Circulatory System: The Heart

Location and Structure of the Heart

• Located within the mediastinum (inferior), between the 2nd and 5th intercostal spaces
• 2/3 of its mass lies to the left of the body midline
• Size: approximately 12 cm (5 inches) in length, 9 cm (3.5 inches) in width, and 6 cm (2.4 inches) in thickness
• Shape: blunt cone, with the apex directed anteriorly and the base directed posteriorly

Pericardium

• Fibrous pericardium: dense, irregular connective tissue
• Serous pericardium:
  • Parietal layer: lines the fibrous pericardium
  • Visceral layer (epicardium): covers the heart
• Pericardial cavity: filled with pericardial fluid, which reduces friction between the heart and surrounding tissues during contraction and relaxation

Layers of the Heart

• Epicardium (visceral layer of serous pericardium):
  • Faces the pericardial cavity
  • Composed of mesothelium and delicate connective tissue
• Myocardium:
  • Thickest layer
  • Composed of cardiac muscle and blood vessels
  • Unique cardiac muscle tissue with branching cardiocytes and intercalated disks
• Endocardium:
  • Endothelium bound to the myocardium by subendothelial connective tissue
  • Contains part of the cardiac conduction system

Heart Chambers

• 2 Atria:
  • Right atrium
  • Left atrium
  • Separated by the interatrial septum
  • Fossa ovalis: a remnant of the foramen ovale in the fetal heart
• 2 Ventricles:
  • Right ventricle
  • Left ventricle
  • Separated by the interventricular septum
  • Papillary muscles: attached to tendon-like structures called chordae tendineae

Heart Valves

• 2 Atrioventricular (AV) valves:
  • Tricuspid valve (right AV): between the right atrium and right ventricle
  • Bicuspid valve (mitral or left AV): between the left atrium and left ventricle
• 2 Semilunar valves:
  • Pulmonary valve (right semilunar): between the right ventricle and pulmonary trunk
  • Aortic valve (left semilunar): between the left ventricle and aorta

Blood Flow

• Blood leaving the heart:
  • Pulmonary trunk: receives blood from the right ventricle and sends it to the lungs for oxygenation
  • Aorta: receives blood from the left ventricle and distributes it to the body
• Blood entering the heart:
  • Right atrium: receives blood from the superior and inferior vena cavae and the coronary sinus
  • Left atrium: receives blood from the pulmonary veins

Cardiac Conduction System

• Specialized cells found within the endocardium
• Sinoatrial (SA) node: the "pacemaker" of the heart, responsible for regulating heart contractions
• Internodal pathways: send signals to the right and left atria
• Atrioventricular (AV) node: relays signals from the SA node to the ventricles
• Atrioventricular (AV) bundle or bundle of His: splits into the right and left bundle branches
• Purkinje fibers: initiate contraction of the ventricles

Cardiac Cycle

• Diastole:
  • Heart is at rest
  • Atria fill with blood
  • AV valves are open, and semilunar valves are closed
• Systole:
  • Atria contract
  • AV valves are open, and semilunar valves are closed
  • Ventricles contract
  • AV valves are closed, and semilunar valves are open

Blood Pressure

• Systolic blood pressure: pressure of blood in the vessels when the heart is contracting
  • Normal: 120 mm Hg
  • High: > 130 mm Hg
  • Low: < 90 mm Hg
• Diastolic blood pressure: pressure of blood in the vessels when the heart is at rest
  • Normal: 80 mm Hg
  • High: > 90 mm Hg