Final answer:
Heparin affects the common coagulation pathway by inhibiting thrombin activity, preventing the formation of a stable blood clot.
Step-by-step explanation:
The coagulation pathway affected by heparin is the common pathway. Heparin is an anticoagulant that works by inhibiting the activity of thrombin, which is an enzyme involved in the conversion of fibrinogen to fibrin during blood clot formation. By inhibiting thrombin, heparin prevents the formation of a stable blood clot.