text: A very interesting question, really.\n\nFirstly, we need to look at what causes the blood clot.  The answer lies in the platelet.  An irregularly shaped, colorless body that is present in the blood.  It survives for 7-10 days before being broken down in the liver/spleen by phagocytes.  \n\nIndeed, blood clots when it is exposed to air.  The body has amazingly found a loophole in blood being exposed to air (for the 2 conditions)  This loophole lies in the loss of blood.  If the body detects a loss of blood and the presence of air at the same time, the platelets immediately kick into action.  The tiny platelets immediately rush to the wound, form a layer (scab) by bursting and spilling its contents.  Its contents, along with calcium, the vitamin K and a protein, fibrinogen, forms a clot.  \n\nAll these components forms a clot by first forming fibrin.  These fibrin are actually tiny threads which later form a wire-like mesh.  This mesh help to hold the red blood cells inside the body.  This explains why our scab is red initially but turn brown; such is the color of our blood.