1. ABO group is divided into four groups
2. A group consists of antigen A and antibody B
3. Group B consists of antigen B and antibody A
4. Group AB contains antigens A and B, but no antibodies
5. Group O has no antigens, but has antibodies A and B
6. It is called universal recipient as it can accept blood from all groups in group AB
7. O group is called universal donor as it can give blood to all groups

In 1901, biologist Karl Landsteiner classified human blood based on the presence or absence of antigens in the blood cells and called ABO blood groups. ABO blood group is divided into four groups.
1. A group: 23% of blood is A group.
2. B group: 35% of blood is B group.
3. AB group: 8% of blood is AB group.
4. O group: 34% of blood is O group.

The word hemoglobin is formed from heme meaning iron and globin meaning protein. The iron-containing substance in the cytoplasm of red blood cells is called hemoglobin. Due to this the blood is red in color. 33% of the weight of a red blood cell is hemoglobin. Each human red blood cell contains about 270 million hemoglobin molecules. 100 ml of blood contains 15/16 grams of hemoglobin. Each gram of hemoglobin can carry 1.36-1.40 ml of oxygen. If hemoglobin is low in the body, it is called oligocythemia. Again, if the amount of hemoglobin is increased, it is called polycythemia.

The word erythropoiesis is derived from the Greek words erythro meaning red and poiesis meaning producing. The process by which red blood cells are produced is called erythropoiesis. Red blood cells are produced from bone marrow, liver, spleen and thymus. Again, it gets destroyed in the liver and spleen. There, the red blood cells change and produce bile acids called bilirubin and biliverdin. 10 million red blood cells are produced every second, while 10 million red blood cells are destroyed every second.

The yellow fluid that remains after blood clots is called serum. The molecule contains serotonin. Serotonin is released into the serum as the molecule breaks down. It is like plasma. It contains serotonin which causes constriction of blood vessels. The branch of biology that deals with serum is called Serology. Serum does not contain fibrinogen, prothrombin, labile and antihemophilic factors.

1. Blood is always flowing at high speed.
2. Blood contains 50 types of procoagulant factor (blood clotting) and anticoagulant factor (prevents clotting).
3. Because the blood vessel walls are very smooth.
4. Anticoagulant factors include heparin and thrombomodulin, which prevent blood clotting.
5. Because active coagulant factors are always removed by the liver.

Substances that prevent blood from clotting are called anticoagulants. Blood contains 50 types of procoagulant and anticoagulant factors. Heparin (mast cells), hirudin (leeches), thrombomodulin, sodium citrate, sodium oxalate etc. are anticoagulants. Heparin, hirudin and thrombomodulin inhibit the generation of thrombin from blood prothrombin. Sodium citrate reacts with calcium ions in the blood to form calcium citrate. Calcium citrate is degraded. Sodium oxalate reacts with calcium ions in the blood to form calcium oxalate. Calcium oxalate is excreted and causes kidney stones.

1. Factor-I or Fibrinogen: Fibrinogen is a globulin like protein. It is converted to fibrin during blood clotting.
2. Factor-II or Prothrombin: Prothrombin is a plasma protein. It is converted to thrombin during blood clotting.
3. Factor-III or Thromboplastin: Thromboplastin is released from the broken microspheres. It converts prothrombin to thrombin in the presence of calcium ions.
4. Factor-IV or Calcium: Calcium ion helps in the production of thromboplastin. It converts prothrombin to thrombin.
5. Factor-V or Labile: Labile is a protein-like substance present in plasma. It converts prothrombin to thrombin.
6. Factor-VI or Axillarin: It is known as labile factor.
7. Factor-VII or Proconvertin: Proconvertin is currently not available. It is projected.
8. Factor VIII or Antihemophilic: Antihemophilic is a protein-like substance present in plasma. It helps in the production of thromboplastin.
9. Factor-IX or CRISTMAS: CRISTMAS is a protein-like substance present in plasma. It helps in the production of thromboplastin.
10. Factor-X or Stewart: Deficiency of Stewart leads to impaired blood clotting.
11. Factor-XI or Plasma Thromboplastin: It is a type of protein. It participates in the formation of thromboplastin.
12. Factor-XII or Hegman: Hegman activates kallikrein. It produces a substance called plasmakinin. Plasmakinin increases the permeability and dilatability of blood vessels.
13. Factor-XIII or Stabilizing: With the help of stabilizing calcium ions, it transforms soft tissue particles into insoluble hard fibers.

Bleeding occurs when a blood vessel is cut in any part of the body and the volume of blood in the body decreases. It disrupts food supply, gaseous transport, excreta removal etc. When the blood volume decreases, problems occur in physiological functions. Sometimes the organism may die. Bleeding must be stopped to prevent these problems.

The time it takes for the blood to clot is called blood clotting time. Human normal bleeding time is 1-4 minutes. Blood collection time is 4-5 minutes. If the blood does not clot within this period, it must be assumed that one or more factors are missing. Absence of any secretion factor results in profuse bleeding from the wound site. This is called hemophilia.