To understand how problems arise we must first learn about how the system works.
firstly Haemostasis, when we get a cut or injury that exposes our blood stream, our body goes through a process to cover the area in as quick time as possible to prevent loss of blood.
Primarily the blood vessels constrict to slow the blood flow and the loss of blood, even more this allows the broken extracellular matrix (ECM*) and endothelial lining to the blood flow and its components.
Platelets then flood to the scene, which have specific attachment proteins complementary to the collagen*. This holds the tissue of the broken blood vessel together. Now bound, The platelets get aggregated and change shape and more easily adhere to each other. The platelets work further by secreting more factors which in turn bring more platelets to the wound site. Now we have a Primary Platelet Plug.
To aid this 'sticky nature' among the platelets, is Fibrinogen which is present from the blood plasma. Fibrinogen acts as a glue and sticks platelets together.
Forming a Thrombus*.
Coagulation can now occur, The enzyme thrombin is then made through a series of factor activations and reactions. It also activates platelets to bind, increasing the positive feedback pathway. Thrombin breaks down fibrinogen to fibrin, this fibrinogen is a monomer that goes through polymerisation to form a fibrin polymer, using the help of the catalyst, Thrombin. The fibrin wraps itself around the platelets and other blood components stabilising the clot. As the fibrin traps some blood cells, it will seem to be red under the skin.
So when the clot comes into contact with the air it will dry and scab up.
In some, defects occur and this may lead to unwanted coagulation.
Usually diseases can stimulate this, such as Atherosclerosis*, where the atheroma ruptures causing a break in the cell wall of the blood vessel and so this causes the platelets to be exposed to the collagen in the ECM and becomes aggregated and forms a thrombus.
This can cause a lack of blood flow and may even stop it, causing decreased oxygen saturation to the tissues.
This thrombus may become very dangerous in another way, it may embolise* and if large enough they can cause significant damage. This blocks blood flow into tissues in limbs or even more dangerous in the coronary arteries of the heart and also may end up in the capillaries in the alveoli, where it may cause heart attacks or problems with gas exchange.
ECM - a network of intricate components that link to form the mechanical functions of the tissue
collagen - In this case, strengthens the tissue in the ECM, platelets bind to this.
Thrombus - A blood clot that forms in a vessel.
Atherosclerosis - A build up of fats and cholesterol in the artery wall of the blood vessel
Embolism - A blockage of an artery or blood vessel that is blocked by a rogue blood clot normally broken off from a thrombus.
NCBI - Physiology, Haemostasis
Khan Academy - Extraccellular Matrix and the cell wall