•    Circulatory system consists of blood, blood vessels and heart.

BLOOD : Blood has following components - 

•    Blood is a liquid, which has cells of various kinds suspended in it. The fluid part of the blood is called plasma.
•    One type of cells are the red blood cells (RBC) which contain a red pigment called haemoglobin. Haemoglobin binds with oxygen and transports it to all the parts of the body and ultimately to all the cells. It will be difficult to provide oxygen efficiently to all         the cells of the body without haemoglobin. The presence of haemoglobin makes blood appear red.
•    The blood also has white blood cells (WBC) which fights against germs that may enter our body. Another type of cells in the blood are called as platelets, which helps in the clotting of blood.

Function of Blood
•    Blood is the fluid connective tissue, which flows in blood vessels. 
•    It transports substances like digested food from the small intestine to the other parts of the body. 
•    It carries oxygen from the lungs to the cells of the body. 
•    It also transports waste for removal from the body. 
•    It maintains equal temperature throughout of the body.

Blood vessels
      In body three types of blood vessels are present. These are arteries, veins and capillaries.
•    Arteries carry oxygen-rich blood from the heart to all parts of the body except pulmonary artery. Since the blood flow is rapid and at a high pressure, the arteries have thick, elastic walls.
•    Veins are the vessels, which carry carbon dioxide rich blood from all parts of the body back to the heart except pulmonary vein. The veins have thin walls. 
•    There are valves present in veins which allow blood to flow only towards the heart or prevent backward flow of blood.
•    The arteries divide into smaller vessels. On reaching the tissues, they divide further into extremely thin tubes called capillaries. The capillaries join up to form veins, which empty into the heart.