Heart Function – Cardiac Cycle and the Electrocardiogram (ECG)

R. Atrium L. Atrium R. Ventricle L. Ventricle Aorta Vena Cava Pulmonary Artery Pulmonary Vein Lungs TCV ASLV PSLV BCV Pathway of Blood – Pulmonary and Systemic Circuits

Cardiac Cycle All the changes that occur during one heartbeat – one cycle of contraction/ relaxation (systole/diastole) Measurable changes include: Pressure Volume Heart sounds Electrical impulses Blood flows through the heart based on pressure differences Pressure differences caused by changes in volume Changes in volume caused by heart contractions Heart contractions caused by depolarization of muscle cells

Cardiac Cycle Electrical Changes Cause Contraction Contraction Decreases Volume Volume Change Increases Pressure Pressure Change Opens Valve Blood Moves High to Low Pressure

Cardiac Cycle Atrial contraction/ventricular relaxation  ventricular contraction (“Lubb”) /atrial relaxation  atrial and ventricular relaxation (“Dupp”)  Atrial Systole = atrial contraction  atrial pressure > ventricular pressure  Bicuspid and tricuspid valves open  Ventricular Systole = ventricular contraction  ventricular pressure > atrial pressure  semi-lunar valves open  Bicuspid and tricuspid valves close (“Lubb”)  Atrial and Ventricular Diastole = atrial and ventricular relaxation  Arteries pressure and atrial pressure > ventricular pressure  Semi-lunar valves close (“Dupp”)

Cardiac Cycle

Click here for video clip of Cardiac Cycle Click here for a video clip of a normal heart beat

Heart Conduction System Intrinsic Conduction (conduction from within)  Heart contractions occur independently of the nervous system.  Cardiac muscle fibers connected in 2 networks: 1) in atrial walls, 2) in ventricular walls. When any part of a network is stimulated, all fibers in it contract as a unit.  Electrical stimulation begins in the SA node, the ‘ pacemaker ’. SA node  AV node  Right and Left Bundle Branches  Purkinje Fibers

Heart Conduction System Electrical Pathway: SA node  (atria contract together)  AV node  AV bundle  (ventricles contract together)  Purkinje Fibers

Heart Conduction System Click here for video clip of Heart Conduction System  Electrical stimulation causes muscles to contract  Polarization in muscle cell is due to distribution of Sodium and Potassium ions  Electrical stimulation  voltage sensitive proteins rearrange Sodium and Potassium ions  Calcium ions are released  bind to troponin  opens myosin binding sites on actin  muscle contraction

Heart Conduction System How an Impulse is Conducted-The Action Potential 1. Heart Muscle Cell--Polarized Heart Muscle Cell -- Depolarized *caused by movement of ions Heart Muscle Cell – -( Re)polarized *When cells depolarize, they contract. *When many cells depolarize, the heart contracts. Inside cell Outside cell

The Electrocardiogram (ECG) Measures electrical conduction throughout the cardiac cycle Works by measuring electrical changes of de- and re-polarization Sequence: SA node triggers Atrial depolarization (P-wave, contraction, blood to ventricles) Atrial repolarization (QRS waves, relaxation) AV node triggers Ventricles depolarization (QRS waves, contraction, blood to arteries) Ventricles repolarization (T wave, relaxation)

The Electrocardiogram (ECG) Abnormal ECG ’ s Figure 13.15

ECG and Cardiac Cycle Click here for a video clip of a normal heart beat Click here for a video clip of an abnormal heart beat – ventricular fibrillation