This is the first of our two posts on the absolute basics of clotting which can be covered by two swear word innuendos,
- taking the Pxxs
- the ‘F’ words
We hope to make the science of coagulation simple, by going at it in bite sized pieces that will in time build into a whole picture of coagulation enabling you to understand the science behind why you had a clot and how your medication works.
Today we’re learning about the first things that happen to stop the bleeding if you cut yourself, or have minor damage to the lining of your nose, bladder or bowel. This is called Primary Haemostasis
P for Primary haemostasis
there are two parts to primary haemostasis, luckily both starting with a P!
there is vasoconstriction or ‘pinching’ of the blood vessel that has been cut. Vaso means blood vessel
We discussed platelets in our ‘What’s in your blood?’ Q and A. Usually they can be thought of as smooth bits of broken plates but once they’re activated they change in shape and character.
Usually this wall (the endothelium) keeps platelets separated from proteins that would activate them, but suddenly the underlying wall structure is exposed. The jargon for this again starts with a ‘P’ and is the Procoagulant subendothelial matrix (consisting of proteins such as collagen, laminin, and fibronectin). This matrix causes platelets to
- stick to the exposed vessel wall
- become activated – once they’re activated they attract more platelets and produce a phospolipid called phosphatidylserine (note the ‘P’s again) which is a binding site for the proteins in the coagulation cascade (the second stage of clotting)
As a reaction to the damage to the vessel wall, muscle around the vessel contracts. It is smooth muscle which means muscle we can’t consciously control. This causes vasoconstriction, a narrowing of the vessel width, which slows blood flow.
Slower blood flow causes yet more activation of platelets as they spend longer in contact with the activation proteins from the exposed vessel wall.
3. A platelet plug forms
The activated platelets stick to the vessel wall and use their spikes to stick to each other. This happens very fast, within 20 seconds of cutting yourself. The plug formed is very soft and delicate, not a proper blood clot.
What can go wrong?
Primary haemostasis is a very temporary measure – literally a stop-gap measure! It needs to be followed by secondary haemostasis, (what we’re calling the F words of clotting) or the vessel will dilate again and the platelet plug will wash away so that bleeding occurs.
Some people don’t have enough platelets. This is known as thrombocytopenia. These people have poor primary haemostasis so may have nose bleeds, blood in their urine or pin-point bleeds in their skin.