Heart

• CV system:

  • heart = pump
  • arteries = outflow consuits
  • capillaries = drop off/pick up sites
  • veins = return flow conduits
• Circulatory circuit:

  • start at right atrium and end at right atrium
• Heart:

  four chambers:

  • left and right atria = receiving chmabers
  • left and right ventricles = pumping chambers

  right heart = pulmonary circulation:

  • pumps deoxygenated blood from body to lungs

  left heart = systemic circulation:

  • pumps oxygenated blood from lungs to body
• Cardiac muscle:

  • contracts as single unit
  • fibres interconnected via intercalated discs
  • electrically charged
• Coronary circulation:

  • primary blood supply to heart provided by coronary arteries from aorta
  • cardiac veins return deoxygenated blood to inferior and superior vena cava
• Heart and exercise:

  • heart generates pressure to drive oxygenated blood through vessels to skeletal muscles
  • driven by demands of muscle for O2

  what does it do:

  • removes CO2 and wastes
  • transports hormones and molecules
  • supports temperature balance and fluid regulation
  • maintains acid-base balance
• Matching systemic O2 supply with O2 demand:

  • VO2 = Q x a-vO2difference
  • Q = HR x SV
  • a-vO2difference = CaO2 - CvO2
  • important to increase Q with increase intensity because we would not be able to generate big enough VO2max
  • Q allows us to manage higher intensity
  • CvO2 decreases with increased intensity because more O2 is dropped off at tissue
• Q and O2 utilization:

  • as VO2 increases, Q increases in proportion
  • increase demand = increase CO2
  • ~ 6:1 ratio
  • for every 1 L increase in VO2, we get a 6 L output of CO2
• Cardiac output (Q):

  • total volume of blood pumped by ventricle each minute
  • influenced by HR and SV
• HR control:

  2 mechanisms to control HR:

  intrinsic control:

  • cardiac muscle generates own signal
  • pacemaker (SA node) establishes sinus rhythm
  • without external control (~100 bpm)

  extrinsic control:

  • systems modulate intrinsic
  • causes HR to speed up or slow down
  • adjusts HR to ~35-40 bpm at rest
  • maximal effort, HR = ~220 bpm
• Intrinsic regulation of HR:

  • SA node —> atria —> AV node —> purkinje fibres —> ventricles
  • SA node = spontaneous depolarization and repolarization to provide innate heart stimulus
  • AV node = delays impulse to provide time for atria to finish contracting and finish giving blood to ventricles
  • bundle of His
  • purkinje fibres = carry impulse rapidly through ventricles
• Extrinsic regulation of HR:

  3 systems:

  parasympathetic:

  • releases ACh to slow down HR

  sympathetic:

  • releases NE to speed up HR and increase contractility
  • innervates heart at SA node

  endocrine:

  • release of Epi to affect SA node and increase HR
• HR during exercise:

  • at low intensities, increase in HR due to decrease in PSNS activity
  • as intensity increases, increase in HR due to SNS activation
• HR vs O2 uptake:

  • tight connection between HR, Q, and exercise intensity up to VO2max
• SV:

  • amount of blood left in heart after contraction
  • EDV - ESV = SV

  ejection fraction (EF):

  • percent of EDV pumped
  • SV/EDV = EF
• Components of SV:

  preload:

  • volume of blood in heart during diastole (before contraction)
  • end-diastolic filling

  Frank Starling Law of Heart:

  • relationship between contractile and resting length of heart muscles
  • force of contraction equals initial muscle length
  • preload stretched ventricles in diastole to produce more forceful ejection
• Components of SV:

  contractility/inotropy:

  • enhanced contractile force to increase SV and facilitate emptying

  myocardial contractility:

  • inotropy = increasing muscle tension for given preload and rate of muscle tension development
  • increase inotropy = increase SV

  During exercise, increase inotropy:

  • increases SNS
  • decreases PSNS
  • increases endocrine (catecholamines)
• Components of SV:

  afterload:

  • Pressure heart must generate to open aortic valve
  • increase afterload = more pressure
  • decrease afterload = less pressure
  • exercise = reduction in afterload which causes increase in SV, Q, and O2
• SV vs O2 uptake:

  • rapid rise in SV due to decreased afterload, increased inotropy, and increased preload
  • SV plateaus as exercise continues
  • increase exercise = increase HR, which decreases the amount of time we have to fill the heart, therefore decreasing slope at which SV rises as intensity increases = plateau
• Qmax and VO2max:

