Human Circulatory System | Short Essay for Children

December 16, 2017 0 Comment

The circulatory system is an organ system composed of blood, heart and associated blood vessels. The major function of the circulatory system is the circulation of blood, which transports hormones, gases, minerals, nutrients and waste materials in the various parts of the body.

Human beings have a closed circulatory system meaning that their blood flows in blood vessels unlike some lower animals in which blood flows freely in the tissue spaces present in their body as they lack any blood vessels.  


The blood is red coloured connective tissue which carries materials to the various parts of the body. It is pumped by the heart. The red colour of the blood is due to the presence of iron containing pigment, Haemoglobin.

The Haemoglobin helps in transport of oxygen. One haemoglobin molecule carries 4 oxygen molecules. Blood is composed of Plasma, also known as the mother liquid and formed elements.  


Plasma is a straw coloured viscous liquid which has all the formed elements floating in it. It contains 90% water, 7-8% proteins (globulins, albumins, fibrinogen etc.), 1% inorganic salts (cations and anions) and 1-2% other substances such as anticoagulants, antibodies, cholesterol, gases, hormones, waste materials such as urea.

There are many functions of plasma some of which are transport of food, gases, waste products, regulation of pH, temperature, lymph formation and prevention of blood loss. 

Formed elements: The formed elements consist of blood cells and platelets. They are

Red Blood Cells

Also known as erythrocytes, the red blood cells are red in colour due to the presence of Haemoglobin. The RBCs do not have nucleus so that more of oxygen molecules can be carried by it. They are biconcave, circular and have a life span of 120 days.

The normal RBC count in a healthy human is 4.5-5 million RBCs per cubic millimetre of blood. Anaemia is a condition in which the number of red blood cells decreases abruptly.

Abnormal rise in RBC count is known as polycythaemia. The RBCs are formed in the Reticuloendothelial cells of the red bone marrow. It is destroyed in the liver and spleen. Thus, Spleen is also known as the ‘graveyard of RBCs’. 

White blood cells

Also known as Leucocytes. They lack haemoglobin. The average WBC count is 6000 to 8000 cells per cubic millimetre of blood. They are synthesised in lymph nodes, thymus, spleen and red blood marrow. The white blood cells are of two types- Granulocytes and Agranulocytes.

Granulocytes contains granules in their cytoplasm and are further of 3 types: Basophils which have S- shaped nucleus and contain histamine and is responsible for allergic reactions; Eosinophils which have a bilobed nucleus and have antihistamine properties, they are also phagocytic; Neutrophils which have a many lobed nucleus and have hydrolytic enzymes.

The Agranulocytes do not contain granules in their cytoplasm and are further of 2 types: Monocytes which have a bean shaped nucleus and are phagocytic in nature and Lymphocytes (B lymphocytes and T lymphocytes) which secrete antibodies and are responsible for the immune response of the body. 


Also known as thrombocytes. They also lack haemoglobin and hence, are colourless. They are formed in the red bone marrow by the process of thrombopoietic.

They are not exactly cells but are cell fragments. The platelet count in a healthy human is 250000 platelets per cubic millimetre of blood. They are mainly involved in blood clotting. 


The human heart is muscular organ which is involved in pumping of blood to the various parts of the body. It is roughly the size of a clenched fist. The heart is located almost in the middle of the thoracic cavity and is slightly tilted to the left. It is covered by a tough sac known as the pericardium. It is protective in function.  

Human Circulatory System essayThe internal structure of the heart consists of four chambers. 2 auricles and 2 ventricles. There is a left auricle and right auricle. In the similar way there is a left ventricle and right ventricle. The auricles act as the collecting chambers for the blood.

The right auricle receives deoxygenated blood from superior and inferior vena cava and the left auricle receives oxygenated blood from the lungs via the pulmonary vein. In the similar way, the ventricles act as pumping chambers.

The right ventricle receives the deoxygenated blood from the right auricle and pumps it to the lungs via the pulmonary artery for oxygenation. The left ventricle received the oxygenated blood from the left auricle and pumps it to the rest of the body via the aorta. As ventricles have the role of pumping the blood, they are composed of thick muscles.  

