What exactly happens in the human body during a blood transfusion? How is the extra blood accommodated?
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New blood is inserted into circulation and flows throughout the body just as the old blood does. So long as you’re replacing the amount of blood and blood products that were lost, there is really no need for accommodation. If you’re overdoing it on red blood cells, increased viscosity and clotting can occur. Overdoing it on plasma or saline to a mild extent can be filtered out by the kidneys; but to an excessive extent can cause fluid overload, resulting in backup into the tissues (including the lungs) and strain the heart.
Your blood vessels aren’t rigid. They stretch and contract with each heartbeat to allow blood to flow past, and your blood volume can (and does) vary over the course of the day based on things like how much water you’re drinking, salt intake, and other factors. (This is part of why high blood pressure is bad: it keeps the vessels stretched out in a way that makes them less elastic.)
Transfusions are also usually used in cases where you’ve *lost* blood, so your total blood volume isn’t actually higher than it previously was in most cases.
Yes you are absolutely right in thinking that fluid overload is a possible complication of receiving blood, because it is.
When a medical team is administering blood to a patient they are attentive to the possible complications that can arise. The one that most people think of is ABO incompatibility but that is just one of the many possible complications and it actually occurs very infrequently.
Volume overload following a transfusion is much more common however. I just looked it up and found it carries a risk of happening in one in 100 transfusions, according to one article I found on Medscape. It happens more in patients who already have heart or kidney problems or in patients who receive blood too quickly. In these patients the excess fluid will overload the circulatory system and may leak into the lungs causing the person to feel like they can’t breath.
Of course in the majority of people this won’t happen but it is a very real complication and cause of death following a blood transfusion.
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