Human Heart: Structure, Function and Cardiac cycle Easy Notes

Human Heart:

The human heart is a muscular organ, which is present in the thoracic region of the human body. In this article you can understand about the structure, function and cardiac cycle of human heart in easy words.

Shape of Heart Human Heart:

Heart is a hollow, conical shaped, a muscular structure that lies in the thoracic cavity above the diaphragm and in between the two lungs.

Shape and Weight of Human Heart:

The size and the weight of human heart is a about the size of a closed fist measuring about 12 centimeters in length and 9 centimeters in a breath. The weight human heart is about 300 grams. Heart is a four-chambered organ-two atria and two ventricles.

Atrial Systole

Transportation of oxygenated and deoxygenated blood through heart that is given below:

Deoxygenated Blood:

It received into the right atrium by superior vena cava from the anterior region and inferior vena cava from posterior of the body. This vena cava opens directly into the right atrium as there is no sinus venosus.


Right atrium of mammalian including humans also gets blood from coronary veins from the heart muscles itself.

The right and left atria are totally separated by an inter-atrial septum. Similarly, the right and left ventricles are also separated by the interventricular septum. 

Deoxygenated blood is then passed from right atrium to right ventricle through the atrioventricular aperture which is guarded by tricuspid valves (having three cusps).

The blood is then pumped into the lungs for oxygenation of the deoxygenated blood via the pulmonary artery. After oxygenation, the blood is brought back into the left atrium.

Oxygenated Blood:

Oxygenated blood now goes to the left ventricle through the atrioventricular aperture and this opening is regulated by bicuspid (having two cusps) or mitral valve. 


The ventricle also has chordae tendineae and papillary muscles, which prevent the valves from being pushed into atria, at the time of the ventricular contraction. 

The muscular walls of the left ventricle are thicker than the walls of the right ventricle of the heart.

The oxygenated blood from the left ventricle is then distributed to all parts of the body with the help of aorta

The openings of the aorta and other major arteries are guarded by semi-lunar valves that prevent the back flow of the blood.

The Function of Human Heart:

The human heart contracts 72 times per minute at rest and pumps over 7,000 liters of blood daily. Therefore, it takes five to six liters per minute per ventricle

During each heartbeat, the heart chambers contract and relax in a specific sequence. The contraction and relaxation of a cardiac chamber are known as systole and diastole respectively.

The movements of cardiac chambers are repeated in a cyclic manner during each heartbeat. These movements in each heartbeat constitute a cardiac cycle.

Cardiac Cycle:

The cardiac muscles of the heart are equipped with a special characteristic of rhythmic contraction and relaxation. This includes two distinct events, the systole or contraction of heart chambers one after the other and the diastole i.e. their relaxation. The complete sequence of events during one systole and one diastole or during one heartbeat represents the cardiac cycle. In mammals, all parts of the heart are simultaneously in diastole, for a brief period.

The systole then starts by the contraction of sinus Venosus in lower vertebrates and the part of the right auricle at the sino-auricular node in mammals. This is followed by contraction of auricles, then of the ventricles and finally the base of the aorta. Thus, the wave of contraction passes from one chamber of the heart to the other end is followed by the diastolic wave.

Therefore, the wave of relaxation of chambers one after the other. Auricles receive blood during their diastole. When auricles are in diastole the ventricles are in systole and auriculo-ventricles valves are closed. This helps in pushing the blood from ventricles into the aorta, whose base contracts and pumps the blood into different arteries. Soon ventricles being their diastole, the auriculo-ventricular valves open and the empty ventricles are again filled with blood. 

The time of the cardiac cycle depends upon the rate of the heartbeat. The number of beats or number of complete cycles per minute varies from 25 in elephant to about 175 in the mouse. 

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