Structure of human heart and its functioning

The heart is the most vital organ of human body whose main function is to circulate blood and transport various materials within the body. This article provides the detailed structure of heart and the way it works to accomplish its functions in human body. This article also deals with the various important blood vessels associated with the heart and also the characteristic double circulation of blood that occurs in human body. Some frequently asked questions were also provided along with this article.


The heart in the human body is considered to be a pumping machine as it pumps the blood to various parts of the body ranging from the tip of the head to the tip of the toe.

The human heart is roughly triangular in shape and it is broad at its upper end and cone-shaped at its lower end. An adult human heart weighs about 250-350 gms. The heart is located on the median position of the chest region and its base is situated on the diaphragm. The popular belief is that the heart is towards the left side of the chest is due to the fact that while the heart contracts, the lower cone end of the heart gets projected towards the left side. The delicate heart is protected by the bony rib cage and two spongy lungs covered on either side of the heart. The heart is covered and protected by a double membranous sac called Pericardium. A fluid called pericardial fluid fills between the gap of the two membranes of the sac and that acts as a lubricating agent to reduce the friction and mechanical damage to the heart as it contracts and expands during the heartbeat.

Internal structure of human heart
The internal structure of the human heart (

Structure of human heart

In a vertical section view (V.S) of heart, we can find four distinct chambers in it. The upper two chambers of the heart are known as auricles or atria and the lower two chambers are known as ventricles. The right side chambers of the heart are separated from the left side chambers by Inter auricula ventricular septum. This septum helps to separate the deoxygenated blood in the right side of the heart with the oxygenated blood present on the left side of the heart. This feature helps to increase the efficiency of oxygen supply to the entire body as the blood is not mixed like in some other organisms.

The walls of the upper two auricles are much thinner and less muscular when compared to the walls of the two lower ventricles of the heart. The reason for this is, the auricles receive blood from different parts of the body and just push the blood to their nearby ventricles. But the ventricles have to push the blood out of the heart to reach the various parts of the body. Thus the walls of the ventricles are much thicker and more muscular. The wall of the left ventricle of the heart is thicker than the right ventricle because the left ventricle has to push oxygenated blood with great force so that it can reach to the tip of the head to the tip of the toe. But the wall of the right ventricle is thinner than the left ventricle because the right ventricle has to push the deoxygenated blood to the nearest lungs for purification.

Valves of the heart

A valve is an arrangement present in the heart to direct the flow of blood in one direction only. Tricuspid valve or Right auriculoventricular valve is a valve present between the right auricle and the right ventricle. When the right auricle contracts, the deoxygenated blood from the right auricle will be pushed into the right ventricle through the aperture of the tricuspid valve. But this valve helps to prevent the back of blood when the right ventricle contracts. This valve has three cusps (leaf apex like projections) and thus this valve is known as a tricuspid valve.

A Bicuspid valve or Mitral valve or left auriculoventricular valve is present between the left auricle and the left ventricle. It allows the oxygenated blood to flow from the left auricle to the left ventricle through the aperture of this valve but it prevents the backflow of blood. This valve has three cusps and thus this valve is known as the bicuspid valve.

Tricuspid and tricuspid valves are held in position by special structures called Chordae tendinae that connect valves with the walls of the ventricles. These structures hold the valves in position and prevent overturning into atria.

Pocket shaped pulmonary semilunar valves are present at the base of the pulmonary artery in the heart that prevents the backflow of blood and allows the deoxygenated blood to move into the lungs for purification.

Three pocket shaped aortic semilunar valves are present at the base of the aorta prevents the backflow of blood and help to push oxygenated blood to move out of the heart to get distributed to all parts of the body.

Main blood vessels associated with heart

The blood vessels that enter and leave the heart will play an important role in its functioning. The chief blood vessels that enter and leave heart are

  1. Anterior vena cava or Superior vena cava or Precaval

    It is the major blood vessel (vein) that brings deoxygenated blood from the upper parts of the body like head, chest, and arms into the right auricle.
  2. Posterior vena cava or Inferior vena cava

    It brings deoxygenated blood from lower parts of the body like the abdomen and legs and pours into right auricle of the heart.
  3. Pulmonary artery

    This blood vessel originates from right ventricle and carries deoxygenated blood from right ventricle to lungs for its oxygenation. Arteries usually carry oxygenated blood but this is the only artery that carries deoxygenated blood. Pocket shaped semilunar valves are present at the base of the Pulmonary artery to prevent backflow of blood.
  4. Pulmonary veins

    They start from lungs and carries oxygenated blood from lungs to left auricle of the heart. Veins usually carry deoxygenated blood but pulmonary veins carry oxygenated blood.
  5. Aorta

    It is the largest artery that originates from the left ventricle and that carries oxygenated blood from the heart to all parts of the body. This largest artery shows many branches and these branches supply blood to various parts of the body like abdomen, intestine, liver, kidneys, etc.
  6. Hepatic artery

    It is a branch of aorta that carries oxygenated blood to liver.
  7. Hepatic vein

    It carries deoxygenated blood from liver to vena cava.
  8. Renal artery

    It is branch of aorta that carry oxygenated blood to kidney.
  9. Renal vein

    It carry deoxygenated blood from kidneys to vena cava.
  10. Coranary arteries

    Right and left Coronary artery branches that arise from the base of aorta supplies oxygenated blood to the muscle of the heart. Any blockage that occurs in these arteries will lead to the deadening of the muscle tissue of the heart due to a lack of oxygen and glucose supply to the heart muscles. This specific condition is said to be a heart attack or technically known as Myocardial infarction.
  11. Hepatic portal vein

    It is a blood vessel formed by the union of blood capillaries that originated from digestive organs and carries digested nutrients into the liver for their metabolism. Actually a portal vein is one which starts as blood capillaries and also ends as blood capillaries. Hepatic portal vein starts as blood capillaries in digestive organs and it ends as blood capillaries in the liver.

