Bloodworks’ Vein to Vein curriculum makes it easy for educators to create lesson plans around blood and blood donation. It’s one of the many student programs Bloodworks offers to our community.
Have you ever wondered why blood is collected from and transfused into veins and not arteries?
Collection is simple: it’s a whole lot easier to draw blood from a vein than an artery. Veins tend to be closer to the surface of the skin and have thinner walls and fewer nerves than arteries, so it’s a better experience for both the blood collection specialist and the blood donor.
Transfusion requires a little understanding of anatomy.
As blood circulates through your body, it transports oxygen and nutrients and picks up carbon dioxide and waste.
Veins are one of several types of blood vessels in the body. They carry blood to the right section of the heart, where it is pumped into the lungs to receive oxygen and offload carbon dioxide. Most veins transport deoxygenated blood, except for the pulmonary vein, which channels oxygenated blood back to the left side of the heart from the lungs.
From there, arteries shuttle the oxygenated blood from the heart to the body.
The veins and arteries leading to and from the heart branch off into smaller veins and arteries, then venules, and arterioles, and eventually tiny capillaries.
“If you give something into a vein, it gets more and more diluted, because every vein leads into another larger vein and that into another larger vein and eventually to the right heart,” said Dr. Moritz Stolla of Bloodworks Research Institute.
“If you inject something into an artery it gets not diluted. Instead it stays at the same concentration but gets pumped in the microcirculation, so the local effect could be very detrimental.”
For example, platelets stop bleeding by clumping together in aggregates. Prior to transfusion, platelets are kept in constant motion to prevent these aggregates from forming. Platelet units with noticeable aggregates are not transfused.
If an aggregate does slip through, there are measures in place to catch them.
“If you give an aggregate to somebody, there’s a bed-side transfusion filter. That filter should filter anything out below 170-240 microns,” explained Dr. Stolla.
If a small aggregate makes it through the filter and into the vein, it will travel into progressively bigger veins before circulating through the right chambers of the heart and into the lung vessels.
“There are billions and billions of tiny lung vessels, so if you block one tiny lung vessel with one aggregate, you won’t feel it,” Dr. Stolla said. “So, what happens if you give aggregates with platelets? Likely nothing.”
However, if this same unit were transfused into an artery, it would travel through progressively smaller and smaller arteries.
From there, there’s the risk it could travel to organs, such as the brain or kidney, get stuck in a small vessel, and cut off blood flow, causing significant damage or even a stroke.
There are a few rare exceptions. Patients with congenital heart defects may have a shunt (abnormal blood flow) between their right and left atria in their heart. In these cases, blood doesn’t follow the typical pattern through the heart and lungs, and an aggregate could travel into the arteries.
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