Human Heart and circulatory system

Are you looking for information about the Human Heart and the circulatory system? Are you looking for information about how it works the structure and functioning of the heart and the blood vessels and how can this system get diseased? Or are you looking for information about the medicinal treatment available in the different medicinal practices available? Read this article to know complete information about how the Human Heart works and follow the table of contents to read information about the exact information that you want to know about the CardioVascular system.

Introduction to the Human Heart and the circulatory system

The Human Heart is one of the most vital organs in the whole human body. Acting as a pump, the human heart is one of the most significant organ which is hollow and muscular with its main function being to pump the blood through the arteries to the whole body and to the lungs for getting oxygenated through a different channel of blood vessels. The heart facilitates provision of nutrition, oxygen, removal of metabolic waste products through life. The heart never rests and stopping of the heart and other vital organs can lead to stopping of functioning of the muscles, organs and related systems leading to death and cessation of the human being.

History, discovery and development about the heart

The Ancient beliefs

The heart was not a fully understood organ until quite recent. Research on the heart started quite early, but was misunderstood as most of the work was done on dead bodies. The arteries and the heart are usually empty as the blood gets back into the veins because of gravity, thus leaving the arteries in a empty state. Researchers of the ancient days thought that these vessels were used to carry air throughout the body. Such strong was the belief that arteries carried air in them, that many philosophers and physicians thought that the air which escaped out of the arteries was replaced by blood which went into them through very small vessels called as capillaries.

Heart as a air sucking organ

It was thought in those days that as the Chyle from the Stomach and intestines reached the Liver, it formed venous blood over there and heart was the site where arterial blood was formed when it gets mixed with air over there. The great physician of 2nd century AD used to think that arterial blood was formed when the Venous blood flowed from the left heart to the right heart through pores and then the sucking ability of the heart was the one responsible to mix air with blood and the arterial blood created vapors which were then exhaled.

Modern advances

It was only in 1628, through the writings of William Harvey in De Motu Cordis, that a complete description of the system circulation and pulmonary circulation was mentioned and accepted. William Harvey also mentioned about the mechanical forces of the heart. The discovery of the Purkinje Fibres, the Frank-Starling mechanism, the Sino-atrial and Atrioventricular node and the electrocardiogram led to many changes in the actual beliefs of the Human heart and how the heart and blood system worked in the human body.

Advances towards heart surgeries and transplantation

In the mid 20th century, a much proper and detailed understanding of the heart, led to various surgical procedures such as Heart transplantation. While, the rate of success has been improving substantially than before, research is an ongoing process and the effects of various influences, diet, exercise, common medications are still being understood to a large extent.

Location of the Heart

The heart is located in the Upper part of the trunk, also called as chest. The mediastinum of the chest occupies the heart in its middle compartment. The location corresponds to the T5 to T8 – Fifth to Eight vertebrae of the Thoracic spine.

Relations to the Heart

The relates of the heart are as follows :

  • Anterior: The Sternum is located immediately anterior to the heart. The Sternum is also known as the Breastbone and is centrally located in between the rib cage. The anterior rib cage and the intercostals muscles and rib cartilage also form a part of the anterior relations.
  • Posterior: The Vertebral column with the Spinal code form the posterior relation of the Heart. While the heart is located with a left sided offset, still the major bulk of posterior relations are formed by the above. Other minor relations are
    • Ribs
    • Intercostal muscles
    • Inferior Vena Cava
    • Oesophagus
  • Superior: The Superior mediastinum with:
    • Esophagus
    • Bronchi
    • Thymus gland
    • Superior Vena Cava
    • Further away Thyroid Gland
  • Inferior: The inferior relation is formed by the Diaphragm and lower part of the left lung
  • Lateral: Lateral relations are formed by the Left and Right Lungs respectively.

Structure of the Heart

The Human heart is pinecone shaped and is said to be of the shape of one’s palm and fingers if they are kept straight. It is also said that if you keep your hand in your trouser’s pocket, the direction in which you keep your hand, is the direction in which the heart sits in the chest.


