Why does PTT prolong when heparin is administered?

Heparin is an anticoagulant that works primarily by enhancing the activity of antithrombin III, a protein in the blood that inhibits clotting factors, specifically thrombin and factor Xa. When heparin is administered, it binds to antithrombin III, causing a conformational change that increases its ability to inactivate these clotting factors more effectively.

As thrombin and factor Xa play crucial roles in the coagulation cascade, their inhibition leads to a slower clot formation. Consequently, this prolongs the Partial Thromboplastin Time (PTT), which is a test used to measure the efficacy of the intrinsic and common pathways of coagulation. A prolonged PTT indicates that the blood takes longer to clot, which is a direct result of the therapeutic action of heparin in inhibiting these key factors.

Thus, the correct understanding rests on the mechanism where heparin interferes with critical components of the coagulation process, consequently extending the time it takes for clotting to occur.