The heart

• Atrioventricular (AV) valves

  • When open, cusps of valve push into the ventricles allowing blood to flow from atrium to ventricles
  • When ventricles contract, pressure in ventricles increases pushing blood back toward the atria thus, pushing the cusps of valves back towards the atria closing the valves

  
• Semilunar valves

  • Include aortic and pulmonary valves, which allow blood to pass from the ventricles into the aorta and pulmonary veins
  • As pressure in the ventricles exceed that in the arteries, blood passes from the heart to the arteries
  • As ventricles relax, backflow of blood catches the cusps causing these valves to close preventing movement of blood to the ventricles

  
• Cardiac muscle

  • Involuntary, striated muscle
  • MF connect with neighboring fibers via intercalated discs (thickening of sarcolemma)

  
• Intercalated discs

  • Contain desmosomes that hold fibers together
  • Possess gap junctions that allow AP to move among cardiac MF and allow cardiac MF to act as syncytium, thus atria and ventricles contract as a unit
• Heart conduction system

  • Heart contains specialized cardiac MF that can self-generate an action potential (autorhythmic fibers)
  • Autorhythmic cells act as a pacemaker, establishing the basic electrical activity of the heart
  • Helps the heart pump in a coordinated manner so that blood can be pumped throughout the body

  
• Heart conduction system

  1. Sinoatrial (SA) node

  2. Atrioventricular (AV) node

  3. Atrioventricular (AV) bundle

  4. Purkinje fibers (PF)

  
• Sinoatrial (SA) node

  • Located in right atrial wall which spontaneously depolarize 75 times/min
  • As a pacemaker, it establishes the sinus rhythm
  • SA node depolarization generates AP which is propagated throughout the atria

  
• Atrioventricular (AV) node

  • Located in inferior portion of interatrial septum, above tricuspid valve
  • Delays AP about 0.1 sec before it travels to the ventricles
  • A delay which allows the atria to complete their contraction before the ventricles contract
• Atrioventricular (AV) bundle

  1. AP moves to AV bundle (bundle of His) in the superior portion of interventricular septum

  2. AP continues to the right and left branches

  3. AP proceeds through inferior portion of interventricular septum to the apex of the heart
• Purkinje fibers (PF)

  1. From R and L branches of bundle of His, AP continues to Purkinje fibers

  2. PF supply the papillary and ventricular muscles

  3. AP proceeds to heart apex to the outer walls of ventricles toward the atria
• Factors affecting SA node

  • Epinephrine (SNS) accelerates SA node
  • Acetylcholine (PNS) slows down SA node
• Mechanisms of heart contraction

  1. Depolarization

  2. Plateau

  3. Repolarization

  4. Refractory period

  
• Depolarization

  As cardiac MF is stimulated by AP, voltage-gated fast sodium channels will open followed by rapid influx of sodium from ECF, depolarization of cardiac MF from -90mV to +30mV, and sodium channels become inactivated and close

  
• Plateau

  Voltage-gated slow calcium channels (sarcolemma and SR) open which result to influx of calcium from extracellular space (20%) causes release of calcium from SR (80%), membrane permeability to potassium decreases, and membrane remains depolarized (0mV for about 0.25 sec)
• Repolarization

  Opening of voltage-gated potassium channels, potassium ions flow out of the cell and the membrane repolarizes, cell returns to resting membrane potential (-90mV)
• Refractory period

  Time during which the next contraction cannot be triggered, longer in cardiac muscle compared to skeletal muscle, prevents cardiac muscle from developing tetanus and allows heart to act as an effective pump
• Electrocardiogram (ECG or EKG)

  Propagation of AP through the heart produces electrical currents that can be detected on the surface of the body, recording of these electrical activities

  
• P wave

  Small upward deflection reflecting atrial depolarization, generated as SA node depolarizes and AP spreads throughout the atria
• QRS complex

  Begins with downward deflection, then rises sharply and ends with a downward deflection, represents ventricular depolarization, movement of wave of depolarization through the ventricle changes direction throughout the wave
• T wave

