Cardiovascular System and The Heart Help

By — McGraw-Hill Professional
Updated on Aug 18, 2011


The heart is a hollow, four-chambered muscular organ that is specialized for pumping blood through the vessels of the body. It is located in the mediastinum where it is surrounded by a tough fibrous membrane called the pericardium. The parietal pericardium is a loose sac composed of an outer fibrous layer that protects the heart and an inner serous layer that secretes pericardial fluid. The visceral pericardium is a serous membrane that makes up the outer layer of the wall of the heart (the epicardium). The space between the parietal pericardium and the visceral pericardium is the pericardial cavity. Pericardial fluid is found within this cavity and functions to lubricates the surface of the heart.

The heart is composed of three layers from superficial to deep:

  • Epicardium:   Serous membrane of connective tissues covered with epithelium
      Outer coating that lubricates
  • Myocardium:   Cardiac muscle tissue and connective tissues Contractile layer, thickest layer
  • Endocardium:   Epithelial membrane and connective tissues Strengthened protective inner lining of heart

Internal structure

The heart is a four-chambered double pump (Figure 15-1). It consists of upper right and left atria that pulse together, and lower right and left ventricles that also contract together. The atria are separated by the thin, muscular interatrial septum, while the ventricles are separated by the thick, muscular interventricular septum. Atrioventricular valves (AV valves) are located between the upper and lower chambers of the heart, and the semilunar valves are at the bases of the two large vessels (the pulmonary trunk and aorta) that leave the heart.

Internal Structure

Each cusp of the atrioventricular valves is held in position by strong tendinous cords, the chordae tendineae, which are secured to the ventricular wall by cone-shaped papillary muscles. All the valves in the heart prevent the backflow of blood to the previous chamber upon contraction of the muscular walls of the heart.

Blood Flow through the Heart

Deoxygenated blood returning from the body fills the right atrium while oxygenated blood returning from the lungs fills the left atrium. Blood flows from the atria into the ventricles, the atria contract, emptying the remaining blood into the ventricles. The ventricles then contract, forcing blood from the right ventricle into the pulmonary trunk and from the left ventricle into the aorta.

The pulmonary circuit refers to the circulation of blood from the heart to the lungs and then back to the heart. The structures that are part of the pulmonary circuit are the right ventricle, pulmonary trunk and pulmonary arteries, the capillary network in the lungs, the pulmonary veins returning blood to the heart, and the left atrium, which receives this oxygenated blood. The systemic circuit refers to the circulation of blood to and from all of the other body tissues. The components of this circuit are the left ventricle, the arteries, capillaries, and veins going to all the body tissues, and the right atrium, which receives deoxygenated blood as it returns from all the body tissues.

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