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About Blood
Blood Types
Although blood is made of the same basic elements, not all blood is alike. In fact, there are 8 common red blood cell types, which are determined by the presence or absence of certain antigens. Since some antigens can trigger a patient's immune system to attack the transfused blood cells, safe therapy with blood depends on careful blood typing and cross-matching.
How Blood Travels
Blood moves in two large, continuous circles through a network of blood vessels. The 'right circuit' moves blood from the right side of the heart through the lungs back to the heart (left side). The 'left circuit' moves blood from the heart to the rest of the body and then back to the heart. There are different types of blood vessels: arteries, capillaries, and veins. 
Arteries carry blood away from the heart. They branch out into smaller arteries, which connect to capillaries. The capillaries are very narrow — only one cell wide. Inside the capillaries, the red blood cells release oxygen, which passes through the thin capillary walls and into the surrounding tissue. The tissue releases waste products, like carbon dioxide, which passes through the thin capillary walls into the blood. Blood returns to the heart in the veins. Veins contain one-way valves to keep low-pressure blood flowing toward the heart, even against the pull of gravity. Because the blood in veins contains so little oxygen, it appears bluish in comparison to the bright red of oxygenated blood.
Blood Groups
Complex chemical substances found on the surfaces of red blood cells are different for each blood group. The two most important blood group systems in transfusion work are the ABO and Rh systems. Within the ABO system people can be one of four types – O, A, B or AB, whilst in the Rh system they can be either Rh positive or Rh negative. Each system is inherited independently of the other. Thus, there are eight main blood groups. They are, from the most common to the rarest:
O Positive
A Positive
O Negative
B Positive
A Negative
B Negative
AB Positive
AB Negative

Percentage of blood in the body
Approximately 8% of the body weight are blood. Thus a person weighing 50 Kg would have about 4000-ml blood. The average adult has about 5000 ml of blood. Blood volume changes slightly in the same person from time to time, varying with hot weather, muscular exercise, high altitudes and pregnancy
General information
Quality of blood drawn above 45 kg – 350 cc for whole blood above 50 kg – 450 cc for component separation
3 weeks for the RBC to get replenished. Volume of blood given replaced within 48 hours. Blood donation immediately after a meal or on an empty stomach is not avdvisable. Only disposable blood collection sets are used for collecting blood.