Human Heart Anatomy and Blood Flow Mechanics

Circulatory System Components and Structure

Veins and Arteries Function

Veins empty into the atria. They have thin and rigid walls. Valves prevent reverse blood flow. Veins carry blood from tissues to the heart.

Arteries: These originate from the ventricles. The major arteries are elastic and contain significant muscle tissue. They lack valves and carry blood to the tissues.

Heart Location and Boundaries

The heart is located in the thoracic cavity within the mediastinum. It has the following boundaries:

• Anteriorly: Chest muscles, sternum, and ribs.
• Posteriorly: Ribs, muscles, spine, and lungs.
• Inferiorly: Below the diaphragm.
• Superiorly: Over veins and arteries.
• Laterally: Ribs, intercostal muscles, and lungs.

Heart Wall Layers

The heart has three main layers:

• Pericardium: The outer serous membrane that forms a tough coat of connective tissue.
• Myocardium: Heart muscle tissue (site of infarction).
• Endocardium: The endothelium lining the heart cavities.

The heart contains valves (folds of the endocardium consisting of two or three cusps) that automatically close, preventing backflow of blood.

Cardiac Automatisms and Conduction

Heart Automatisms: The heart rhythm is coordinated by an intrinsic conduction system. This system causes automatic contraction of the heart, produced by specialized cells of the myocardium.

The sinoatrial (SA) node initiates a contraction via electrical signals that travel along the atrial walls, causing atrial systole. The SA node is located near the opening of the superior vena cava.

This wave stimulates the atrioventricular (AV) node located at the base of the right atrium. From there, the wave causes the Bundle of His, located in the interventricular septum, which continues with the branches of the Purkinje fiber network. These fibers radiate throughout the ventricles, causing ventricular systole.

The Cardiac Cycle

The walls of the heart contract and relax, called systole and diastole, respectively.

• Atrial Systole: The atria contract while filled with blood and pump it into the ventricles.
• Ventricular Systole: The ventricles contract after being filled with blood, closing the atrioventricular valves and pumping blood into the arteries.
• Diastole: The semilunar valves close, and the ventricles relax as blood slowly fills the atria.