The heart measures around 5 inches (12 cm) in length, 3.5 inches (8 cm) in width and 2.5 inches (5 cm) in thickness. People who do regular exercise and are involved in sport activities can have a larger heart as they have higher demands of blood supply and thus a positive capacity growth effect on the heart.

Weight of the Heart

The heart weighs around 250 to 350 grams. Weight of the heart can be more in heavy workers or sports people. Similarly the heart weight may be low in people who are dwarfs, have low activity or are weak.

The Pericardium

The heart is enclosed in a sac like structure. The sac covers the muscles of the heart, the various structures inside the muscular walls. It also gets attached to the Pericardiac Pleura and then to the sternum, by which, it provides anchorage and stability to the heart from the anterior aspect. The Pericardium is a two layered structure. These two layers lie in close proximity to each other but are not directly attached. They contain and release a fluid which is called as the Pericardial fluid in between them. This liquid maintains the lubrication, smoothen the constant relaxation and contraction movements of the heart and prevents from any shocks to the heart.

Vessels entering into the Heart

The heart is a hollow organ and works as a pump for blood to flow throughout the body. The heart has blood vessels which bring the blood into the heart and there are blood vessels which take blood out of the heart. And apart from these there are blood vessels which feed blood to the heart vessels. Thus the heart has the following:

  1. The two great veins:
    • Superior Vena Cava
    • Inferior vena Cava
  2. The two great arteries:
    • Aorta
    • Pulmonary trunk
  3. Coronary circulary: This supplies blood to the cells of the heart

Shape of the Heart

The heart is conical in shape. The broad part of the cone forms the top of the narrow pointed portion appears at the bottom part. The bottom part of the heart is also known as the apex.

Direction and placement of the heart

The heart is present more on the left side of the mediastinum. The left lung is smaller in size and occupies the heart. The right side of the heart can be seen as pointing forwards while the left side of the heart is pointing towards the left, downwards and backwards.

Chambers of the Human Heart

The human heart has four chambers in it. These chambers are hollow spaces inside the organ which gets filled with blood through the muscular movements. These movements are broadly categorised as Contraction and Relaxation. There are two types of chambers in the heart. The two chambers which are located on the top are called as Atria and the ones located below are called as Ventricles.


Atria are responsible for receiving the blood into the heart.


Ventricles are responsible for discharging the blood out of the heart.
The Atria and Ventricles are separated by a groove called as the Coronary Sulcus. These grooves have flap like structures in them. These flaps act as trapdoors and are called as the Atrioventricular valves. They have specific names – Mitral Valve and Tricuspid valve. Mitral Valvce is also called as Bicuspid valve.

The Atria have small ear shaped appendages which are called as Auricles.

The Left Atrium and the Left Ventricle are together known as the Left Heart. The Right Atrium and the Right Ventricle are known together as the Right Heart.

The left heart and the right heart are separated by a groove. This groove is known as the groove is known as the Posterior Interventricular Sulcus.

Skeleton of the heart

The heart is supported by a dense and strong tissue. This tissue is formed of Collagen type of Connective tissue fibers. They formed the AtrioVentricular septum. This septum separates the Right heart from the Left. The four heart valves are also formed by this collagen tissue. The components of the AtrioVentricular septum are the interatrial septum for dividing the atria and the interventricular septum to divide the ventricles. The septum in between the ventricles is thicker than the septum in between the atrium.

The collagen tissue further forms cord like structure in the Ventricles. These chords are known as Chorda tendinae. They support the Atrioventricular valves and provide proper functioning and prevent regurgitation of blood.

The septum are also carry a special electrical system which powers the contraction and relaxation movements of the muscle layers of the heart. The muscle layer of the heart is known as the Myocardium.

Components of the Electrical System of the Heart

The electrical system consists of the

  • Sino-Atrial Node (SA Node)
  • Atrio-Ventricular node (AV Node)
  • Bundle of His
  • Left Bundle Branch (LBH)
  • Right Bundle Branch (RBH)
  • Distributing Purkinje Fibres

Layers of the Heart

The Heart walls are formed of multiple layers. Each layer has its own specific function.

