Blood does not coagulate inside the body due to the presence of :

Blood remains in a fluid state within the circulatory system primarily due to the presence of heparin, a natural anticoagulant. Heparin is a sulfated polysaccharide that prevents excessive coagulation by inactivating key enzymes like thrombin and factor Xa through an antithrombin-dependent mechanism. While the body possesses a complex hemostatic system involving platelets and plasma proteins to repair vascular damage [1], systemic anticoagulants like heparin ensure that these pro-coagulant responses do not lead to dangerous internal clot formation. In contrast, fibrin is the protein that actually forms the structural mesh of a blood clot [4], and haemoglobin is a respiratory pigment responsible for oxygen transport [3]. Plasma is the liquid medium of blood containing various clotting factors, but it requires anticoagulants to prevent spontaneous coagulation [2]. Therefore, heparin is the specific substance that inhibits clotting inside the body.

Sources

1. [1] https://www.ncbi.nlm.nih.gov/books/NBK538247/
2. [2] Science , class X (NCERT 2025 ed.) > Chapter 5: Life Processes > Activity 5.7 > p. 91
3. [4] Science , class X (NCERT 2025 ed.) > Chapter 5: Life Processes > Do You Know? > p. 90
4. [3] https://www.sciencedirect.com/topics/neuroscience/blood-plasma