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What is Circulatory System?

The circulatory system can be defined as the system of organs and tissues, including the heart, blood and the blood vessels. It helps in transportation of minerals, nutrients, respiratory gases, excretory products, hormones and other substances throughout the body.

  • The heart and the blood vessels through which the blood is passed throughout the body constitute a system called circulatory system.

  • The lymph , lymph nodes and lymph vessels forms the lymphatic system.

  • Two fluids in our body are blood and lymph that moves through circulatory system.

Human Heart

It is a pulsatile organ that pumps the blood to all the parts of the body by rhythmic contractions and relaxations

Shape: it is triangular in shape.

Size: size of adult human heart is about 12cm in length and 9 cm in breadth. It is equal to the size of our closed fist. The average weight of the heart is about 350g in males and 300g in females.

Position: it is located in the thoracic cavity between the lungs protected by the rib-cage tilted towards the left side. The base of the heart rests on the diaphragm.

External Structure Of Heart

Externally heart is protected by the double layered membrane called pericardial membrane. The outer layer is called epicardium and inner layer is called pericardium. The space between the two layers is called pericardial space of pericardial cavity which is filled with the fluid called pericardial fluid.

Function of pericardial fluid: It acts as a cushion and protects the heart from external jerks, injuries, physical shocks and internal aberrations.

Internal Structure Of Heart

Chambers of the heart: Internally heart is divided into four chambers by septums:

  • Auricles ( right auricle and left auricle) – they acts as receiving chambers because they receive the blood coming from the body and lungs. Right auricle receives de-oxygenated blood from the body and left auricle receives oxygenated blood from the heart. Auricles are upper smaller chambers of the heart.

  • Ventricles ( right ventricle and left ventricle) – They act as distributing chambers because they send the blood through the arteries for distribution. Right ventricle pumps the de-oxygenated  blood to the lungs and left ventricle pumps the oxygenated blood to all the parts of the body, therefore the walls of left ventricle is thicker then the right ventricle.

Septums of the heart: Septums that divide the heart into four chambers are:

  • Interauriculo-ventricular septum – it divides the heart into auricles and ventricles.

  • Inter auricular septum present between the two auricles i.e it divides the auricles into left and right.

  • Inter ventricular septum present between the two ventricles i.e it divides the ventricles into left and right.

During the fetal development for proper blood circulation there is a hole in the inter auricular septum that helps in the proper communication between left and right auricles .this hole is known as foramen ovale.

After the fetal development foramen ovale closes by a fibrous tissue which remains as a depression on the interatrial septum. This depression on the right side of septum is called fossa ovalis.

 Blood vessels of the heart:

  • Superior and inferior vena cava: both the veins open in right auricle and brings deoxygenated blood. Superior vena cava brings de-oxygenated blood from the upper parts ( head and arms) of the body. Inferior vena cava brings de-oxygenated blood from lower parts ( trunk and legs) of the body.

  • Pulmonary artery: carry de-oxygenated blood from right ventricle to the lungs.

  • Pulmonary vein: it brings oxygenated blood from lungs to left auricle.

  • Systemic aorta: it is the largest artery emerging from the left ventricle. It carry oxygenated blood from left ventricle and distribute it to various parts of the body.

  • Coronary arteries: these are the first divisions of the aorta. These two arteries divide into many branches.

Valves of the heart:

  • Bicuspid or mitral valve: it is located between the left auricle and left ventricle. It has two cusps hence it is called bicuspid valve.

  • Tricuspid valve: it is located between right auricle and right ventricle. It has three cusps that is why it is known as tricuspid valve.

  • Pulmonary valve: it is located at the opening of right ventricle and pulmonary artery and prevents the backflow of blood from pulmonary artery into the ventricle.

  • Systemic valve: it is located at the opening of left ventricle and systemic aorta and prevents the backflow of blood into the ventricle and systemic aorta.

Pulmonary valve and systemic valve are collectively known as Semi lunar valves.