  Represents ventricular repolarization, longer than QRS complex
• Cardiac cycle

  1. Mid-to-late diastole

  2. Atrial systole

  3. Ventricular systole

  4. Early diastole
• Mid-to-late diastole

  Heart is relaxed, blood returns to the atria and into the ventricles, ventricular filling occurs (about 70%)

  
• Atrial systole

  Begins with SA node depolarization causing AP spread throughout the atria, generation of P wave, remaining 30% of blood is forced through AV valves to the ventricles, volume of blood in the ventricles is called end diastolic volume (EDV)

  
• Ventricular systole

  As the atria relaxed, the ventricles contract generating the QRS complex, as volume in the ventricles decreases, ventricular pressure increases closing the AV valves, all heart valves are closed resulting in isovolumetric contraction phase, ventricular pressure exceeds the pressure in large arteries, thus semilunar valves open leading to ventricular ejection phase

  
• Early diastole

  Immediately following T wave, ventricles relax leading to end systolic volume (ESV) or the amount of blood remaining in the ventricles, as ventricular pressure decreases, blood in the aorta and pulmonary arteries begins to return to the heart, closure of semilunar valves causing transient increase in aortic blood pressure (dicrotic notch)

  
• Heart sounds

  Auscultation (listening to body sounds with a stethoscope), first heart sound (lub, closing of AV valves which occurs at beginning of systole, louder and longer than the second sound), second heart sound (dup, closing of semilunar valves occurring at the beginning of ventricular diastole)

  
• Heart murmurs


  Include clicking, rushing or gurgling sounds, indicate a valve disorder, if valve is stenotic, a click may be audible
• Cardiac output (CO)

  Amount of blood pumped by either the right or left ventricle per minute, equal to stroke volume (SV), the amount of blood pumped by the ventricle per heart beat multiplied by the heart rate, cardiac reserve is the difference between an animal's maximum CO and its resting CO

  
• Regulation of heart rate (HR)

  1. Factors that increase HR (positive chronotropic factors)

  2. Factors that decrease HR (negative chronotropic factors)

  3. Most important factor (ANS)

  
• ANS regulation of HR

  Cardiovascular center (CVC) in medulla oblongata influences HR, CVC receives input from sensory receptors, limbic system and cerebral cortex, CVC directs output from both PNS and SNS

  
• ANS regulation of HR

  CVC receives sensory inputs from proprioreceptors, chemoreceptors, and baroreceptors
• SNS activation of CVC

  CVC stimulate HR via fibers from SC, activate cardiac accelerator nerves (SC), affect SA node, AV node, and myocardium

  
• Sympathetic nerve fibers

  Release norepinephrine (NE) which binds to β1 adrenergic receptors in the heart, NE accelerates rate of depolarization of SA node, NE increases calcium influx into cardiac myofibers thus increasing contractility

  
• PNS activation

  PNS sends signals to the heart via vagus nerve, vagus nerve terminates in SA node, AV node, and atrial myocardium, acetylcholine decreases rate of depolarization of SA node

  
• Chemical regulation of HR

  Hormones (E, NE, and thyroid hormones) increase HR and contractility, Cations (elevated blood sodium and potassium concentrations) decrease HR and contractility

  
• Blood pressure

  Hydrostatic pressure exerted by blood against the blood vessel wall, highest in the arteries and decreases as blood moves through the circulatory system

  
• Mechanisms which control blood pressure

  1. Neural regulation

  2. Chemical regulation (Short-term control)

  3. Renal regulation (Long-term control)

  
• Neural regulation

  CVC sends sympathetic signals (increase HR and contractility), parasympathetic signals from CVC (decrease HR and contractility), CVC sends signals to blood vessels via vasomotor nerves (vasomotor tone)

  
• Neural regulation (CVC)

  1. Baroreceptor reflex

  2. Chemoreceptor reflex

  
• Baroreceptor reflex

  Pressure-sensitive mechanoreceptors in carotid sinuses, aortic arch and walls of most large arteries in the neck and thorax, in response to stretch, baroreceptors send signals inhibition of CVC, vasodilation and decrease in BP, important in making rapid adjustments in BP in response to acute changes