  • Pericardium

    Pericardium is the double layered membrane and surrounds the heart and the base of the great vessels into it. It is also called as the Pericardial sac.

    • Outer Fibrous Pericardium

      This layer is made up of dense and strong connective tissue. It works to protect the heart from injuries and is located in the Thoracic cavity of the Rib cage.

    • Visceral Serous Pericardium

      This layer is responsible for secretion of pericardial fluid, lubrication and other functions. It is composed of:

      • Parietal Pericardium

        This layer is fused with the Fibrous pericardium.It forms the internal lining to secrete Pericardiul Fluid.

      • Inner Visceral Pericardium / Epicardium

        This layer is also called as the Epicardium. It secretes Pericardial fluid in the Pericardial Cavity. This layer consists of different types of cells. These are:

        • Simple Squamous epithelium called as Mesothelium
          These cells are responsible for production of the pericardial fluid. Pericardial fluid is a serous fluid which has lubricating action. It reduces the friction as the heart contracts and relaxes. The pericardial fluid also facilitates the heart to move in accordance the movements of the diaphragm and the lungs.
        • Areolar connective tissue
          The areolar connective tissue are present in a loose and irregular manner. They form discrete connections with the parietal Pericardium.
  • Myocardium

    The myocardium is the main layer present beneath the Epicardium. It is basically a framework of collagen supporting fibers. Onto these collage fibers, the Heart muscle fibers grow and are present in a specific pattern. The muscle fibers of the atria curve from both sides towards the inter atrial septum, while the muscle fibers of the ventricles are present in swirling form. The myocardium also houses the blood vessels that supply the heart itself along with the nerves carrying electrical impulses and feedback. The muscle fibers are present in the figure of 8. Along with these, there are muscle fibers which wrap the ventricles as well.

  • Endocardium

    The endocardium forms the inner lining for the heart. It is made of simple squamous epithelium cells and is connected to the Myocardium with the aid of a thin layer of connective tissue. The functions of the endocardium are:

    • Regulation of growth patterns of the Heart cells.
    • Secretion of Endothelins
    • Environment creation to regulate concentrations of ions and contraction states.
    • Enable causation of vasoconstriction and maintain a balance in between vasoconstriction and vasodilation.

The Heart Cycle

The heart cycle represents the working of the heart by which the blood is pumped and circulated through the two systems in the body – the Systemic circulation and the Pulmonary circulation.

The Systemic Circulation

The Systemic circulation is the pathway or channel through which pure and oxygenated blood is circulated throughout the body and impure blood is collected back to the heart.

The Pulmonary circulation

The Pulmonary circulation is the pathway or channel through which impure blood is circulated to the lungs to get oxygenated and then which is collected back to the Heart.

The Cycle

The heart cycle is approximately about 0.8 seconds in average adults. The cycle can be less than 0.8 seconds in infants and young children. The same can be more than 0.8 seconds in adults and geriatric people.

While the cycle is a continuous phase and there is no starting point, we will take the incoming of the blood into the Left atrium as the starting point.

Flow of blood in the Left Heart

As the Atrium relaxes, blood gets pulled into the Atrium from the four pulmonary veins The Atrium gets inflated with the incoming blood and until optimal pressure gets built up in the Atrium, the Bicuspid valve – also called as the Mitral valve – remains closed. As the pressure gets built up, the Mitral valve open and the pressurized Atrium releases the blood into the Left ventricle. This action happens approximately in about 0.3 seconds.

Blood from the Left Atrium flows first rapidly because of the built up pressure in the Atrium and then flows passively until the Mitral valve close. The passive flow is also because of the empty Ventricle which also gets relaxed.

Now as the Mitral valve close, one can feel or hear the first heart sound, which is quite intense and can be heard as “LUBB”.

Now as the Mitral valve gets closed, blood being in the Ventricle, and the semicircular valve opening into the Aorta also being closed, pressure starts building inside the Ventricle. The ventricle also starts contracting because of incoming electric impulses through the Purkinje fibers.

