Three layers of the heart

• Endocardium- thin inner lining
• Myocardium-layer of muscle
• Epicardium- outer layer

Pericardium

• A fibrous sac that covers the heart and is composed of two layers
• Visceral layer- inner layer of the pericardium (epicardium)
• Parietal layer- outer layer
• Small amount of pericardial fluid (10 to 15 mL) lubricates the space between the pericardial layers (pericardial space) and prevents friction between surfaces as the heart contracts

Structure of the heart

• Divided vertically by the septum.
• Interatrial septum- creates a right and left atrium.
• Interventricular spetum- creates a right and left ventricle.
• Atrial myocardium is thinner than ventricle myocardium and left ventricular wall is 2 to 3 times thicker than the right ventricular wall
• The thickness of the left ventricle is necessary to generate the force needed to pump blood into the systemic circulation.

Cardiac valves

• Four valves in the heart function to keep blood flowing in a forward direction.
• Cusps of the mitral and tricuspid valves (atrioventricular valves) are attached to thin strands of fibrous tissue termed chordae tendinae.
• Chordea are anchored in the papillary muscles of the ventricles.
• This support system prevents the eversion of the leaflets into the atria during ventricular contraction.
• Pulmonic and aortic valves (semilunar valves) prevent blood from regurgitating into the ventricles at the end of each ventricular contraction.

Blood supply to myocardium

• Has its own blood supply through the coronary circulation.
• Blood flow into the two major coronary arteries occurs primarily during diastole (relaxation of the myocardium).
• Left coronary artery arises from the aorta and divides into two main branches.
• Left anterior descending artery
• Left circumflex artery
• These arteries supply the left atrium, the left ventricle, the interventricular septum, and a portion of the right ventricle.
• Right coronary artery also arises from the aorta.
• Its branches suppley the right atrium, the right ventricle, and a portion of the posterior wall of the left ventricle.
• 90% of people's atrioventricular node and bundle of His receive blood supply from the right coronary artery.
• Obstruction of this artery often causes serious defects in cardiac conduction
• The division of coronary veins prallel the coronary arteries.
• Most of the blood from the coronary system drains into the coronary sinus, which empties into the right atrium near the entrance of the inferior vena cava.

Conduction system

• Specialized nerve tissue responsible for creating and transporting the electrical impulse, or action potential. This impulse initiates depolarization and subsequently cardic contraction.
• The impulse is initiated by the sinoatrial (SA) node (the pacemaker of the heart). Each impulse generated at the SA node travels through intratrial pathways to depolarize the atria resulting in contraction.

Electrical impulse of the SA node

• The electrical impulse travels from the atria to the AV node through internodal pathways.
• The excitation then moves through the bundle of His and the left and right branches.
• the left bundle has two fasicles (divisions)
• Anterior
• Posterior
• Action potential diffuses widely through the walls of both ventricles by means of purkinje fibers.
• The ventricular conduction system delivers the impulse within 0.12 second. This triggers a synchronized ventricular contraction. 

The cardiac cycle

• The climax of the cycle is the ejection of blood into the pulmonary and systemic circulations.
• It ends with repolarization when the contractile fiber cells and the conduction pathway cells regain their resting polarized condition.
• Cardiac muscle cells have a compensatory mechanism that makes them unresponsive or refractory to restimulation during the action potential.
• During ventricular contraction there is an absolute refractory period. Cardiac muscle does not respond to any stimuli. After this period, cardiac muscle gradually recovers its excitability and a relative refractory period occurs by early diastole.