heart physiology

• Nodal tissues


  Modified cardiac muscle with 3 primary characteristics: Autorhythmicity, Excitable, Conduction of impulses

  
• Cardiac conduction system


  1. Sinoatrial Node

  2. Atrioventricular Node

  3. Bundle of His

  4. Left & Right Bundle branches

  5. Apex

  6. Purkinje Fibers

  
• Sinoatrial node (SA)


  Initiates beat at rate of 60-80 bpm, Pacemaker potential with low resting potential, membrane permeable to Na+, Na+ leaks into cell = decay, Depolarization, Repolarization, Hyperpolarization

  
• Cardiac conduction system


  1. Depolarization wave from SA node contracts atria from top down

  2. Atrioventricular node (AV) intercepts signal from nodal tissues in atria, delays signal to allow atrial contraction to maximize ventricular filling, relays depolarization wave to AV bundle (Bundle of His)

  3. Bundle of His (a.k.a. AV bundle) at junction of atria and ventricles, A-V bundle branches carry impulse to apex of heart

  4. Purkinje fibers carry impulse to ventricular myocardium, Depolarization wave from apex up, Ventricle contracts from bottom up

  
• Cardiac muscle


  • Anastomoses: branched cells
  • Intercalated discs: modified gap junctions, shared cytoplasm, quick transmission of impulse

  
• Cardiac muscle


  • Slower contraction speed than skeletal muscle, fewer myofibrils
  • All-or-none response
  • Slow refractory period, 9X longer than skeletal muscle, Plateau effect - slow Ca+2 channels allow slow leak of calcium, Counteracts K+ outflow, Extends into relaxation phase, Insures time for ventricles to fill, No summation, voltage-gated K+ channels cause rapid repolarization
• Frank-Starling Law of the Heart


  The more the cardiac fibers stretch, the stronger the contraction, More blood will be pumped out with greater filling, Less filling, lighter contraction, Regulates for different end diastolic volumes, Allows adjustment for right vs. left ventricle

  
• Cardiac Output


  Volume of blood pumped per minute by each ventricle, CO = HR x SV, At rest: HR = 70 bpm, SV = 75 ml/beat, CO = 5.25 liters/min
• Factors affecting Cardiac Output


  • Exercise: ↑ HR (120 bpm), ↑ SV (120 ml/beat), ↑CO = 14.5 liters/min
  • Chronic tachycardia: Resting HR ~ 170-250 bpm, CO will diminish due to decreasing SV
  • Venous return: Influences end-diastolic volume, Frank-Starling Law of the Heart
  • Peripheral resistance: Increase resistance lowers stroke volume, ↑ resistance = ↑ blood pressure, Vasoconstriction ↑ peripheral resistance, vasodilation ↓ peripheral resistance, Healthy heart will compensate via Frank-Starling Law, Congestive heart failure - inability to compensate
  • Neurotransmitters (ANS): Norepinephrine affects βeta-receptors, increases HR, Acetylcholine decreases HR
  • Chemicals: Chronotropic - affect HR, Inotropic - affect SV

  
• Cardiovascular structures


  • Elastic arteries - large, dampen shock of ventricular systole, recoil to propel blood forward
  • Muscular arteries - medium, regulate blood pressure, respond to vasomotor commands, thick tunica media
  • Arterioles - smallest arteries, lead into capillaries, pre-capillary sphincter
  • Capillary bed - area of majority of gas & nutrient exchange via diffusion, extensive branching, ~5% of blood, more filtration at arteriole end, osmotic return at venule end
  • Venules - drain capillary beds
  • Medium veins
  • Large veins - venae cavae

  
• Blood Flow


  Volume of blood that passes through a vessel at a given time
• Pressure


  Energy exerted on the vessel wall by the blood
• Resistance


  Friction generated by blood as it moves past the vessel walls, Flow = pressure / resistance, ↑diameter by 2 = cut resistance in half, ↓ diameter by 2 = ↑ resistance by 2
• If we combine ALL capillaries the total cross section is greater than that of any other vessel type, Low resistance, lowest flow rate = ↑ diffusion time
• Blood Flow Factors


  • Cardiac output - establishes pressure and flow of blood, relationship between volume and pressure
  • Peripheral resistance - related to friction, vasodilation decreases resistance, vasoconstriction increases resistance, athrosclerosis increases resistance
  • Elasticity of arteries - arteriosclerosis "hardening of arteries", less recoil to push blood forward, increased pressure with constant volume
  • Circulating volume - fluid retention increases volume, increases blood pressure, increase in salts increases blood pressure
  • Viscosity - thickness of blood, thicker blood increases pressure, polycythemia increased RBC's, blood doping & EPO
• Regulation of Blood Pressure

  • Baroreceptors - aortic arch and carotid sinuses, vasomotor and cardiac control centers in medulla oblongata
  • Kidneys - regulation of urine volume and concentration, Renin – Angiotensin – Aldosterone cycle