Each auricle and ventricle is separated by atrioventricular septum. In the similar way, left and right auricles are separated by an interarticular septum and left and right ventricles are separated by interventricular septum.

The flow of blood from right auricle to right ventricle is guarded by tricuspid valve and the flow of blood from left auricle to left ventricle is guarded by bicuspid or the mitral valve.

Also, the flow of blood from right ventricle to the pulmonary artery is guarded by pulmonary valves and flow of blood from left ventricle to the aorta is guarded by semi lunar valves.  

The heart consists of three layers: Endocardium, Myocardium and Epicardium. The heart also consists of the Sino atrial node or the SA node which helps in initiating a wave which in turns spread to the whole musculature of the heart, hence resulting in the ‘beating’ of the heart.

It is also known as the pacemaker of the heart. The SA node is the part of the nodal tissue of the heart along with the Atrioventricular node, bundle of His and Purkinje fibres. The rate of heart beat is controlled by the central nervous system (parasympathetic and sympathetic) and the hormones (epinephrine and norepinephrine). 

Blood vessels

The muscular, elastic tubes which carry blood are known as blood vessels. There are two types of blood vessels- Arteries and Veins. They are joined together by very thin blood vessels known as capillaries.  

Artery has wall composed of 3 layers- tunica intima, tunica media and tunica externa. They carry blood away from the heart for distribution to the body. The flow of blood in them is fast. 

Veins are also composed of 3 layers just like arteries. They carry blood from the body back to the heart. The flow of blood in veins is slow and steady. 

Blood groups  

Human Circulatory SystemThere are 4 types of blood groups under the ABO blood grouping system given by Karl Landsteiner. Those are- A, B, AB and O. The blood groups occur due to the presence of specific antigens present on the surface of RBCs. When the antigen A is present on the surface of RBC, the blood group is A and hence, the antibodies present in the plasma are anti- B.

In the same way, blood group B will have antigens B on the surface of RBCs and anti- A antibody in plasma. Blood group AB has both A and B antigens on RBC surface and no antibodies in plasma.

On a similar line, Blood group O has no antigens on RBC surface and has both anti- A and anti- B antibody in plasma. Thus, AB blood group is the universal receiver and blood group O is the universal donor. 

Another system of blood grouping is Rh blood grouping due to which blood groups are either Rh positive or Rh negative. It is due to the presence or absence of certain antigen, known as the Rh antigen on the surface of RBCs. 

Heart and rate and its measurement: The heart rate or the number of heart beats per minute is 72. It can be measured by checking the pulse of a person. 

Blood pressure

The pressure caused by blood against the walls of the blood vessels is known as blood pressure. The temporary rise in blood pressure due to contraction of the heart is known as systolic pressure.

The temporary fall in blood pressure due to relaxation of the heart is known as diastolic pressure. Blood pressure is determined by an instrument known as sphygmomanometer. A healthy adult person has systolic/diastolic pressure of 120.80 mm of Hg.  

Disorders of the Circulatory system

  • Hypertension: Emotional stress such as fear, anxiety, anger etc. may cause a permanent increase of blood pressure. It is also known as a silent killer and can cause damage to the eyes, kidneys and various other internal organs. In worst cases, it can also cause stroke. 
  • Angina Pectoris: It is the pain in heart muscles due to the oxygen deprivation in them. It can also be a warning of heart attack. 
  • Heart Attack: Formation of clot or deposition of fats in the walls of the artery which stops supply of blood to the heart. Hence the muscle cells die at that particular place. This condition is known as heart attack. 

Prevention of heart diseases: The heart diseases can be prevented by taking certain precautions. Some of them are: 

  • Stopping the usage of drugs, tobacco, and alcohol. 
  • Doing regular exercise. 
  • Consulting doctor in case of any chest pain. 
  • Avoid consumption of too much oily food. 
  • Keeping the body weight normal and avoid being obese. 
  • By avoiding tension and stress. 

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