Blood transports food and oxygen to all the cells of the body. It is made of fluid plasma and cells. Plasma which transports glucose, enzymes and hormones to cells, carries waste material from liver to kidneys for removal.

Blood Cells

RBC (Red blood cells or Erythrocytes) for transport of O2 to cells by combining with haemoglobin. RBCs are red as they contain the red pigment Haemoglobin. They have no nuclei. In lungs, haemoglobin of RBC combines with oxygen and forms a compound known as oxy-haemoglobin. Blood carrying oxygen reaches all the cells of body where gas exchnage takes place. Oxygen goes into cells and CO2 comes into it from cells.

WBC (White blood cells or Leucocytes) are nucleated cells and eat up bacteria and foreign bodies or fight them by producing antibodies 

Blood Platelets (Thrombocytes) are small fragments of cells without nuclei and help in the clotting of blood.

Blood vessels: Blood flows in arteries and veins which are joined by thinner blood vessels called capillaries. Arteries carry away blood from heart to parts of body and veins bring blood to heart from other parts of the body.

Heart and Blood circulation: Heart is made of cardiac muscle fibres which rhythmically contract and relax all the time throughout life. 

One cycle of contraction and relaxation of muscles in wall of ventricle constitutes a heart beat. In one minute, heart beats 72 times which can be felt as a pulse on the wrist or neck.

Heart is four chambered: with two atria and two ventricles. Heart receives blood from all parts of the body and also pumps blood to all parts. Vena cava (largest vein) brings CO2 laden blood to heart. Pulmonary artery carries it to lungs. In the lungs, capillaries on alveoli exchange CO2 laden blood for O2 laden blood which goes again to heart via pulmonary vein.

Aorta (largest artery) takes oxygenated blood to all parts of the body. Since blood crosses the heart twice in one round the circulation is called double circulation.