Pacemaker or Sinoatrial node (SAN)

It is a patch of muscle tissue present at the base of the right atrium near a tricuspid valve on the inter auricular septum. It is the structure that initiates a heartbeat. It generates a wave of cardiac impulse that spreads from auricles to ventricles. This impulse is conducted to ventricles with the help of special conducting fibers called Purkinje fibers. These cardiac impulses can be recorded by an instrument called Electrocardiograph. The graphical record of these changes in the heart is called Electrocardiogram (ECG). Sometimes the Pacemaker of the heart may become defective or damaged and in such cases, an artificial Pacemaker is implanted in such persons to regulate the functioning of the heart.

Cardiac Cycle

The heart alternatively contracts and relaxes in a systematic way. The contraction phase of the heart is known as systole and the relaxing phase of the heart is known as diastole. One systole and one diastole together constitute a cardiac cycle or heartbeat. Each heartbeat or cardiac cycle lasts for about 0.85 seconds. Systole of atria takes around 0.15 seconds, systole of ventricles takes around 0.30 seconds, and all the chambers in diastole condition for about 0.40 seconds that altogether constitute 0.85 seconds.

Heart sounds

When we place a Stethoscope on the chest of a person and if we hear the sound of the heartbeat, we can here LUB and DUB sounds. The LUB sound produced in a heartbeat due to the closer of tricuspid and bicuspid valves closed sharply at the start of ventricular systole. Similarly, the LUB sound can be heard due to the closer of the semilunar valves at the base of the pulmonary artery and aorta at the beginning ventricular diastole.

The heartbeat rate of humans is 72 times per minute. Organisms that are smaller in size will have a higher heartbeat but organisms that are larger in size will a less heartbeat rate. For example, an infant child's heartbeat rate is 140 times per minute, rat 250 times per minute, Elephant 25 times per minute, and Whale 15 times per minute. Smaller the body the metabolic rate will be higher and larger the body the metabolic rate will be lesser. Thus, smaller organisms will be compared to be very active when compared to large-sized organisms.

Double circulation of blood in humans

Double circulation of blood is a characteristic feature of the human circulatory system. Circulation of blood twice through the heart before it gets circulated throughout the body is known as 'Double Circulation'. The two circulations that occur in the human circulatory system are Pulmonary circulation and Systemic circulation. These two circulations are together known as double circulation.

  1. Pulmonary circulation

    It is known as the shortest route of blood circulation. This circulation of blood is related to the lungs (pulmonary). The deoxygenated blood from the heart moves from the right ventricle through the pulmonary artery and pours it into the lungs for oxygenation. The oxygenated blood from the lungs then moves into the left auricle with the help of pulmonary veins. Here the blood one time goes out of the heart and reaches the lungs and again enters into the heart. This completes one circulation of blood through the heart.
  2. Systemic circulation

    In this circulation, the oxygenated blood from the left ventricle moves through the aorta and then gets distributed to all parts and tissues of the body with the help of arteries and blood capillaries that arise from aorta. Later the deoxygenated blood from all parts and tissues of the body again moves into the right auricle of the heart through vena cava. Here the oxygenated blood moving out of the heart for the second time and gets distributed to all parts of the body and then the deoxygenated blood gets returned to the heart again.

Functions of the heart

  1. Pumping oxygenated blood to all parts of the body
  2. Pumping deoxygenated blood from heart to lungs for oxygenation
  3. Maintaing blood pressure
  4. Help to push the blood containing metabolic wastes to kidneys for purification
  5. Recieves deoxygenated blood from all parts of the body through superior and inferior vena cava
  6. Helps to circulate and distribute hormones and other vital substance to all parts of the body.

Frequently asked questions

  1. What is the significance of double circulation in humans?
    Ans. Due to double circulation in humans, there is no mixing of oxygenated and deoxygenated blood. This makes to increase the efficient supply of oxygen to all parts of the body as well as the maximum removal of carbon dioxide at lungs.
  2. What is Pacemaker? State its chief function.
    Ans. A patch of muscle tissue at the base of the right atrium is known as Pacemaker. Pacemaker initiates a heartbeat. A pacemaker is also known as Sino-atrial node (SAN).
  3. Name the events in a cardiac cycle.
    Ans. A cardiac cycle includes systole and diastole of both atria and ventricles. In a cardiac cycle auricular systole, ventricular systole, and joint diastole are involved. In auricular systole the auricles contract, in ventricular systole, the ventricles will contract and in joint diastole both auricles and ventricles get relaxed. For one cardiac cycle, it takes 0.85 seconds.
  4. What is the role of valves present in the heart?
    Ans. They allow one direction of flow of blood and prevent backflow of blood.
  5. Name the four valves present in the heart. Ans.
    1. Tricuspid valves present between right auricle and right ventricle.
    2. Bicuspid or Mitral valve present between left auricle and left ventricle
    3. Three pocket shaped pulmonary semilunar valves present at the base of pulmonary artery
    4. Three pocket shaped aortic semilunar valves present at the base of aorta.
  6. What are chordae tendinae? State its chief role in heart.
    Ans. Tough strands of connective tissue that connects the valves of the heart to the wall of ventricles are known as chordae tendinae. Their main function is to hold the valves in position and prevents the overturning of valves into the atrium.


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