Double Circulation in Human Heart


In human beings blood flows twice through the heart to complete one cycle of circulation of blood, hence the name double circulation. It has two types of circulations: pulmonary circulation and systemic circulation.


Pulmonary Circulation: this type of circulation includes movement of de-oxygenated blood from heart to lungs and oxygenated blood back to heart from the lungs.

De-oxygenated blood from right ventricle goes to lungs through pulmonary artery where blood is enriched with oxygen. The oxygenated blood from lungs goes to left auricle through pulmonary vein.

What is Circulatory System?
Human Heart
External Structure Of Heart
Internal Structure Of Heart
Double Circulation in Human Heart
Pulmonary Circulation

Systemic Circulation: This type of circulation includes the movement of oxygenated blood from heart to body parts and  deoxygenated blood from body parts to back to heart.

Oxygenated blood from left auricle of the heart is distributed to all the parts of the body through systemic aorta. Oxygen is used by the body and carbon dioxide is produced which binds with the blood cell and blood becomes deoxygenated. The deoxygenated blood from the body parts enters in the right auricle through superior and inferior vena cava

Systemic Circulation

Cardiac Cycle


The sequence of events which takes place for the completion of one heartbeat is called cardiac cycle. The events of cardiac cycle are as follows:

  • Auricular systole

  • Ventricular systole

  • Auricular diastole

  • Ventricular diastole

Auricular diastole and ventricular diastole are together known as joint diastole.

Auricular systole: (contraction of auricles )

  • In this phase of cardiac cycle auricles contracts and the blood from the auricles is forced into the ventricles through tricuspid and bicuspid valve . In this phase ventricles are in relaxed state.

  • At the end of auricular systole , auricles start relaxing due to which more venous blood is filled in the auricles through great veins ( pulmonary vein and superior inferior venacava)

Ventricular systole: (contraction of ventricles)

  • In this phase ventricles contract and the blood is forced out of the heart through great arteries

  • On the onset of auricular diastole ventricular systole starts. Contraction of ventricles creates pressure in the blood due to which tricusoid and bicuspid valve closes while semilunar valves open and the blood is passed through great veins.

Joint diastole ( relaxation of auricles and ventricles)

  • At the end of ventricular systole ventricles start relaxing as well as auricles also get relaxed

  • At this stage blood continues to flow in the auricles through great veins. As soon as the auricles fill, it again starts contracting and cycle continues.

Heart Beat

  • One complete contraction and relaxation of heart muscles forms one heartbeat.

  • Healthy human heart beats for about 72 to 78 times per minute.

  • The first sound of heartbeat is “lubb”. It is a low pitched sound which is produced by sudden closure of tricuspid and bicuspid valve.

  • Second sound of heartbeat is “dub”. It is high pitched sound produced by the sudden closure of semilunar valves (pulmonary valve and systemic valve).

A single heartbeat starts with an electrical signal from a region of specialized tissue called sinoatrial node or SA node( SAN) located on the walls of right auricle. It acts as a pacemaker or heartbeat regulator. A second node called AV node (atrioventricular node) picks up the signal from SA node and transfer it to the ventricular septum through a group of special muscles called Bundle of His. From Bundle of His the signal spreads throughout the walls of ventricles through another muscles called Purkinje fibres.

Blood Pressure

  • Blood pressure is the pressure or force exerted by the blood on the walls of the arteries.

  • The instrument used to measure blood pressure is “Sphygmomanometer”.

  • Blood pressure is measured in terms of systolic and diastolic pressure.

Normal blood pressure of a healthy person is 120/80 mm Hg.

Systolic pressure is the pressure in the arteries when the ventricles contracts. Its normal range is 120mm Hg. Diastolic pressure is measured when the ventricles relax. Its normal range is 80mm Hg.

