cardiovascular system physiology

• Cardiovascular system


  • 1. The heart (the pump). The heart is located in the thoracic cavity, separated from the abdominal cavity by the diaphragm
  • 2. Blood vessels (series of tubes)
  • 3. Blood (fluid medium, accounting for 7-8% of body weight)

  
• Functions of the cardiovascular system


  • Transport of substances
  • Oxygen and nutrients to cells
  • Wastes from cells to liver and kidneys
  • Hormones, immune cells, and clotting proteins to specific target cells
• Heart size and weight


  Size of a fist, weighs 250-350 grams
• Pericardium


  Membranous sac surrounding the heart, lubricates the heart and decreases friction

  
• Layers of the heart wall


  • Epicardium (outer layer)
  • Myocardium (middle layer)
  • Endocardium (inner layer)

  
• Myocardium


  • Concentric layers of cardiac muscle tissue
  • Atrial myocardium wraps around great vessels

  
• Endocardium


  Epithelium, provides protection for valves and heart chambers
• The wall of the left ventricle is thicker than the wall of the right ventricle
• Valves and unidirectional blood flow


  • Pressure difference drives blood flow from high pressure to low pressure
  • Valves prevent backward flow of blood
• Atrioventricular (AV) valves


  • Right AV valve = tricuspid valve
  • Left AV valve = bicuspid valve = mitral valve
• Papillary muscles and chordae tendinae


  • Keep AV valves from everting
• Semilunar valves


  • Aortic valve
  • Pulmonary valve
• Types of cardiac muscle cells (cardiocytes)


  • Contractile cells (myocardium)
  • Autorhythmic cells
• Contractile myocardial cells


  • Small, bifurcate, single centrally located nucleus, aerobic, high in myoglobin and mitochondria, extensive blood supply, involuntary but contract similar to skeletal muscle
  • Must contract together to generate enough force
• Characteristics of cardiac muscle


  • Striated, contain sarcomeres, have short, wide T tubules, less SR with no terminal cisternae, under SNS and PNS control, single nucleus, have intercalated discs to connect cells
• Intercalated discs


  Have gap junctions to electrically connect adjacent cardiocytes, and desmosomes to provide 'glue' that holds the cells together
• The conducting system


  1. Sinoatrial node (pacemaker)

  2. Atrioventricular node

  3. Internodal pathways

  4. Bundle of His

  5. Purkinje fibers
• The spread of excitation between cells coordinates the contraction of the atria and then the ventricles
• Initiation and conduction of an impulse


  1. AP initiated in SA node, signals spread through atrial muscle via interatrial pathways

  2. Signal travels to AV node via internodal pathway, AV nodal delay

  3. Atrioventricular bundle (bundle of His)

  4. Splits into left and right bundle branches

  5. Purkinje fibers
• Autorhythmic cells


  • Cells that depolarize themselves without an external stimulus
  • No steady resting potential
  • After an action potential, depolarization begins again slowly
  • This initiates another action potential
  • Autorhythmic cells can initiate action potentials in the heart, but the central nervous system regulates and modulates the heart rate and force
• Ionic basis of autorhythmic cell electrical activity


  • Na+, K+, and Ca2+ channels with unusual characteristics
  • Leak K+ out and Na+ in, making the membrane more positive
  • Ca2+ then contributes the remaining depolarization
• 1. type channels


  • Contribute to the slow depolarization (pacemaker potential)
  • Contribute to the rapid depolarization (action potential)
• Cardiac contractile cell action potential


  • Long duration: 250-300 ms (skeletal muscle: 1-2 ms)
  • Five phases of electrical activity
• Phases of cardiac contractile cell action potential


  1. Phase 0: increased permeability to Na+

  2. Phase 1: decreased permeability to Na+

  3. Phase 2: increased permeability to Ca2+, decreased permeability to K+

  4. Phase 3: increased permeability to K+, decreased permeability to Ca2+

  5. Phase 4: resting membrane potential
• Myocardial action potentials


  • Twitch summation is not possible
  • Long absolute refractory period
  • Tetanus is not possible
  • Relaxation is imperative for heart filling
  • Myocardial action potential duration is much longer than neuron or skeletal muscle (200 ms vs 1-5 ms)
• Excitation-contraction coupling in cardiac muscle


  Similar to skeletal muscle (T-tubules, sarcoplasmic reticulum Ca2+, troponin-tropomyosin regulation)

  Similar to smooth muscle (gap junctions, extracellular Ca2+)
• Steps to Excitation-contraction coupling in cardiac muscle


  1. Depolarizing current spreads through gap junctions

  2. Action potentials travel along plasma membrane and T-tubules

  3. Ca2+ channels open in plasma membrane and sarcoplasmic reticulum

  4. Ca2+ induces Ca2+ release from sarcoplasmic reticulum

  5. Ca2+ binds to troponin, exposing myosin-binding sites

  6. Crossbridge cycle begins, muscle fiber contracts

  7. Ca2+ is actively transported back into the sarcoplasmic reticulum and extracellular fluid

  8. Tropomyosin blocks myosin-binding sites, muscle fiber relaxes
• Cardiac cycle and heart rate


  • At 72 bpm, cardiac cycle lasts about 800 ms (0.8 s)
  • When heart rate increases, all phases shorten, particularly diastole
  • Systole (contraction) is ~0.3 s
  • Diastole (relaxation) is ~0.5 s, about 65% of the cycle duration and 2/3 at rest
• Excitation-contraction coupling in cardiac contractile cells


  • Properties similar to skeletal muscle
  • Properties similar to smooth muscle
• Electrical communication


  Begins with AP in autorhythmic cell, depolarization spreads through gap junctions in intercalated disks
• Cardiac cycle


  Events associated with the flow of blood through the heart during a single complete heartbeat
• Phases of cardiac cycle


  • Systole: ventricle contraction
  • Diastole: ventricle relaxation
• Valves


  Open passively due to pressure gradients, AV valves open when atrial pressure > ventricular pressure, Semilunar valves open when ventricular pressure > arterial pressure
• Systole


  Contraction: ~0.3 sec
• Diastole


  Relaxation: ~0.5 sec, about 65% of duration, 2/3 time at rest
• Phases of cardiac cycle


  1. Phase 1 - Ventricular filling (DIASTOLE): blood returning to the heart enters relaxed atria, passes AV valves, into ventricles

  2. Phase 2 - Isovolumetric contraction (SYSTOLE): ventricles begin to contract, AV valves close, semilunar valves still closed, no blood flow, pressure continues to increase

  3. Phase 3 - Ventricular ejection (SYSTOLE): blood is expelled from ventricles, pressure reaches a peak, ventricular volume decreases

  4. Phase 4 - Isovolumetric relaxation (DIASTOLE): heart is resting, some blood still in ventricles, remains constant, pressure still present
• Atrioventricular valves


  Open in phases 1 and 3, closed in phases 2 and 4
• Aortic and pulmonary (semilunar) valves


  Open in phase 3, closed in phases 1, 2 and 4
• Heart sounds


  Produced by the opening and closing of the heart valves
• Electrocardiogram


  Measures the electrical activity of the heart