Stephen J. Schueler, M.D.

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Third Degree Heart Block Anatomy

To better understand third degree heart block, it helps to understand the anatomy of the heart.

The heart is a muscular pump that contains four chambers: right atrium, left atrium, right ventricle and left ventricle. The two small atria make up the top of the heart, and the two large ventricles make up the bottom of the heart. The right atrium pumps blood to the right ventricle, and the left atrium pumps blood to the left ventricle. A wall, called the septum, separates the right atrium and right ventricle, from the left atrium and left ventricle.

Blood flows through the heart in the following manner:

  • The right atrium receives oxygen-poor blood from the body, and then pumps the blood through the tricuspid valve and into the right ventricle.
  • The right ventricle pumps the blood through the pulmonic valve and to the lungs, where it picks up more oxygen.
  • The left atrium receives the oxygen-rich blood from the lungs, and then pumps the blood through the mitral valve and into the left ventricle.
  • The left ventricle pumps blood through the aortic valve and to the rest of the body.
  • The blood supplies oxygen to the body and the cycle starts again.

Anatomy examples:
  • Normal circulation through the heart
  • The human heart video
  • Cross-section of heart at the level of the atria
  • Cross-section of heart at the level of the ventricles
  • The heart sits inside the pericardium
  • The heart valves

Coronary Arteries
The coronary arteries supply oxygen to the heart muscle.

The heart has three main coronary arteries:
  • Right coronary artery: supplies the right ventricle
  • Left coronary artery: supplies the left ventricle
  • Posterior circumflex artery: supplies the posterior aspect of both ventricles

Anatomy examples:
  • Cardiac catheterization video
  • Front view of the heart and coronary arteries
  • Back view of the heart and coronary arteries

Cardiac Conduction System
An electrical impulse stimulates the muscle fibers in the heart to contract. The impulse spreads through the heart in a very organized manner, so that the atria contract first, followed by the ventricles.

The electrical impulse proceeds in the following manner:
  • The electrical impulse originates at the sinoatrial (SA) node, which is located in the wall of the right atrium.
    • The SA node is the heart's natural pacemaker: it regulates the heart rate.
  • The impulse proceeds through the atria, stimulating them to contract.
  • After the atria are stimulated to contract, the atrioventricular (AV) node slows the electrical impulse before it proceeds to the ventricles. This pause allows the ventricles to fill with blood before they contract.
    • The AV node is located between the atria and the ventricles.
  • After the pause, the impulse then proceeds through the ventricles, stimulating them to contract.

Anatomy examples:
  • The cardiac conduction system
  • EKG showing electrical activity of heart

Last Updated: Oct 20, 2010 References
Authors: Stephen J. Schueler, MD; John H. Beckett, MD; D. Scott Gettings, MD
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PubMed Third Degree Heart Block References
  1. Bevilacqua LM, Berul CI. Advances in pediatric electrophysiology. Curr Opin Pediatr. 2004 Oct;16(5):494-9. [15367841]
  2. Liautaud S, Khan AJ, Nalamasu SR, Tan IJ, Onwuanyi AE. Variable atrioventricular block in systemic lupus erythematosus. Clin Rheumatol. 2005 Apr;24(2):162-5. [15517446]
  3. Manfredini R, Gallerani M, Boari B, Maraldi C, la Cecilia O, Rafael Ferreira AB, Smolensky MH, Portaluppi F. Morning preference in onset of symptomatic third-degree atrioventricular heart block. Chronobiol Int. 2002 Jul;19(4):785-91. [12182503]
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