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Human Heart

Updated on May 22, 2012

Pericardium and heart walls

The human heart is one of the most vital organs of the human body. It is a muscular organ that pumps the blood to different parts of the body to complete their need of oxygen and nutrients through cardiac cycle. Carbon dioxide and waste material are also removed from the body tissues during the cardiac cycle.

The human heart is mesodermal in origin. It is located in the pericardial cavity located in another cavity called the Thoracic cavity (present on the chest region). Heart is present between the two lungs, slightly tilted to the left. Pericardium, a double walled layer (outer parietal pericardium and inner visceral pericardium), covers the heart. In-between the two layer of pericardium is present the pericardial cavity filled with pericardial fluid.


The wall of the heart is three layered: outermost epicardium, middle myocardium (consisting of cardiac muscles) and the inner endocardium. The human heart is four chambered. The upper two chambers are called the atria (or auricles) and the lower two are called the ventricles. The walls of the ventricles are thicker than that of atria. The two atrium are divided by a thin walled interatrial septum into left and right atrium whereas the ventricles are divided by thick walled interventricular septum into left and right ventricles. The atrium and ventricle on the right side is separated by a thick fibrous tissue called the tricuspid valves (with three muscular flaps or cusps) whereas on the left side they are separated by bicuspid valve (or mitral valve). Together the two valves are called atrio-ventricular valve. The opening of the right and left ventricles into the pulmonary artery and the aorta respectively are provided with the semilunar valves. The function of these valves is to prevent the backflow of the blood from atria to ventricles and from ventricles to pulmonary artery or aorta.

Nodes of heart

Cardiac muscles

The working of the heart is regulated by the cardiac muscles. Cardiac muscles consist of contractile cell-regulating systole and diastole, and autorhythemic conducting tissues- sinoatrial node, atrioventricular node, bundle of HIS and purkinje fibres.

Sino atrial (SA) node is also called heart of heart and pacemaker. It is located in the walls of right atrium near the opening of the superior vena cava. The impulse is generated here. Atrio-ventricular (AV) node located at the base of the interatrial septa are also called pacesetter. Impulse is transferred here from SA node. Bundle of HIS is located on the interventricular septa and Purkinje fibres are present on the walls of ventricles.

Artificial pacemaker, made up of lithium halide battery along with electrode and lead wire, are inserted at the base of the interatrial septa near AV node in case of SA node failure and in case of AV node failure, inserted in the right ventricular wall.

Cardiac cycle

The heart shows alternate contraction and dilation of its chamber. Contraction is called systole and dilation is called diastole. The heart is said to be in joint diastole when all the four chambers are in relaxed state. The right atrium supplies the deoxygenated blood to the right ventricle which it receives through superior vena cava (carries deoxygenated blood from head portion) and inferior vena cava (carries deoxygenated blood from rest of the body) due to atrial systole that starts just after the end of joint diastole. During atrial systole, blood cannot pass back into superior and inferior vena cava because they are compressed by the atrial contraction. The right ventricle pumps the deoxygenated blood to the pulmonary trunk (due to ventricular systole after atrial systole) arising from it. The pulmonary trunk bifurcates to form right and left pulmonary arteries which supplies deoxygenated blood to the lungs of the respective side. At the lungs the blood gets reoxygenated and the reoxygenated blood is transferred to the left atrium from the lungs. The left ventricle receives oxygenated blood from the left atrium (due to atrial systole after joint diastole) and pumps the blood to the aorta (due to ventricular systole after atrial systole) which put the blood into coronary artery which supplies the blood to the body except to the lungs. [Note: after atrial systole, the atrial muscles relax and atrial diastole and ventricular systole starts. During atrial diastole, both the atriums again gets filled with respective type of blood]. The cuspid valve closes as soon as the ventricular systole starts. And this closure of atrio-ventricular valve (cuspid valves) produces a sound "lubb" called the first heart sound. The blood do not flows to the artery and aorta until a sufficient pressure is produced in the ventricles due to the presence of semilunar valves at the opening of pulmonary artery and aorta. At the end of the ventricular systole, ventricular diastole starts. And as the atria still continues its diastole, the joint diastolic condition is created again. This ventricular diastole results in lowering of ventricular pressure which results in closure of semilunar valve (at the beginning of the diastole) produces a sound "dupp" called the second heart beat. The whole process is repeated again and again.

The sound of "lubb" and "dupp" can be heard using stethoscope.

Cardiac Disorders

  • Hypertension - Increase in the pressure of blood.
  • Angina pectoris - Pain in heart muscles.
  • Tachycardia - Increased rate of heart beat.
  • Bradycardia - Decreased rate of heart beat.
  • Heart attack - When symptoms such as breathlessness, palpitation, pain in chest, unconsciousness appear suddenly. Nitroglycerine is used for the treatment.
  • Heart block - When heart beat is not passed to the ventricles properly.
  • Arrhythmia - Irregular heart beat.
  • Coronary thrombosis - Caused due to formation of clot in coronary artery.


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      Thomas Stokes 

      7 years ago

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