  • increase Q = increase VO2max
  • endurance athletes that have high VO2max also have high Q
  • HR = limiting factor for ability to sustain higher intensities
• Fick principle:

  • increase in VO2 is determined by Q
  • VO2max related to maximum ability of heart to pump blood
• Central delivery of O2 blood:

  • SV fills chambers during diastole —> increases with increased intensity
• CV system:

  • arteries = carry blood away from heart
  • arterioles = control blood flow
  • capillaries = area of exchange
  • venules = collect blood from capillaries
  • veins = carry blood to heart
• Arteries and arterioles:

  • high-pressure tubing
  • pressure = high on arterial side, low on venous side
  • connect left ventricle to tissue
  • walls contain circular layers of smooth muscle that constrict and relax to control blood flow
  • innervated by SNS (NE released = constriction)
  • no gas exchange
• Capillaries:

  • exchange
  • gas, nutrients, and waste products
  • velocity decreases as blood moves to and through capillaries because blood is moving away from area of high pressure (heart) to low pressure (veins)
• Blood flow and VO2:

  • they follow each other
  • as one rises/lowers, the other rises/lowers
  • blood flow and VO2 rise to match O2 demands
  • relationship with metabolic rate of muscles
• Hemodynamics:

  pressure:

  • force that drives flow
  • provided by heart contraction
  • from region of high pressure to low pressure (no gradient = no flow)

  resistance:

  • force that opposes flow
  • provided by physiological properties of vessels —> pressure differential
  • modification of vessel radius
• Blood flow (Q):

  • total volume of blood pumped through vessel per minute
  • Q = MAP / TPR
  • increase pressure and decrease resistance = increase Q
  • decrease pressure and increase resistance = decrease Q
• MAP:

  P = arterial pressure - venous pressure

  arterial pressure = 2/3 DBP + 1/3 SBP
• Arterial blood pressure:

  • systolic = highest pressure in artery during contraction
  • diastolic = lowest pressure in artery during relaxation
  • mean arterial pressure (MAP) = average pressure on arterial vessel walls over entire cardiac cycle
• Facts about pressure:

  • diastolic pressure stays the same with varying intensities
  • but systolic pressure changes (ex. increases with greater intensity)
• MAP:

  • venous pressure = CVP (central venous pressure)
  • represents blood coming back to heart
• Venous blood pressure:

  CVP:

  • blood pressure taken in vena cava or right atrium pressure
  • reflects amount of blood returning to heart and ability of right heart to pump blood into pulmonary circulatory
  • range = 0-8 mmHg.
  • right atrial pressure doesn't change much during exercise
• TPR:

  resistance = n x L x r^4

  • we only really manipulate radius (r^4) because viscosity (n) and length of vessel (L) don’t change as often
  • r^4 has a powerful impact on resistance
  • increase r^4 = decrease resistance
  • decrease r^4 = increase resistance
• TPR:

  • resistance to Q offered by all systemic vasculature, excluding pulmonary vasculature

  3 factors affect resistance of Q:

  • Poiseuille’s Law —> n, L, r^4
  • we can only acutely change r^4 (vasoconstriction and vasodilation)
  • arterioles = resistance vessels (they control TPR; responsible for 70-80% of pressure drop from left ventricle to right atrium)
  • TPR = MAP / Q
• Blood flow and exercise:

  • at rest, muscles receive ~ 1L/min of Q
  • with increased intensity, blood to muscle rises (80-85% of total Q)
  • brain always receives steady flow of blood
  • vessels constrict to redirect blood from areas that do not need it to areas that do (kidneys and splanchic)
• Distribution of blood flow:

  • blood to sites that most need it

  at rest:

  • liver and kidneys receive 50% of Q
  • muscles receive 20% of Q

  exercise:

  • muscles receive 80% of Q
• Q and muscle blood flow:

  • active muscle = decrease resistance
  • non-active muscle = increase resistance
• Muscle blood flow:

  • resistance = most powerful tool to modify blood flow
• Arterioles and smooth muscle:

  • arterioles have strong muscular walls of smooth muscle
  • smooth muscle uses cross-bridge cycling, actin and myosin, and Ca2+
  • increase Ca2+ = contraction
  • decrease Ca2+ = relaxation
  • innervated by SNS and controlled by hormones