The heart is enclosed in a pericardial sac. This sac is lined by a thin membrane which secretes pericardial fluid that acts as a lubricant to prevent friction between the pericardial layers as the heart beats. The outer layer is tough and fibrous and keeps the heart anchored and positioned within the chest.

The heart consists of three layers:

  • the Endocardium - which is the inner thin layer,
  • the Myocardium - which is the middle layer composed of cardiac muscle,
  • the Epicardium - which is the outer most layer covering the heart.


The heart is made up of four chambers:

  • Right Atrium - receives deoxygenated blood from the body.
  • Right Ventricle - receives deoxygenated blood from the Right Atrium.
  • Left Atrium - receives oxygenated blood from the lungs.
  • Left Ventricle - receives oxygenated blood from the Left Atrium.

The atria receive blood from the body and directs it into the ventricles which then pump the blood away from the heart. These chambers are separated by the interatrial / interventricular septum respectively. The flow of blood from the atria to the ventricles is passive and occurs during diastole (rest) while the flow of blood from away from the heart occurs during systole (myocardial contraction).  

The Left Ventricular wall is thicker than the Right Ventricular wall in order to withstand the high pressures that occur during the Cardiac Cycle i.e the ejection of oxygenated blood to the rest of the body.

Labelled diagram showing the structure of the Heart.Image courtesy of

 The blood vessels that carry blood to the heart are the veins:

  • the Venae Cavae (superior and inferior) - carries deoxygenated blood from the body to the Right Atrium.
  • the Pulmonary Vein - carries oxygenated blood from the lungs to the Left Atrium.

The blood vessels that carry blood from the heart are the arteries:

  • the Pulmonary Artery - pumps deoxygenated blood from the Left Ventricle to the lungs. 
  • the Aorta - pumps oxygenated blood from the Left Ventricle to the rest of the blood.


The Atria and Ventricles are seperated by valves, known as Atrio-ventricular valves, which prevent the back flow of blood to Atrium during ventricular systole (contraction). These valves are connected to the papillary muscles by the Chordae Tendineae.

  • the Tricuspid Valve: is on the right side of the heart and has three leaflets and three papillary muscles.
  • the Bicuspid/Mitral Valve: is on the left side of the heart and has two leaflets. 

The Ventricles and their respected arteries are separated by valves, known as Semi-lunar Valves, which prevent back flow of blood pumped from the Ventricles to the Arteries:

  • the Pulmonary Valve - is located between the Right Ventricle and Pulmonary Artery.
  • the Aortic Valve - is located between the Left Ventricle and Aorta.

 The Atrio-Ventricular and Semi-lunar valves are made from endocardium and connective tissues.

 The wave of excitation intiates and conducts through specialised nodes:

  • the Sino-Atrial Node: is placed on the top of the right atrium and initiates the wave of excitation or action potential. 
  • the Atrio-Ventricular Node: is located in the septum between the right and left atrium through which the excitation is transmitted from the atrium to purkinje fibres.