A search for clotting’s accelerator found its brake
Blood clotting is useful when a vessel is damaged, but dangerous when a clot forms inside a vessel or during surgery. In 1916, Jay McLean was studying substances in canine liver that might promote clotting. The liver extract he examined did the opposite: it delayed clot formation. The unexpected result mattered because it pointed toward a way to control clotting, not merely encourage it.
The substance became known as heparin, a name drawn from hepar, the Greek word for liver. McLean’s finding was an early discovery, not yet a ready-to-use medicine. Turning a biological extract into a dependable treatment would require further work to separate and prepare the active material.
How heparin changes the clotting process
A clot forms through a chain of chemical reactions. One step activates proteins that help make fibrin, threadlike strands that reinforce a clot. Heparin works by strengthening the action of antithrombin, a natural blood protein that switches off key clotting proteins, including thrombin and factor Xa. With those steps restrained, the body is less able to build a clot.
That effect makes heparin valuable when clinicians need to reduce clot formation, but it also creates a tradeoff: too much anticoagulation can cause bleeding. A treatment that suppresses clotting therefore has to be given and monitored with care.
From laboratory surprise to practical tool
McLean’s observation opened a path rather than completing it. Researchers later developed ways to obtain heparin in forms suitable for medical use, making anticoagulation possible in settings where an unwanted clot could be life-threatening. The twist in the original experiment is the lasting point: while searching for a substance that would make blood clot more readily, McLean found a way to hold that process back.
