Video Transcript
Consider the equations. (1) Blood platelets plus destroyed
cells forms thromboplastin. (2) Prothrombin, in the presence of
thromboplastin, forms thrombin. (3) X, in the presence of thrombin,
forms fibrin. Which of the following should
replace label X? (A) Heparin, (B) albumin, (C)
fibrinogen, or (D) globulin.
Platelets are cell fragments that
circulate in the blood and are responsible for forming blood clots. Blood clot formation helps to
prevent excess blood loss when blood vessels are damaged. This allows our vital organs and
tissues to continue to receive the oxygen and nutrients they require, promoting our
survival. This series of equations shows a
helpful process that occurs when platelets are activated due to damage to a blood
vessel. Let’s take a closer look at the
mechanism of blood clotting so we can work out what the substance replaced by the
letter X is.
When the cells that line the walls
of blood vessels are damaged, platelets are attracted and adhere to the site of
damage and become activated. The activated platelets release
factors that attract even more platelets, causing them all to aggregate together and
block up the damaged site with a platelet plug, preventing the other components of
the blood from leaking out of the damaged vessel.
The formation of the platelet plug
triggers a cascade of biochemical reactions, some of which are shown to us in the
question. First, the damaged blood vessel and
the tissues surrounding it express the tissue factor thromboplastin. Thromboplastin converts
prothrombin, a protein produced by the liver, into an active enzyme called
thrombin. This reaction must take place in
the presence of calcium ions.
Fibrinogen is another protein
produced by the liver, which is usually found dissolved in blood plasma. In the presence of thrombin,
fibrinogen is converted into a substance called fibrin, which is insoluble in blood
plasma. Strands of fibrin precipitate out
of the blood forming a net, or mesh, which reinforces the platelet plug by trapping
other red blood cells and activated platelets. The formation of this strong clot
prevents excess blood loss while the blood vessel is repaired.
Now we know the correct answer to
our question. The factor that should replace X is
(C) fibrinogen.
