Week 3: Cardiovascular System 1

Week 3: Cardiovascular System 1

1. Atria: Receiving chambers

   • Vessels entering right atrium
     • Superior vena cava
     • Inferior vena cava
     • Coronary sinus
   • Vessels entering left atrium
     • Right and left pulmonary veins

2. Ventricles: Discharging Chambers

   • Papillary muscles project into ventricular cavities
   • Vessel leaving the right ventricle - pulmonary trunk
   • Vessel leaving the left ventricle - aorta

3. Heart Valves

   • Ensure unidirectional blood flow through the heart
   • Atrioventricular valve
     • Prevent backflow into atria when ventricles contract
     • Tricuspid valve (right)
     • Mitral valve (left)
   • Chordae tendineae anchor AV valve cusps to papillary muscles
   • When atrial pressure is higher than ventricular pressure, the AV valve opens
   • Semilunar valve
     • Prevent backflow into ventricles when ventricles relax
     • Aortic semilunar valve
     • Pulmonary semilunar valve
     • When ventricular pressure exceeds the blood pressure in the aorta and pulmonary trunk, the semilunar valves open

4. Pathway of blood through the heart

   • Right side is pump for pulmonary circuit
     • Vessels that carry blood to and from the lungs
   • Left side is the pump for systemic circuit
     • Vessels that carry the blood to and from all body tissues
   • Right atrium - tricuspid valve - right ventricle - pulmonary semilunar valve - pulmonary trunk - pulmonary arteries - lungs
   • Lungs - pulmonary veins - left atrium - bicuspid valve - left ventricle - aortic semilunar valve - aorta - systemic circulation
   • Equal volumes of blood are pumped to pulmonary and systemic circuits
   • Pulmonary circuit is a short, low pressure circulation
   • Systemic circuit blood encounters much resistance in long pathways

5. Microscopic anatomy of cardiac muscle

   • Cardiac muscle cells are striated, short, fat, branched and interconnected
   • Branching cardiac muscle cells are held together by connective tissue
   • T Tubules are wide but less numerous; SR is simpler than skeletal muscle
   • Numerous large mitochondria
   • Intercalated discs: junctions between cells anchor cardiac cells
     • Desmosomes prevent cells from separating during contraction
     • Gap junctions allow ions to pass; electrically couple adjacent cells
     • Heart muscle behaves as a functional syncytium

6. Cardiac muscle contraction

   • Contraction of the heart is triggered by action potential
   • About 1% of cardiac cells have automaticity (self-excitable)
   • Depolarization of heart is rhythmic and spontaneous
   • Gap junctions ensure the heart contracts as a unit

7. Electrical events

   • Intrinsic cardiac conduction system
   • Network of noncontractile (autorhythmic) cells that initiate and distribute impulses to coordinate the depolarization and contraction of the heart
   • Sinoatrial node (pacemaker)
     • Generates impulses about 75 times/minute
     • Depolarizes faster than any other part of myocardium
   • Atrioventricular (AV) node
     • Smaller diameter fibres, fewer gap junctions
     • Delays impulses approx 0.1 second
     • Depolarizes 50 times per minute in absense of SA node input
   • Atrioventricular bundle
     • Only electrical connection between the atria and ventricles
   • Right and left bundle branches
     • Two pathways in the interventricular septum that carry the impulses toward the apex of the heart
   • Purkinje fibers
     • Complete the pathway into the apex and ventricular walls
     • AV bundle and Purkinje fibres depolarize only 30 times per minute in absense of AV node input

8. Extrinisic Innervation of the Heart

   • Heartbeat is modified by ANS
   • Cardiac centes are located in the medulla oblongata
   • Cardioacceleratory centre innervates SA and AV nodes, heart muscle and coronary arterties through sympathetic neuron
   • Cardioinhibitory center inhibits SA and AV nodes through parasympathetic fibres in vagus nerves

9. Mechanical Events: Cardiac cycle

   • Cardiac cycle: all events associated with blood flow through the heart during one complete heartbeat
   • Flow of blood through the heart is controlled by pressure changes
   • Blood flows from higher to lower pressure through an opening
   • Systole - contraction: pressure inside chamber increases and there is emptying of blood
   • Diastole - relaxation: pressure inside chamber decreases and there is filling of blood
   • Duration of cardiac cycle depends upon heart rate
   • During a cardiac cycle of 0.8 sec, the ventricles are in systole for 0.3 sec and in diastole for 0.5 sec

10. Cardiac cycle

    • Inflow Phase: inlet valve open, outlet valve closed
    • Isovolumetric contraction: both valves closed, no blood flow
    • Outflow Phase: outlet valve open, inlet valve closed
    • Isovolumetric relaxation: both valves closed, no blood flow
    • Step1: At beginning all chambers are relaxed. The atria fill with blood from the veins. AV valves open between atria and ventricles. 80% of blood passively flows from atria to ventricles
    • Step 2: An AP fires in SA node and atrial cell depolarize. The atria contract following the wave of depolarization.
    • When the depolarization wave reaches the ventricles, they contract and the pressure in the ventricle rises rapidly
    • As the ventricles contract they eventually produce enough pressure to open the semilunar valves and push blood into arteries
    • Ventricles begin to repolarize and relax. Ventricular pressure decreases
    • When ventricular pressure falls below atrial pressure, the AV valves open

11. Cardiac output

    • Heart must pump
      • adequate amount of blood (flow)
      • At a pressure high enough to drive it through the tissues
    • CO = heart rate x stroke volume
    • Stroke volume = blood volume before contraction (EDV) - Blood volume after contraction (ESV)

12. Frank-Starling Law

    • Heart is filled with more blood, the heart is stretched more
    • Cause the end of the sarcomere to move out, which means that it will have a greater distance to travel
    • This causes an increase in force of contraction

13. What affects venous return

    • Skeletal muscle pump
      • Skeletal muscle surrounding the veins contracts and increases blood flow back to the heart
    • Respiratory pump
      • During inhalation, the diaphragm descends and abdominal pressure increases
      • The increasing pressure squeezes veins and moves blood towards the heart
    • Body position
      • Gravity opposes the return of blood from the feet to heart during sitting or standard. Effect is lost when we lie down
    • Sympathetic Activity
      • Veins are innervated by sympathetic nerves. Activation of SNS will constrict veins and push more blood towards the ehart

14. Regulation of stroke volume

    • Parasympathetic nerves
      • parasympathetic nerves innervate the atrial muscle. Sparse innervation to ventricular muscle. Release acetylcholine onto muscarinic receptors
      • Weakens atrial contraction and reduces stroke volume
    • Sympathetic nerves
      • Sympathetic nerves innervate the atrial and ventricular muscle. Release noradrenaline onto beta receptors
      • Increases the force of contraction of atria and ventricles and increases stroke volume