Stem cells introduced into the bloodstream do pass through the lungs, but their behavior in the body depends on several factors such as the type of stem cells, the method of administration, their size, and the condition of the organism.
- Passing through the lungs: The lungs play an important role as a “filter” for cells introduced into the general circulation. When administered intravenously, many stem cells, especially larger ones (such as mesenchymal stem cells), can become temporarily trapped in the lungs. This is referred to as the “pulmonary first-pass effect.” However, over time, some cells may leave the lungs and distribute to other parts of the body.
- Remaining in the bloodstream: After passing through the lungs, some cells may remain in circulation and move toward other organs, but many stem cells tend to settle in tissues or target organs, particularly if those tissues are damaged or inflamed.
- Crossing the blood-brain barrier (BBB): The blood-brain barrier is a highly selective barrier that restricts the entry of substances and cells from the bloodstream into the central nervous system. Most stem cells cannot freely cross this barrier. However, in cases of inflammation or brain injury, when the BBB is compromised, cell penetration becomes possible. There are also certain strategies that allow stem cells to bypass the BBB, such as direct administration into the cerebrospinal fluid (intrathecal delivery) or the use of carriers to facilitate cell delivery across the barrier.
