The Human Blood System

  A Human Body has about 5 liters of blood

Blood is the red fluid that supplies the cells with the food and oxygen they need for work and growth. A healthy person has about 5 liters of blood in his body. The human blood is made up of plasma and blood corpuscles. Plasma is a light yellow colored fluid which contains 92% water and 8% proteins, sugar, salts and other minerals. Blood contains three kinds of corpuscles they are red blood corpuscles, white blood corpuscles and platelets.

            Red blood corpuscles or erythrocytes are disk shaped, flat and biconcave. They do not contain nuclei. Red blood corpuscles contain a red pigment called hemoglobin which makes the blood look red. New red cells form in the bone narrow. There are about 5000 million red cells in 1 cc of blood. Red blood cell lives from 50 to 120 days. Their size is about 0.007 mm.

            White blood cells or leucocytes are mostly formed in the bone narrow while some of them in the spleen and lymph glands. White cells are larger than the red cells. Their average size is 0.007 to 0.12 mm in diameter. There are about 11 million white cells in 1 cc of blood. White cells have a nucleus. They are actually transparent and white as they do not contain hemoglobin. White cells protect the body against diseases and help in fighting infection lymphocytes are white cells that control immunity.

            Platelets or Thrombocytes are tiny disks with a diameter of 0.002 to .004 mm. there are 150,000 to 400,000 platelets in one cubic millimeter of blood. They have no nucleus. If a small blood vessel is cut. Platelets stick to the damaged edges and to each other. As they pile up, they form a temporary seal over the injury. At the same time, they release a substance that starts the process of blood clotting. Blood clotting prevents the loss of blood. People having a less number of platelets in their blood bleed excessively before the blood clotting takes place.

  • Red cells carry oxygen
  • White cells fight infection
  • Platelets stop bleeding in injuries

Blood Group

            A person bleeds profusely due to some injury or during an operation, the blood is transfused into his body. But it cannot be transfused without matching the blood groups. Karl Land Steainer in 1931 classified the human blood into three groups. Later, De Castello and Stulri identified one more group. Human blood is divided into four different groups based on the presence or absence of certain antigens in red blood cells. These blood groups are A, B, AB and O.

            Antigens are special proteins that stimulate the production of antibodies. If antibodies of one blood group are mixed with another group, they react with antigens and cause the red blood cells to clump together. Clumping can block small blood vessels and result in serious illness or death. The distribution of antibodies is shown below: 

Blood Group
Antigen in RBC
Antibodies in Plasma
Only A
Only B
Only B
Only A
Both A, B
Both A, B
          Doctors prefer to use similar ABO blood types during transfusion to avoid any possibility of clumping. Type O is the universal donor ad can be transfused into anyone. Type AB is the universal recipient and can receive blood of any ABO type.

In 1940, an additional rhesus (Rh) factor was discovered, which must be considered before the blood transfusion.



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