Hepatic Portal System

Portal system is the system that begins and ends in capillaries. Hepatic portal system is circulation blood between liver and other related organs. In this capillaries carries deoxygenated blood from stomach, intestine and spleen to the liver. The vein that carries blood to the liver is called hepatic portal vein as it is formed by the joining of capillaries.


Blood is a fluid connective tissue that plays an important role in transportation of substances from one part of the body to the other parts.

Components of Blood: Blood consist of two components – Blood Plasma and Blood Corpuscles.

Blood Plasma: It is a pale yellow or straw colored liquid part and constitutes about 55-65% of blood.

Plasma consist of:

a) 90-92% of plasma is water

b) 8% to 10% is organic and inorganic constituents which includes :

  • some salts

  • proteins (prothrombin, fibrinogen ,albumin, globulins, immunoglobulins etc.)

  • digested nutrients (glucose, fats, fatty acids, amino acids, vitamins etc.)

  • excretory substances (ammonia, urea, uric acid, creatine, creatinine etc.)

  • hormones (chemicals secreted from the endocrine glands)

  • dissolved gases (water in plasma contain dissolved gases like oxygen and carbon dioxide)

  • defense compounds (antibodies, lysozyme etc.)

  • it contains an anticoagulant named Heparin.

Blood Corpuscles : cellular part of blood is called blood corpuscles which constitute about 40-45%.

Blood cells are of three types- Red Blood Cells , White Blood Cells and Platelets.

a) RED  BLOOD CORPUSCLES:  RBCs are known as erythrocytes and constitute about 99% of blood part.

  • Shape – RBCs are disc like biconcave shape, flat at the centre and thick rounded at the periphery. Nucleus is absent in mature RBCs.

  • Size – RBCs are very small in size so that they pass through very fine blood capillaries throughout the body and their small size helps them to absorb more oxygen.

  • Life span –the average life span of RBCs is 120 days after that the dead RBCs are destroyed in the spleen and new RBCs are formed in haemopoietic tissues and bone marrow of long bones.

  • Structure – RBCs contains an iron containing pigment called haemoglobin. The red color of blood is due to the presence of haemoglobin. It consist of iron and globular proteins.

  • Functions – most important function of RBCs is to transport respiratory gases to all the parts of the body. It receives oxygen from the respiratory surface (lungs) and deliver it to all the cells of the body where energy and carbon dioxide are produced by using oxygen. The carbon dioxide produced is again transported by the RBCs to the respiratory surface (lungs) and is removed from the body. This function of RBCs is performed by haemoglobin.

  • Haemoglobin combines with oxygen present in the lung alveoli and forms an unstable compound called OXYHAEMOGLOBIN which on reaching the tissues where oxygen concentration is low breaks into oxygen and haemoglobin. Oxygen is then used by the cells and carbon dioxide thus produced again combines with haemoglobin forming another unstable compound CARBAMINOHAEMOGLOBIN which on reaching lungs releases carbon dioxide which is removed out of the body. In this way blood transports oxygen and carbon dioxide .

WHITE BLOOD CORPUSCLES:  Also known as leucocytes and are colorless.

  • Shape - WBCS are irregular in shape they have nucleus and are capable of showing amoeboid movement.

  • Size - they are much larger than RBCs.

  • White blood cells regularly move through the walls of capillaries and venules to enter into connective tissue and lymphatic tissue where they perform their specific function related to defence mechanism.  WBCs move through the walls by a special mechanism called ‘DIAPEDESIS’ i.e squeezing of WBCs to pass through the walls of capillaries.

Types of WBCS – on the basis of their shape and size, shape and size of nucleus and cytoplasm WBCs are of following types:

Blood Pressure
Heart Beat
Cardiac Cycle
Blood carpuscles
blood plasma
hepatic portal system
Types of White Blood Corpuscles

FUNCTIONS OF WBCs – White blood cells plays an important role in fighting against the disease causing pathogens either by phagocytosis of by producing antibodies. Hence WBCs provide immunity.

PLATELETS: also known as Thrombocytes are colorless.