The Chorda tendinae are also holding the Mitral valve closed as they are pushed upward. Now, when optimal pressure builds up, the Semicircular valves open and the blood flows into the Aorta. Again the flow is fast in the first phase and it slightly reduces as the Ventricle empties itself into the Aorta. As the pressure diminishes, the Semicircular valves fall down and the Aorta gets closed from the Ventricle. Closure of the Semicircular valve creates the second heart sound which is audible as “DUBB”. This prevents regurgitation of the blood back into the Ventricle.

Flow of blood in the Right Heart

Now as the blood gets circulated throughout the body, impure and de-oxygenated blood gets collected through the capillaries and is ultimately collected into the Superior Vena Cava and Inferior Vena Cava.

These two cavae open into the Right Atrium of the heart. Again as the right atrium gets filled with blood from the two cavae, the tricuspid valves open and the blood then flows into the Right ventricle. When the tricuspid valve closes, pressure starts building up in the Right Ventricle, the Semicircular valves to the Pulmonary artery open and the blood starts flowing into the vessel, first at a higher pressure and then followed with a comparatively slower speed until a stage when the pressure is low enough for the valves to close.

After the blood enters the Pulmonary Artery, it goes through the Lungs, get purified and oxygenated. After the blood gets oxygenated, it returns back to the Left Ventricle of the heart through the 4 pulmonary veins.

How does the Heart contract and relax?

The contraction and relaxation as seen in the heart cycle above paragraphs, takes place according to the flowing electric currents in the heart. Studies have shown that the Left heart and the right side of the heart are able to have their own signals, contraction and relaxations. Still, both of the cycles happen at the same time and thus are nearly similar. The heart of an adult is able to produce its own electric signals. In such a state, when proper stimulus is not received from the Brain, the Heart can still continue to pump blood on its own.
The components mentioned below are responsible for generation, distribution and conduction of the Electical impulses throughout the heart.

The Cardiovascular centers for the Heart in the Brain

The Cardiovascular centers are a pair of specialised cells in the Medulla Oblongata of the Brain which give rise to the Sympathetic and Parasympathetic nerves which get connected to the SA Node and various surfaces of the Heart. The Sympathetic nerves are responsible for increasing the contraction of the heart while the Parasympathetic nerve (Vagus nerve) is responsible for reducing the contraction of the heart.
The parasympathetic and sympathetic nerves form a network called as the Cardiac Plexus near the base of the heart. The impulses get moderated over here and the Sino Atrial node receives the electric impulse which is then passed further to the Heart fibres.

The Sino Atrial Node (SA Node)

The Sino Atrial Node is also known as the Pacemaker of the heart. The cells in the SA Node are a specialized type of Cardiomyotes which are able to create, store and depolarize electric charge after receiving stimulus from the Vagus and the Sympathetic Cardiac nerves. From the SA Node the electric currents flow through the Atria in a radial manner and reach the AtrioVentricular Node.

The Atrioventricular Node (AV Node)

The AV Node is the second cluster of specialised Myocardial conductive cells. They are situated beloe the SA Node and in between the Atria and the Ventricles. The major function of the AV Node is to prevent the electric impulses to immediately enter into the Ventricles. The AV Node gets depolarised and the impulses are passed on to the interventricular section through the Bundle of His.

Bundle of His


The Bundle of His is the end portion of the AV Node. After the AV Node, the Bundle of His splits into two branches. These two branches are known as the Left Bundle Branch (LBB) and the Right Bundle Branch (RBB). As these Bundle branches pass through the Interventricular septum, the LBB activates the Left Ventricle, while the RBB activates the Right Ventricle. The LBB splits further into the Left Anterior Fascicle and the Left Posterior Fascicle.

Punkinje Fibers

The LBB and RBB further split into distinct fibers and spread out through the ventricles. These fibers are called as the Purkinje Fibers.

This whole system is responsible for the conduction of the electrical impulse and carry out the rhythmic heart cycle.

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