The Heart

• Arteries: Carry blood away from the heart
• Veins: Return blood to the heart
• arteries carry oxygenated blood and veins carry deoxygenated blood except for in the pulmonary circulation
• the pulmonary artery carries deoxygenated blood to the heart for oxygenation which is returned to the heart via the pulmonary vein
• Pulmonary circulation: The closed circulation of blood between the heart and lungs
• Systemic circulation: The circuit of vessels carrying blood from the heart and other body systems
• The heart contains four chambers that divide the heart horizontally and vertically
• The left and right atria become passively filled with blood
• The left and right ventricles project blood
• Unidirectional flow of blood within the heart is facilitated by the opening and closing of four valves
• What are the four valves of the heart?

  Left and right AV valves, semilunar valves
• The heart is predominantly composed of cardiac muscle, but also have autorhythmic cells that set the heart beat
• The septum is dividing muscle that prevents oxygenated and deoxygenated blood from mixing within the heart
• Atrioventricular (AV) valves separate the atria from the ventricles
• The left AV valve is also known as the bicuspid valve
• The right AV valve is also known as the tricuspid valve
• The pulmonary valve controls the flow of blood from the right ventricle into the pulmonary circulation
• Semilunar valves include the pulmonary valve and aortic valve
• The aortic valve controls the flow of blood from the left ventricle into the systemic circulation
• There are two types of cardiac muscle cells that are both excitatory:

  1. Myocardial contractile cells
  2. Myocardial autorhythmic cells
• Autorhythmic cells contain very few contractile fibres and do not have an organised sarcomere
• Myocardial cells branch and join neighbouring cells via intercalated discs
• Intercalated discs consists of:

  1. Gap junction channels
  2. Desmosomes, which transmit force between cells
• Gap junction channels enable electrical conductance through the flow of ions
• Desmosomes transmit force between cells
• Myocardial autorhythmic cells provide the signal that causes the heart to beat
• Myocardial autorhythmic cells do not require input from the nervous sytem
• The heart beats rhythmically from action potentials that it generates by itself
• Myocardial autorhythmic cells are able to generate an AP independently due to having a pacemaker potential
• What is the pacemaker potential?

  An unstable membrane potential that begins at -60mV and drifts upwards towards threshold (-40mV)
• Action potentials of myocardial autorhythmic cells

  1. The cell begins at -60mV, where If channels are open and allowing the influx of Na+
  2. Influx of Na+ increases the membrane potential to -50mV, causing If channels to close and Ca2+ channels to open
  3. Ca2+ influx causes the membrane potential to reach threshold (-40mV)
  4. At threshold, lots of Ca2+ channels open and Ca2+ causes rapid depolarisation
  5. Depolarisation causes Ca2+ channels to close and K+ channels to open, with the resulting efflux of K+ causing the cell to repolarise
  6. At -60mV, K+ channels close and If channels open
• The speed of depolarisation of myocardial autorhythmic cells, which can be altered by the nervous system, sets the heart beat
• If channels = funny channels
• Sinoatrial (SA) node

  • Region of autorhythmic myocardial cells
  • Located in the right atrial wall near the opening of the superior vena cava
• Internodal pathways

  • Pathways between the SA node and AV node
• Atrioventricular (AV) node

  • Small bundle of specialised cardiac cells
  • Located at the base of the right atrium near the septum
• Bundle of His

  • Originates at the AV node and travels down the septum
  • Divides to form left and right branches that travel down the septum, curves around the ventricles, and travels along the outer walls of the atria
• Bundle of His = AV bundle
• Purkinje fibres

  • Small, terminal fibres of the bundle of His
  • Spread throughout the ventricle myocardium
• AP from an autorhythmic cell

  1. SA node depolarises
  2. Electrical activity travels via internodal pathways to the AV node, causing the atria to contract
  3. The AV node is delayed before it transmits depolarisation through the bundle of His to the bottom of the heart
  4. Purkinje fibres undergo depolarisation to spread depolarisation upward from the bottom of the heart, causing the ventricles to contract