Red blood cells...common causes of loss.?

Trauma of course is number one. Hemorrhage in the gastro-intestinal tract from many causes - esophageal varices, gastric ulcers, A/V malformation, etc. There are many types of hemolysis - you can look up the many types of hemolytic anemias. Bleeding disorders such as hemophilia and von Willebrands disease. A low platelet count - "thrombocytopenia" - which you can look up if you want to see the dozens of causes for this

The requirement of three units of red cells is not extremely high at all.

A three unit transfusion does not equate with a 70% loss of rbc's.

Are you sure you have your facts straight ?

Actually there are very few facts / information in this question.

If you ask specific questions you will receive better answers.

Losing Red Blood Cells

Very serious. They might have some blood disease such as leukemia or a bad infection.

Also loss of red blood cells can be due to diet. You might be anemic, or guess you would be anyway. But the foods that help build up red blood are red meats, eggs, green veges and anything with iron in it. You can take iron pills, or molasses to help.

No it won't. Your body will replenish the lose of blood with making more blood. When the heart does that, the oxygen will be replaced as well, at the same time.