Final answer:
Evacuated tubes automatically fill with blood due to a built-in vacuum for blood collection, but in the context of ectopic pregnancies, internal bleeding occurs when an embryo implants within the uterine tube due to blockage by scar tissue.
Step-by-step explanation:
When discussing what can cause evacuated tubes to fill with blood automatically, the context provided suggests that the student might be referring to the process of blood collection using vacuum tubes in a medical setting, or possibly about physiological processes such as those occurring during a tubal or ectopic pregnancy. If the former, evacuated tubes fill with blood automatically due to the vacuum created inside the tubes, which draws the blood in when the tube is connected to a needle inserted in a vein. However, given the information provided about tubal pregnancies, this seems to be the actual topic.
An ectopic pregnancy can occur when scar tissue from a sexually transmitted bacterial infection causes blockage or impediment within the uterine tube. This scar tissue can snag or completely block an embryo from reaching the uterus, sometimes leading to a tubal pregnancy where implantation occurs within the tube itself. This can cause internal bleeding, which in turn triggers smooth muscle contractions that may expel the embryo. If an ectopic pregnancy is detected early enough, methotrexate may be administered to cease the embryo's development by inhibiting folic acid metabolism. In severe cases where the tube is ruptured, surgical repair is critical.