Human Circulatory System :

  • The circulatory system in humans is the main transport system
  • The blood carries oxygen, digested food and other chemicals like hormones and enzymes to all parts of the body.
  • It also takes away waste materials like urea and CO 2
  • The human circulatory system consists of the heart (the organ which pumps and receives the blood ), blood  and the blood vessels or tubes through which the blood flows in the body.

Human Heart :

Process of Excretion in Humans - Teachoo.jpg

  • Human heart is a four chambered structure having 2 atria (left and right) and 2 ventricles (left and right)
  • It is a double pump , which pumps blood around our body.
  • The left side of the heart (left atrium and left ventricle ) acts as one pump which pumps blood to the body , except the lungs.
  • The right side of the heart ( right atrium and right ventricle )only pumps blood into the lungs.
  • The left side of the heart is separated from the right side, by a partition called septum.
  • This separation is important to avoid mixing the oxygenated blood on the left side with the deoxygenated blood on the right.



  • Blood is a fluid connective tissue.  
  • It consists of three components: Blood cells, plasma and platelets.
  • It is the blood which circulates around the whole body and delivers oxygen and nutrients to the cells and tissues.
  • Blood is red in color because of the presence of the pigment called  hemoglobin  which has the ability to bind oxygen. 

Red Blood Cells:

Most of the cells in the blood are RBCs.

  • Red blood cells are red due to the presence of a red pigment called hemoglobin .
  • Hemoglobin in the RBCs carries oxygen in the blood from one part of the body to another.
  • RBCs are circular in shape. They do not have nuclei.
  • Red blood cells have a very short lifetime , due to which they have to be produced in the bone marrow very quickly. 

White blood Cells:

White blood cells fight infection and protect us from diseases .

  • They fight against germs and other foriegn bodies which cause diseases. 
  • Some WBCs can digest foriegn bodies . Others manufacture antibodies to fight infection and provide immunity .
  • WBCs are circular or irregular in shape.
  • They have nuclei , though the shape differs according to the functions of the cell.
  • They are lesser in number than RBCs.


The liquid or fluid part of blood is called plasma.

  • It is a colourless liquid which consists of 90%water and some other substances dissolved in it.
  • It contains Red blood cells, White blood cells and Platelets .
  • It also contains proteins , digested food , common salt , waste products (CO 2 and urea and hormones .


Platelets are tiny fragments  of special cells formed in the bone marrow. 

  • They do not have nuclei
  • They help in the coagulation of blood in cuts or wounds.

**All blood cells are made in the bone marrow from cells called stem cells

Blood Vessels:

The blood vessels are of three types :


Arteries : 

  • They carry blood from the heart to all parts of the body. 
  • Since they receive the blood directly from the heart which has a very high pressure , they are thick walled.
  • The main artery is called Aorta , carries oxygenated blood from the left ventricle to all parts of the body
  • The Pulmonary artery is connected to the right ventricle , which carries deoxygenated blood from the right ventricle to the lungs .

Veins : 

  • These blood vessels carry blood (deoxygenated) from the body back to the heart. 
  • They are thin walled because of the low pressure of the blood which flows in them
  • These have valves present in them which prevents backflow of blood.  
  • The main vein , called Vena Cava, is connected to the right atrium and carries deoxygenated blood from all parts, back to the heart
  • The pulmonary vein carries oxygenated blood from the lungs back to the heart.


Capillaries : 

  • These are thin walled, extremely narrow tubes which connect arteries to veins
  • The various dissolved substances like oxygen, food etc. present in blood pass through the capillaries. 
  • At the same time, the waste materials also enter the capillaries . Thus, an exchange takes place.


Double Circulation in the heart :


  • The left atrium of the heart receives oxygenated blood from the lungs. It further moves down to the left ventricle.
  • This blood is then pumped through the aorta to the entire body , to supply cells and tissues with oxygen.
  • Oxygen poor blood (deoxygenated blood) from the various body parts, returns to the heart. 
  • This blood enters the right side of the heart, through the right atrium and passes down to the right ventricle. 
  • These collect and pump the deoxygenated blood to the lungs through  the pulmonary artery. The lungs refresh the blood with a new supply of oxygen, making it 

The valves in the heart ensure that the blood does not flow backoxygenated .

  • Oxygenated blood then again enters the left side of the heart, through the left atrium and left ventricle. 

  when the atria or ventricles contract.

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Maninder Singh

CA Maninder Singh is a Chartered Accountant for the past 14 years and a teacher from the past 18 years. He teaches Science, Economics, Accounting and English at Teachoo