Blood does not coagulate inside the body due to the presence of which substance?

Blood has a natural ability to coagulate, or form clots, to stop bleeding when blood vessels are injured. However, under normal conditions inside the body, blood does not coagulate spontaneously. This is due to the presence of heparin, a naturally occurring anticoagulant.

Heparin is a substance produced by certain cells in the liver, lungs, and mast cells. It works by inhibiting the action of thrombin and other clotting factors in the blood, which prevents the formation of fibrin, the protein responsible for creating a blood clot. By controlling the clotting process, heparin ensures that blood flows smoothly through vessels without forming unwanted clots that could block circulation.

Other substances listed in the options have different roles. Haemoglobin is the oxygen-carrying protein in red blood cells. Fibrin is the protein that forms a mesh during clotting, helping to stop bleeding. Plasma is the liquid part of blood that carries cells, nutrients, and hormones, but does not itself prevent clotting.

Heparin’s anticoagulant effect is critical for maintaining normal blood flow and preventing conditions such as thrombosis, where clots form inside blood vessels and can lead to heart attacks or strokes. Medical heparin is also used therapeutically to prevent clot formation during surgeries or in patients at high risk of clotting disorders.

Understanding the role of heparin is a fundamental part of biology, especially in hematology and human physiology, because it explains how the body regulates blood flow and prevents dangerous clotting inside vessels. This knowledge is key to comprehending how blood functions and maintains homeostasis in everyday science.