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Circulatory System
circulatory systemby derek*b on Flickr

Circulatory System

  • Circulatory System is the system in living organisms which provides for transport of chemical substances, heat, etc, throughout the body.

  • In humans, the circulatory system consists of a system of tubes through which blood, the main transporting agent in the human body, travels to each and every part of the human body.

  • Since the human circulatory system is based on the transportation of blood through blood vessels, the blood does not come in direct contact to tissues and muscles of the body. Thus, our circulatory system is classified as a closed circulatory system.


Parts and Function of Circulatory System

Circulatory system consists of the following main parts:-

  • Blood – the main transporting agent

  • Heart – the effector of blood flow

  • Blood vessels – arteries and veins


  • The blood is a bright red viscous fluid.

  • It flows through all the blood vessels in the body.

  • An average male has 5 to 6 liters of blood and an average female has 4 to 5 liters of blood.

  • The blood constitutes about 6% of the total body weight.

  • The temperature of the blood is maintained at 38 degrees Celsius.

  • The blood is made up of two substances: Formed elements and plasma.

  • Formed elements include red blood cells, white blood cells, and platelets.

  • Plasma is a straw-colored liquid consisting of about 90 to 92 % water and 8 to 10 % of dry matter. It includes proteins, food substances, etc.


Heart Diagram for kids

  • The heart is a hollow, muscular organ which pumps blood in the body.

  • It is located between the two lungs.

  • There are four chambers in the heart, through which the blood flows into arteries and veins. These are the left atria, the right atria, the left ventricle, and the right ventricle.

  • It is covered by a membrane. It consists of three layers, the outer (epicardium), the middle (myocardium), and the inner layer


  • The membrane that holds the heart in position is called the pericardium.


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Veinyby Powderruns on Flickr

Blood vessels

Blood vessels are the part of the circulatory system that transports blood throughout the body. 

There are three types of blood vessels:

  • Arteries - Carry the blood away from the heart.
  • Capillaries - Enable the actual exchange of water and chemicals between the blood and the tissues.
  • Veins - Carry blood from the capillaries back toward the heart.

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Veins Inside the Heart by CarbonNYC on Flickr

Veins are the blood vessels that carry blood towards the heart.

  • While arteries are thick and muscular, veins are very thin and contain almost no muscle.
  • The walls of veins are so thin that you can see the blood inside of them.
  • Veins carry blood from the body back to the heart. This blood is low in oxygen, depleted of nutrients, and loaded with waste products. 
  • If arteries are the supply train, veins can be thought of as the garbage trucks.

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Heart coronary arteriesby Patrick J. Lynch on Flickr

Arteries are what we call efferent vessels because they carry blood away from the heart. 

  • Every time your heart beats, blood is pumped into your arteries. 
  • The walls of the arteries stretch as the blood comes in. As it passes by, the walls relax. 
  • The stretching and relaxation of the artery wall is the pulse. 
  • You feel your pulse at your temples, neck and wrists. 

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capillariesby fo.ol on Flickr

Capillaries can be seen as the site in all tissues at which respiratory gases are exchanged between the tissue cells and the blood contained in them. 

  • They have very thin walls consisting of just one membrane.
  • They are located mostly between the veins and the arteries. 
  • Their job is to distribute oxygen rich blood that comes from the arteries out into the tissues of the body.
  • Then, they move the oxygen-depleted blood from the tissues back into the veins. 


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