  • Shape – they are oval or round shaped cells without nucleus

  • Life span – 8 to 10 days. New cells are produced in bone marrow

  • Function – platelets palys a very important role in blood clotting in the presence of vitamin K. Thrombocytes release THROMBOPLASTIN which initiates blood clotting.

Mechanism of Blood Clotting


Clotting of blood occurs only when the blood escapes from the blood vessels. Blood never clots when it is inside the blood vessels because of the presence of strong anticoagulant HEPARIN secreted from the Liver.

Howell has described the mechanism of blood clotting. It is a complex series of sequential changes which is grouped into three major steps:

  • Step 1 - At the site of injury when bleeding starts, the platelets adhere to the damaged tissues and release “PLATELET FACTOR 3” and the traumatized tissue releases “THROMBOPLASTIN”. Thromboplastin and platelet factor 3 combine with calcium and some plasma protein to form an enzyme called “PROTHROMBINASE”.

  • Step 2 - PROTHROMBINASE formed inactivates Heparin (anticoagulant secreted from the liver) and breaks plasma protein- PROTHROMBIN into THROMBIN and small peptide chains.

  • Step 3 - THROMBIN acts as an enzyme and breaks fibrinogen (present in plasma ) into FIBRIN . The thin, long and solid fibres of fibrin form a dense network upon the wound in about 2 to 8 minutes after injury. After few minutes the clot formed by the fibrin fibres contracts and a pale yellow liquid starts oozing out from it. This pale yellow liquid is called SERUM. Serum is blood plasma minus corpuscles and fibrinogen.

The prothrombin and fibrinogen are formed in liver with the help of vitamin K. So vitamin K is important for blood clotting.

Functions of Blood

  • Blood helps in transportation of digested nutrients from small intestine to all the parts of the body.

  • Blood transport respiratory gases to and from the lungs; it transport oxygen from lungs to the tissues and carbon dioxide from tissues to the lungs.

  • All the metabolic waste produced in the body is transported by the blood to the excretory organ for their removal.

  • Blood carries and transport chemicals secretions like hormones from their secretory glands to their target organs.

  • Blood helps in maintaining the body temperature by equally distributing the heat produced by the body to all the parts.

  • Blood helps our body to fight against the pathogens and thus plays a protective role and provide us immunity.

  • Help in clotting of blood to prevent excessive loss of blood.

Blood Vessels

In a closed circulatory system blood flows in a blood vessels. Blood vessels are of three types :

  • Arteries

  • Veins

  • Capillaries


  • These blood vessels carries deoxygenated blood from heart to different parts of the body. Therefore they are known as distributing vessels because they distribute the blood to all the body parts.

  • Blood in the arteries flows with high pressure because of which the walls of the arteries are thick to prevent the walls of the arteries from rupturing.

  • The lumen of the arteries is narrow. The reduced lumen increases the pressure of the blood in them.

  • Arteries are located deeper than the veins

  • Walls of the arteries have all three layers; tunica externa, tunica interna and tunica media.

  • Each Artery divides into hundreds of arterioles and each arteriole divides into hundreds of capillaries to distribute the blood to each and every cell of the body.


  • These are microscopic vessels that carries blood from arterioles to the venules and helps in exchange of materials between blood and cells  .

  • There walls are thin and perforated made up of only single layer, tunica interna with a wide lumen and valves. Their perforated walls allows the escape of white blood cells and plasma whiah is collected in the lymph nodes and is called as lymph.


  • Veins carries deoxygenated blood towards the heart. Deoxygenated blood is collected in the vein through venules (division of veins). Therefore veins are known as collecting vessels because they collect the deoxygenated blood from body parts to the heart.

  • The blood in the veins flows with low pressure and has valves inside them to prevent back flow of blood.

  • The walls of the veins are thin and lumen is wide. All the three layers that is Ttunica externa, tunica interna and tunica media are present but are very thin as compared to arteries .

blood vessels
functions of blood
mechanism of clotting
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