattern of
  electrical activity

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ECGs fig 12-8 p. 480

• Initial P-wave depolarization of atrium
• QRS complex depolarization of ventricle
• T-wave repolarization of ventricle
  (repolarization of atrium is obscured by huge QRS
  complex)
• Varies with species nature/position of
  recording devices nature of contraction

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Transmission of excitation

• Electrical activity initiated in pacemaker region
  is conducted over entire heart as depolarization
  in 1 cell results in depolarization in
  neighboring cells by virtue of current flow
  through gap junctions
• Transmission generally unidirectional because
  impulse spreads away from pacemaker region

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Transmission of excitation cont

• Mammals wave of excitation begins sinoatrial
  node spreads over both atria in concentric
  fashion
• Atria connected electrically to ventricles
  through atrioventricular node on right side of
  heart (in other regions, atria ventricles
  joined only by CT which is nonconductive) fig
  12-4 p. 477
• Excitation spreads via junctional fibers
  connected to nodal fibers connected via
  transitional fibers to bundle of His

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Transmission of excitation cont

• Bundle of His branches into R L bundles which
  subdivide into Purkinje fibers extending into
  myocardium of 2 ventricles (causing all regions
  of ventricular myocardium to contract together
  from internal lining (endocardium) to external
  covering (epicardium)

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Transmission of excitation cont

• Functional significance electrical organization
  of myocardium is its ability to generate
  separate, synchronous contraction sof 1st atria
  then ventricles (1st by slow condcution through
  atrioventricular node allowing atrial contraction
  sto precede ventricular contractions and time for
  blood to move from atria to ventricles

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Catecholaminesepinephrine norepinephrine

• 3 distinct positive effects on heart
• Increase rate of myocardial contractions (HR)
  positive chronotropic effect (mediated via
  pacemaker)
• Increase force of myocardial contraction
  positive inotropic effect
• Increase speed of conduction of wave of
  excitation over heart positive dromotropic
  effect

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Mechanics - terms

• Cardiac output (CO) volume of blood pumped per
  unit time from a ventricle refers to either R
  or L ventricle not combined
• Stroke volume (SV) volume of blood ejected from
  ventricle by each beat (mean stroke volume CO
  divided by HR) SV difference between volume
  of blood in ventricle just before contraction
  (end-diastolic volume) volume in ventricle at
  end of contraction (end-systolic volume)

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Cardiac Cycle p. 484-485 fig 12-12

• During diastole aortic valves are closed
  maintains large pressure difference between
  relaxed ventricles aorta pulmonary artery
  atrioventricular (AV) valves are open blood
  flows directly from venous system into ventricles
• Atria contract pressures within them rise
  blood ejected from them into ventricles

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Cardiac Cycle cont

• Ventricles begin to contract, ventricular
  pressure rise exceed that in atria AV valves
  close, preventing backflow of blood into atria
  ventricular contraction proceeds both AV
  aortic valves are closed so that ventricles form
  sealed chambers no volume change (ventricular
  contraction is isometric)

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Cardiac Cycle cont

• Pressures within ventricles increase rapidly
  exceed those in systemic pulmonary aortas
  aortic valves are pushed open blood ejected
  into aortas resulting in decrease in ventricular
  volume
• As ventricles relax, intraventricular pressures
  fall below pressures in aortas, aortic valves
  close there is isometric relaxation of
  ventricle (once once this happens, AV valves
  pushed open cycle starts again

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Cardiac Cycle final points

• Mammals volume ob flood forced into ventricle
  by atrial contraction 30 of volume of blood
  ejected into aorta by ventricular contraction
  ventricular filling is largely determined by
  venous filling pressure (which forces blood from
  venous system directly through atria into
  ventricles

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2 phases of contraction

• Isometric contraction - tension in muscle
  pressure in ventricle increase rapidly
• Isotonic contraction - tension doesnt change for
  as soon as aortic valves open, blood is ejected
  rapidly from ventricles into arterial system with
  little increase in ventricular pressure
• tension is generated first with almost no
  change in muscle length, then muscle shortens
  with little change in tension

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Coronary Circulation

• Supplies nutrients O2 to heart
• Extensive blood supply
• Cardiac muscle higher capillary density more
  mitochondria than most skeletal muscles
• High myoglobin content (typical red coloration)
• Heart relies on aerobic pathways to generate
  energy very dependent on O2 supply
• Adenosine key metabolite maintaining
  relationship between coronary flow cardiac
  activity

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Pericardium

• CT membrane surrounding the heart
• Magnitude of pressure changes within pericardial
  cavity depends on rigidity of pericardium on
  magnitude rate of change of heart volume