MH 069 Heart

Cardiac Atria

The cardiac atria are the two upper chambers of the heart, acting as low-pressure receiving chambers that collect blood before transferring it to the ventricles.

• Right Atrium: Receives deoxygenated blood from the body and pumps it to the right ventricle
• Left Atrium: Receives oxygen-rich blood from the lungs and pumps it to the left ventricle

Their relatively thin muscular walls reflect their role in blood collection rather than forceful ejection, distinguishing them structurally from the more robust ventricular chambers.

Atria also function as endocrine organs, producing hormones called natriuretic peptides that help regulate intravascular volume and blood pressure.

Right Atrium

The right atrium is the primary chamber that collects deoxygenated blood returning from the systemic circulation. Its inner surface has distinctive pectinate muscles - specialized muscular ridges that protrude from the surface, enhancing the chamber's ability to accommodate varying blood volumes.

The right atrial wall demonstrates the classic three-layered cardiac architecture:

• Endocardium (Inner Layer)
  • Endothelium: Simple squamous epithelium lines the inner surface of the heart
  • Subendothelial Layer: Thin layer of connective tissue of collage and elastic fibers that supports the endothelium
• Myocardium (Middle Layer)
  • Cardiomyocytes: Thickest layer responsible for the chamber's pumping action
  • Other Components: Blood vessels (muscular artery, arteriole, capillaries), nerve fibers, and scattered adipocytes
• Epicardium (External Layer)
  • Mesothelium: Simple squamous epithelium that covers the outer surface of the heart
  • Subepicardial Layer: Dense irregular connective tissue with variable amounts of adipose tissue
  • Relationship to Pericardium: Forms the visceral layer of the pericardium that surrounds the heart

The heart's natural pacemaker, the sinoatrial (SA) node, is in the wall of the right atrium. It generates the electrical impulses that set the rhythm and rate of contraction.

Left Atrium

The left atrium receives oxygenated blood from the lungs. Its smooth-walled architecture facilitates efficient blood flow into the left ventricle during contraction.

The left atrial wall demonstrates the classic three-layered cardiac architecture:

• Endocardium (Inner Layer): More complex than the right atrium
  • Endothelium: Simple squamous epithelium lines the inner surface of the heart
  • Subendothelial Layer: Thin layer of connective tissue of collage fibers interspersed with smooth muscle
  • Subendocardial Layer: Outer layer of loose connective tissue that often contains adipocytes
    • Serves as a transition zone to the myocardium
• Myocardium (Middle Layer)
  • Cardiomyocytes: Thickest layer responsible for the chamber's pumping action
  • Other Components: Blood vessels (muscular artery, arteriole, capillaries), nerve fibers (#1 and #2), and scattered adipocytes
• Epicardium (External Layer)
  • Mesothelium: Simple squamous epithelium that covers the outer surface of the heart
  • Subepicardial Layer: Dense irregular connective tissue
    • Some regions contain an inner zone of loose adipose connective tissue
  • Relationship to Pericardium: Forms the visceral layer of the pericardium that surrounds the heart