Transfusions are not ordered by physicians unless there is a clinical necessity. The benchmark is usually when hemoglobin is less than 70 mg/litre (normal levels are around 120 mg/litre), and the patient is experiencing symptoms from acute anemia (from sudden serious blood loss), like dizziness, shortness of breath, headaches, lethargy, etc. They are not able to function as they normally do and their quality of life is diminished until their hemoglobin levels improve, which can take weeks without blood transfusions. Referring to it as "building up" someone is not the same as "feeding" - the focus is entirely on improving oxygen delivery, and often giving oxygen directly has not been effective simply because there are not enough red blood cells to do the necessary job.
Sometimes we'll see patients who have chronic (long-standing) anemia, who are not symptomatic and have no impairment in their day-to-day functionality, who don't want blood transfusions and they don't have blood transfusions ordered in that case.
The function of blood transfusions is to improve the body's ability to transport oxygen through the body. There's no nutritional value to a blood transfusion as the WTS wants JWs to believe, when it gives the comparison to "feeding" on blood. The idea of drinking blood is offensive to a lot of people, but nobody seems offended when people symbolically "drink the blood of Christ" when they "celebrate" the Eucharist or the Memorial of Christ's death.
It may take quite some time for you to shake off the fraudulent information that you have been programmed with by the WTS regarding blood transfusions. You may need to remind yourself over and over and over that blood transfusions are about improving OXYGEN DELIVERY, not about "feeding" before you no longer get that "sick" feeling.