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The Heart

• The heart is located in the thoracic cavity, surrounded by the pericardium.
• The heart wall consists of three layers: epicardium, myocardium, and endocardium.

Layers of the Heart Wall

• Epicardium: the visceral layer of the serous pericardium.
• Myocardium:
  • Consists of spiral bundles of cardiac muscle cells.
  • Has a fibrous skeleton that limits the spread of action potentials to specific pathways, anchors cardiac muscle fibers, and supports great vessels and valves.
• Endocardium: continuous with the lining of blood vessels, composed of simple squamous epithelium.

Heart Chambers

• Atria: the receiving chambers, with walls ridged by pectinate muscles.
  • Right atrium receives deoxygenated blood from the superior and inferior vena cava, and coronary sinus.
  • Left atrium receives oxygenated blood from the right and left pulmonary veins.
• Ventricles: the discharging chambers, with walls ridged by trabeculae carneae.
  • Make up the largest part of the heart.
  • Deoxygenated blood leaving the right ventricle enters the pulmonary trunk (to the lungs).
  • Oxygenated blood leaving the left ventricle enters the aorta (to the body).

Heart Pathway of Blood Through the Heart

• The heart is two side-by-side pumps:
  • Right side pumps blood to and from the lungs (pulmonary circuit).
  • Left side pumps blood to and from all body tissue (systemic circuit).
• Pathway of blood through the heart:
  • 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.

Coronary Circulation

• The functional blood supply to the heart muscle itself.
• Coronary arteries:
  • Right and left coronary arteries (in atrioventricular groove).
  • Marginal, circumflex, and anterior interventricular arteries.
• Coronary veins:
  • Small cardiac, anterior cardiac, and great cardiac veins.

Heart Valves

• Ensure unidirectional blood flow through the heart.
• Atrioventricular (AV) valves:
  • Prevent backflow into the atria when ventricles contract.
  • Tricuspid valve (right) and bicuspid (Mitral) valve (left).
  • Chordae tendineae support valve flaps during ventricle contraction.
  • Flaps open during relaxation of the ventricles.
• Semilunar (SL) valves:
  • Prevent backflow into the ventricles when ventricles relax.
  • Aortic semilunar valve and pulmonary semilunar valve.
  • Contraction increases pressure, forcing these valves open.
  • Relaxation decreases pressure, causing these valves to close.

Heart Physiology: Electrical Events

• Intrinsic cardiac conduction system:
  • A network of noncontractile (autorhythmic) cells that initiate and distribute impulses to coordinate the depolarization and contraction of the heart.
• Intrinsic cardiac conduction system:
  • Sinoatrial (SA) node (pacemaker) generates impulses about 75 times/minute.
  • Atrioventricular (AV) node delays impulses approximately 0.1 second.
  • Atrioventricular (AV) bundle (bundle of His) is the only electrical connection between the atria and ventricles.
  • Right and left bundle branches are the 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.

Heart Physiology: Sequence of Excitation

• 1. P wave: depolarization of the SA node.
• 2. QRS complex: ventricular depolarization.
• 3. T wave: ventricular repolarization.

Heart Sounds

• Two sounds (lub-dup) associated with the closing of heart valves.
• "Lub" is the first sound, occurring when AV valves close, signifying the beginning of ventricular systole.
• "Dup" is the second sound, occurring when SL valves close at the beginning of ventricular diastole.
• Heart murmurs are abnormal heart sounds, often indicative of valve problems.

Mechanical Events: The Cardiac Cycle

• The cardiac cycle is all events associated with blood flow through the heart during one complete heartbeat.
• Phases of the cardiac cycle:
  • Ventricular filling: takes place in mid-to-late diastole, with AV valves open, and 80% of blood passively flowing into ventricles.
  • Atrial systole occurs, delivering the remaining 20% of blood.
  • End diastolic volume (EDV) is the volume of blood in each ventricle at the end of ventricular diastole.
  • Ventricular systole: atria relax and ventricles begin to contract.
  • Rising ventricular pressure results in the closing of AV valves.
  • Isovolumetric contraction phase (all valves are closed).
  • In the ejection phase, ventricular pressure exceeds pressure in the large arteries, forcing the SL valves open.
  • End systolic volume (ESV) is the volume of blood remaining in each ventricle.